diff --git a/Epplus-FixCalcsEdition/EPPlus/CellStore.cs b/Epplus-FixCalcsEdition/EPPlus/CellStore.cs
index dcaf531..f5b83c1 100644
--- a/Epplus-FixCalcsEdition/EPPlus/CellStore.cs
+++ b/Epplus-FixCalcsEdition/EPPlus/CellStore.cs
@@ -337,6 +337,8 @@ internal class CellStore<T> : IDisposable// : IEnumerable<ulong>, IEnumerator<ul
         internal const int ColSizeMin = 32;
         internal const int PagesPerColumnMin = 32;
 
+        // The column index is resized; its identity must never be used as a monitor.
+        internal readonly object SyncRoot = new object();
         List<T> _values = new List<T>();
         internal ColumnIndex[] _columnIndex;
         internal IndexBase _searchIx = new IndexBase();
@@ -357,203 +359,221 @@ public CellStore ()
 	    }
         internal int GetPosition(int Column)
         {
-            if(Column < ColumnCount && _columnIndex[Column].Index==Column)      //Check if th column is lesser than
+            lock (SyncRoot)
             {
-                return Column;
-            }
-            else
-            {
-                _searchIx.Index = (short)Column;
-                return Array.BinarySearch(_columnIndex, 0, ColumnCount, _searchIx);
+                if(Column < ColumnCount && _columnIndex[Column].Index==Column)      //Check if th column is lesser than
+                {
+                    return Column;
+                }
+                else
+                {
+                    _searchIx.Index = (short)Column;
+                    return Array.BinarySearch(_columnIndex, 0, ColumnCount, _searchIx);
+                }
             }
         }
         internal CellStore<T> Clone()
         {
-            int row, col;
-            var ret=new CellStore<T>();
-            for (int c = 0; c < ColumnCount; c++)
+            lock (SyncRoot)
             {
-                col = _columnIndex[c].Index;
-                for (int p = 0;p < _columnIndex[c].PageCount; p++)
+                int row, col;
+                var ret=new CellStore<T>();
+                for (int c = 0; c < ColumnCount; c++)
                 {
-                    for (int r = 0; r < _columnIndex[c]._pages[p].RowCount; r++)
+                    col = _columnIndex[c].Index;
+                    for (int p = 0;p < _columnIndex[c].PageCount; p++)
                     {
-                        row = _columnIndex[c]._pages[p].IndexOffset + _columnIndex[c]._pages[p].Rows[r].Index;
-                        ret.SetValue(row, col, _values[_columnIndex[c]._pages[p].Rows[r].IndexPointer]);
+                        for (int r = 0; r < _columnIndex[c]._pages[p].RowCount; r++)
+                        {
+                            row = _columnIndex[c]._pages[p].IndexOffset + _columnIndex[c]._pages[p].Rows[r].Index;
+                            ret.SetValue(row, col, _values[_columnIndex[c]._pages[p].Rows[r].IndexPointer]);
+                        }
                     }
                 }
+                return ret;
             }
-            return ret;
         }
         internal int Count
         {
             get
             {
-                int count=0;
-                for (int c = 0; c < ColumnCount; c++)
+                lock (SyncRoot)
                 {
-                    for (int p = 0; p < _columnIndex[c].PageCount; p++)
+                    int count=0;
+                    for (int c = 0; c < ColumnCount; c++)
                     {
-                        count += _columnIndex[c]._pages[p].RowCount;
+                        for (int p = 0; p < _columnIndex[c].PageCount; p++)
+                        {
+                            count += _columnIndex[c]._pages[p].RowCount;
+                        }
                     }
+                    return count;
                 }
-                return count;
             }
         }
         internal bool GetDimension(out int fromRow, out int fromCol, out int toRow, out int toCol)
         {
-            if (ColumnCount == 0)
+            lock (SyncRoot)
             {
-                fromRow = fromCol = toRow = toCol = 0;
-                return false;
-            }
-            else
-            {
-                fromCol=_columnIndex[0].Index;
-                var fromIndex = 0;
-                if (fromCol <= 0 && ColumnCount > 1)
-                {
-                    fromCol = _columnIndex[1].Index;
-                    fromIndex = 1;
-                }
-                else if(ColumnCount == 1 && fromCol <= 0)
+                if (ColumnCount == 0)
                 {
                     fromRow = fromCol = toRow = toCol = 0;
                     return false;
                 }
-                var col = ColumnCount - 1;
-                while (col > 0)
+                else
                 {
-                    if (_columnIndex[col].PageCount == 0 || _columnIndex[col]._pages[0].RowCount > 1 || _columnIndex[col]._pages[0].Rows[0].Index > 0)
+                    fromCol=_columnIndex[0].Index;
+                    var fromIndex = 0;
+                    if (fromCol <= 0 && ColumnCount > 1)
                     {
-                        break;
+                        fromCol = _columnIndex[1].Index;
+                        fromIndex = 1;
                     }
-                    col--;
-                }
-                toCol=_columnIndex[col].Index;
-                if (toCol == 0)
-                {
-                    fromRow = fromCol = toRow = toCol = 0;
-                    return false;                    
-                }
-                fromRow = toRow= 0;
-
-                for (int c = fromIndex; c < ColumnCount; c++)
-                {                    
-                    int first, last;
-                    if (_columnIndex[c].PageCount == 0) continue;                    
-                    if (_columnIndex[c]._pages[0].RowCount > 0 && _columnIndex[c]._pages[0].Rows[0].Index > 0)
+                    else if(ColumnCount == 1 && fromCol <= 0)
                     {
-                        first = _columnIndex[c]._pages[0].IndexOffset + _columnIndex[c]._pages[0].Rows[0].Index;
+                        fromRow = fromCol = toRow = toCol = 0;
+                        return false;
                     }
-                    else
+                    var col = ColumnCount - 1;
+                    while (col > 0)
                     {
-                        if(_columnIndex[c]._pages[0].RowCount>1)
+                        if (_columnIndex[col].PageCount == 0 || _columnIndex[col]._pages[0].RowCount > 1 || _columnIndex[col]._pages[0].Rows[0].Index > 0)
                         {
-                            first = _columnIndex[c]._pages[0].IndexOffset + _columnIndex[c]._pages[0].Rows[1].Index;
+                            break;
                         }
-                        else if (_columnIndex[c].PageCount > 1)
+                        col--;
+                    }
+                    toCol=_columnIndex[col].Index;
+                    if (toCol == 0)
+                    {
+                        fromRow = fromCol = toRow = toCol = 0;
+                        return false;
+                    }
+                    fromRow = toRow= 0;
+
+                    for (int c = fromIndex; c < ColumnCount; c++)
+                    {
+                        int first, last;
+                        if (_columnIndex[c].PageCount == 0) continue;
+                        if (_columnIndex[c]._pages[0].RowCount > 0 && _columnIndex[c]._pages[0].Rows[0].Index > 0)
                         {
-                            first = _columnIndex[c]._pages[0].IndexOffset + _columnIndex[c]._pages[1].Rows[0].Index;
+                            first = _columnIndex[c]._pages[0].IndexOffset + _columnIndex[c]._pages[0].Rows[0].Index;
                         }
                         else
                         {
-                            first = 0;
+                            if(_columnIndex[c]._pages[0].RowCount>1)
+                            {
+                                first = _columnIndex[c]._pages[0].IndexOffset + _columnIndex[c]._pages[0].Rows[1].Index;
+                            }
+                            else if (_columnIndex[c].PageCount > 1)
+                            {
+                                first = _columnIndex[c]._pages[0].IndexOffset + _columnIndex[c]._pages[1].Rows[0].Index;
+                            }
+                            else
+                            {
+                                first = 0;
+                            }
+                        }
+                        var lp = _columnIndex[c].PageCount - 1;
+                        while(_columnIndex[c]._pages[lp].RowCount==0 && lp!=0)
+                        {
+                            lp--;
+                        }
+                        var p = _columnIndex[c]._pages[lp];
+                        if (p.RowCount > 0)
+                        {
+                            last = p.IndexOffset + p.Rows[p.RowCount - 1].Index;
+                        }
+                        else
+                        {
+                            last = first;
+                        }
+                        if (first > 0 && (first < fromRow || fromRow == 0))
+                        {
+                            fromRow=first;
+                        }
+                        if (first>0 && (last > toRow || toRow == 0))
+                        {
+                            toRow=last;
                         }
                     }
-                    var lp = _columnIndex[c].PageCount - 1;
-                    while(_columnIndex[c]._pages[lp].RowCount==0 && lp!=0)
-                    {
-                        lp--;
-                    }
-                    var p = _columnIndex[c]._pages[lp];
-                    if (p.RowCount > 0)
+                    if (fromRow <= 0 || toRow <= 0)
                     {
-                        last = p.IndexOffset + p.Rows[p.RowCount - 1].Index;
+                        fromRow = fromCol = toRow = toCol = 0;
+                        return false;
                     }
                     else
                     {
-                        last = first;
-                    }
-                    if (first > 0 && (first < fromRow || fromRow == 0))
-                    {
-                        fromRow=first;
-                    }
-                    if (first>0 && (last > toRow || toRow == 0))
-                    {
-                        toRow=last;
+                        return true;
                     }
                 }
-                if (fromRow <= 0 || toRow <= 0)
-                {
-                    fromRow = fromCol = toRow = toCol = 0;
-                    return false;
-                }
-                else
-                {
-                    return true;
-                }
             }
         }
         internal int FindNext(int Column)
         {
-            var c = GetPosition(Column);
-            if (c < 0)
+            lock (SyncRoot)
             {
-                return ~c;
+                var c = GetPosition(Column);
+                if (c < 0)
+                {
+                    return ~c;
+                }
+                return c;
             }
-            return c;
         }
         internal T GetValue(int Row, int Column)
         {
-            int i = GetPointer(Row, Column);
-            if (i >= 0)
+            lock (SyncRoot)
             {
-                return _values[i];
-            }
-            else
-            {
-                return default(T);                
+                int i = GetPointer(Row, Column);
+                if (i >= 0)
+                {
+                    return _values[i];
+                }
+                else
+                {
+                    return default(T);
+                }
+                //var col = GetPosition(Column);
+                //if (col >= 0)
+                //{
+                //    var pos = _columnIndex[col].GetPosition(Row);
+                //    if (pos >= 0)
+                //    {
+                //        var pageItem = _columnIndex[col].Pages[pos];
+                //        if (pageItem.MinIndex > Row)
+                //        {
+                //            pos--;
+                //            if (pos < 0)
+                //            {
+                //                return default(T);
+                //            }
+                //            else
+                //            {
+                //                pageItem = _columnIndex[col].Pages[pos];
+                //            }
+                //        }
+                //        short ix = (short)(Row - pageItem.IndexOffset);
+                //        var cellPos = Array.BinarySearch(pageItem.Rows, 0, pageItem.RowCount, new IndexBase() { Index = ix });
+                //        if (cellPos >= 0)
+                //        {
+                //            return _values[pageItem.Rows[cellPos].IndexPointer];
+                //        }
+                //        else //Cell does not exist
+                //        {
+                //            return default(T);
+                //        }
+                //    }
+                //    else //Page does not exist
+                //    {
+                //        return default(T);
+                //    }
+                //}
+                //else //Column does not exist
+                //{
+                //    return default(T);
+                //}
             }
-            //var col = GetPosition(Column);
-            //if (col >= 0)  
-            //{
-            //    var pos = _columnIndex[col].GetPosition(Row);
-            //    if (pos >= 0) 
-            //    {
-            //        var pageItem = _columnIndex[col].Pages[pos];
-            //        if (pageItem.MinIndex > Row)
-            //        {
-            //            pos--;
-            //            if (pos < 0)
-            //            {
-            //                return default(T);
-            //            }
-            //            else
-            //            {
-            //                pageItem = _columnIndex[col].Pages[pos];
-            //            }
-            //        }
-            //        short ix = (short)(Row - pageItem.IndexOffset);
-            //        var cellPos = Array.BinarySearch(pageItem.Rows, 0, pageItem.RowCount, new IndexBase() { Index = ix });
-            //        if (cellPos >= 0) 
-            //        {
-            //            return _values[pageItem.Rows[cellPos].IndexPointer];
-            //        }
-            //        else //Cell does not exist
-            //        {
-            //            return default(T);
-            //        }
-            //    }
-            //    else //Page does not exist
-            //    {
-            //        return default(T);
-            //    }
-            //}
-            //else //Column does not exist
-            //{
-            //    return default(T);
-            //}
         }
         int GetPointer(int Row, int Column)
         {
@@ -600,24 +620,30 @@ int GetPointer(int Row, int Column)
         }
         internal bool Exists(int Row,int Column)
         {
-            return GetPointer(Row, Column)>=0;
+            lock (SyncRoot)
+            {
+                return GetPointer(Row, Column)>=0;
+            }
         }
         internal bool Exists(int Row, int Column, ref T value)
         {
-            var p=GetPointer(Row, Column);
-            if (p >= 0)
+            lock (SyncRoot)
             {
-                value = _values[p];
-                return true;
-            }
-            else
-            {                
-                return false;
+                var p=GetPointer(Row, Column);
+                if (p >= 0)
+                {
+                    value = _values[p];
+                    return true;
+                }
+                else
+                {
+                    return false;
+                }
             }
         }
         internal void SetValue(int Row, int Column, T Value)
         {
-            lock(_columnIndex)
+            lock (SyncRoot)
             {
                 var col = GetPosition(Column);          //Array.BinarySearch(_columnIndex, 0, ColumnCount, new IndexBase() { Index = (short)(Column) });
                 var page = (short)(Row >> pageBits);
@@ -689,7 +715,7 @@ internal void SetValue(int Row, int Column, T Value)
         /// <param name="Value"></param>
         internal void SetRangeValueSpecial(int fromRow, int fromColumn, int toRow, int toColumn, SetRangeValueDelegate Updater, object Value)
         {
-            lock (_columnIndex)
+            lock (SyncRoot)
             {
                 // split row to page groups (pageIndex to RowNo List)
                 Dictionary<short, List<int>> pages = new Dictionary<short, List<int>>();
@@ -775,7 +801,7 @@ internal void SetRangeValueSpecial(int fromRow, int fromColumn, int toRow, int t
         // Set object's property atomically
         internal void SetValueSpecial(int Row, int Column, SetValueDelegate Updater, object Value)
         {
-            lock (_columnIndex)
+            lock (SyncRoot)
             {
                 //var col = Array.BinarySearch(_columnIndex, 0, ColumnCount, new IndexBase() { Index = (short)(Column) });
                 var col = GetPosition(Column);
@@ -840,7 +866,7 @@ internal void SetValueSpecial(int Row, int Column, SetValueDelegate Updater, obj
 
         internal void Insert(int fromRow, int fromCol, int rows, int columns)
         {
-            lock (_columnIndex)
+            lock (SyncRoot)
             {
 
                 if (columns > 0)
@@ -925,7 +951,7 @@ internal void Delete(int fromRow, int fromCol, int rows, int columns)
         }
         internal void Delete(int fromRow, int fromCol, int rows, int columns, bool shift)
         {
-            lock (_columnIndex)
+            lock (SyncRoot)
             {
                 if (columns > 0 && fromRow == 0 && rows >= ExcelPackage.MaxRows)
                 {
@@ -1607,16 +1633,19 @@ public ulong Current
 
         public void Dispose()
         {
-            if(_values!=null) _values.Clear();
-            for(var c=0;c<ColumnCount;c++)
+            lock (SyncRoot)
             {
-                if (_columnIndex[c] != null)
+                if(_values!=null) _values.Clear();
+                for(var c=0;c<ColumnCount;c++)
                 {
-                    ((IDisposable)_columnIndex[c]).Dispose();
+                    if (_columnIndex[c] != null)
+                    {
+                        ((IDisposable)_columnIndex[c]).Dispose();
+                    }
                 }
+                _values = null;
+                _columnIndex = null;
             }
-            _values = null;
-            _columnIndex = null;
         }
 
         //object IEnumerator.Current
@@ -1636,105 +1665,92 @@ internal bool NextCell(ref int row, ref int col)
             return NextCell(ref row, ref col, 0,0, ExcelPackage.MaxRows, ExcelPackage.MaxColumns);
         }
         internal bool NextCell(ref int row, ref int col, int minRow, int minColPos,int maxRow, int maxColPos)
-        {            
-            if (minColPos >= ColumnCount)
-            {
-                return false;
-            }
-            if (maxColPos >= ColumnCount)
-            {
-                maxColPos = ColumnCount-1;
-            }
-            var c=GetPosition(col);
-            if(c>=0)
+        {
+            lock (SyncRoot)
             {
-                if (c > maxColPos)
+
+                if (minColPos >= ColumnCount)
                 {
-                    if (col <= minColPos)
-                    {
-                        return false;
-                    }
-                    col = minColPos;
-                    return NextCell(ref row, ref col);
+                    return false;
                 }
-                else
+                if (maxColPos >= ColumnCount)
                 {
-                    var r=GetNextCell(ref row, ref c, minColPos, maxRow, maxColPos);
-                    col = _columnIndex[c].Index;
-                    return r;
+                    maxColPos = ColumnCount-1;
                 }
-            }
-            else
-            {
-                c=~c;
-                if (c >= ColumnCount) c = ColumnCount - 1;
-                if (col > _columnIndex[c].Index)
+                var c=GetPosition(col);
+                if(c>=0)
                 {
-                    if (col <= minColPos)
+                    if (c > maxColPos)
                     {
-                        return false;
+                        if (col <= minColPos)
+                        {
+                            return false;
+                        }
+                        col = minColPos;
+                        return NextCell(ref row, ref col);
+                    }
+                    else
+                    {
+                        var r=GetNextCell(ref row, ref c, minColPos, maxRow, maxColPos);
+                        col = _columnIndex[c].Index;
+                        return r;
                     }
-                    col = minColPos;
-                    return NextCell(ref row, ref col, minRow, minColPos, maxRow, maxColPos);
                 }
                 else
-                {                    
-                    var r=GetNextCell(ref row, ref c, minColPos, maxRow, maxColPos);
-                    col = _columnIndex[c].Index;
-                    return r;
+                {
+                    c=~c;
+                    if (c >= ColumnCount) c = ColumnCount - 1;
+                    if (col > _columnIndex[c].Index)
+                    {
+                        if (col <= minColPos)
+                        {
+                            return false;
+                        }
+                        col = minColPos;
+                        return NextCell(ref row, ref col, minRow, minColPos, maxRow, maxColPos);
+                    }
+                    else
+                    {
+                        var r=GetNextCell(ref row, ref c, minColPos, maxRow, maxColPos);
+                        col = _columnIndex[c].Index;
+                        return r;
+                    }
                 }
             }
         }
         internal bool GetNextCell(ref int row, ref int colPos, int startColPos, int endRow, int endColPos)
         {
-            if (ColumnCount == 0)
+            lock (SyncRoot)
             {
-                return false;
-            }
-            else
-            {
-                if (++colPos < ColumnCount && colPos <=endColPos)
+                if (ColumnCount == 0)
                 {
-                    var r = _columnIndex[colPos].GetNextRow(row);
-                    if (r == row) //Exists next Row
-                    {
-                        return true;
-                    }
-                    else
+                    return false;
+                }
+                else
+                {
+                    if (++colPos < ColumnCount && colPos <=endColPos)
                     {
-                        int minRow, minCol;
-                        if (r > row)
+                        var r = _columnIndex[colPos].GetNextRow(row);
+                        if (r == row) //Exists next Row
                         {
-                            minRow = r;
-                            minCol = colPos;
+                            return true;
                         }
                         else
                         {
-                            minRow = int.MaxValue;
-                            minCol = 0;
-                        }
-
-                        var c = colPos + 1;
-                        while (c < ColumnCount && c <= endColPos)
-                        {
-                            r = _columnIndex[c].GetNextRow(row);
-                            if (r == row) //Exists next Row
+                            int minRow, minCol;
+                            if (r > row)
                             {
-                                colPos = c;
-                                return true;
+                                minRow = r;
+                                minCol = colPos;
                             }
-                            if (r > row && r < minRow)
+                            else
                             {
-                                minRow = r;
-                                minCol = c;
+                                minRow = int.MaxValue;
+                                minCol = 0;
                             }
-                            c++;
-                        }
-                        c = startColPos;
-                        if (row < endRow)
-                        {
-                            row++;
-                            while (c < colPos)
+
+                            var c = colPos + 1;
+                            while (c < ColumnCount && c <= endColPos)
                             {
                                 r = _columnIndex[c].GetNextRow(row);
                                 if (r == row) //Exists next Row
@@ -1742,77 +1758,100 @@ internal bool GetNextCell(ref int row, ref int colPos, int startColPos, int endR
                                     colPos = c;
                                     return true;
                                 }
-                                if (r > row && (r < minRow || (r==minRow && c<minCol)) && r <= endRow)
+                                if (r > row && r < minRow)
                                 {
                                     minRow = r;
                                     minCol = c;
                                 }
                                 c++;
                             }
-                        }
+                            c = startColPos;
+                            if (row < endRow)
+                            {
+                                row++;
+                                while (c < colPos)
+                                {
+                                    r = _columnIndex[c].GetNextRow(row);
+                                    if (r == row) //Exists next Row
+                                    {
+                                        colPos = c;
+                                        return true;
+                                    }
+                                    if (r > row && (r < minRow || (r==minRow && c<minCol)) && r <= endRow)
+                                    {
+                                        minRow = r;
+                                        minCol = c;
+                                    }
+                                    c++;
+                                }
+                            }
 
-                        if (minRow == int.MaxValue || minRow > endRow)
-                        {
-                            return false;
-                        }
-                        else
-                        {
-                            row = minRow;
-                            colPos = minCol;
-                            return true;
+                            if (minRow == int.MaxValue || minRow > endRow)
+                            {
+                                return false;
+                            }
+                            else
+                            {
+                                row = minRow;
+                                colPos = minCol;
+                                return true;
+                            }
                         }
                     }
-                }
-                else
-                {
-                    if (colPos <= startColPos || row>=endRow)
+                    else
                     {
-                        return false;
+                        if (colPos <= startColPos || row>=endRow)
+                        {
+                            return false;
+                        }
+                        colPos = startColPos - 1;
+                        row++;
+                        return GetNextCell(ref row, ref colPos, startColPos, endRow, endColPos);
                     }
-                    colPos = startColPos - 1;
-                    row++;
-                    return GetNextCell(ref row, ref colPos, startColPos, endRow, endColPos);
                 }
             }
         }
         internal bool GetNextCell(ref int row, ref int colPos, int startColPos, int endRow, int endColPos, ref int[] pagePos, ref int[] cellPos)
         {
-            if (colPos == endColPos)
-            {
-                colPos = startColPos;
-                row++;
-            }
-            else
+            lock (SyncRoot)
             {
-                colPos++;
-            }
+                if (colPos == endColPos)
+                {
+                    colPos = startColPos;
+                    row++;
+                }
+                else
+                {
+                    colPos++;
+                }
 
-            if (pagePos[colPos] < 0)
-            {
-                if(pagePos[colPos]==-1)
+                if (pagePos[colPos] < 0)
                 {
-                    pagePos[colPos] = _columnIndex[colPos].GetPosition(row);
+                    if(pagePos[colPos]==-1)
+                    {
+                        pagePos[colPos] = _columnIndex[colPos].GetPosition(row);
+                    }
+                }
+                else if (_columnIndex[colPos]._pages[pagePos[colPos]].RowCount <= row)
+                {
+                    if (_columnIndex[colPos].PageCount > pagePos[colPos])
+                        pagePos[colPos]++;
+                    else
+                    {
+                        pagePos[colPos]=-2;
+                    }
+                }
+
+                var r = _columnIndex[colPos]._pages[pagePos[colPos]].IndexOffset + _columnIndex[colPos]._pages[pagePos[colPos]].Rows[cellPos[colPos]].Index;
+                if (r == row)
+                {
+                    row = r;
                 }
-            }
-            else if (_columnIndex[colPos]._pages[pagePos[colPos]].RowCount <= row)
-            {
-                if (_columnIndex[colPos].PageCount > pagePos[colPos])
-                    pagePos[colPos]++;
                 else
                 {
-                    pagePos[colPos]=-2;
                 }
+                return true;
             }
-            
-            var r = _columnIndex[colPos]._pages[pagePos[colPos]].IndexOffset + _columnIndex[colPos]._pages[pagePos[colPos]].Rows[cellPos[colPos]].Index;
-            if (r == row)
-            {
-                row = r;
-            }
-            else
-            {
-            }
-            return true;
         }
         internal bool PrevCell(ref int row, ref int col)
         {
@@ -1820,160 +1859,166 @@ internal bool PrevCell(ref int row, ref int col)
         }
         internal bool PrevCell(ref int row, ref int col, int minRow, int minColPos, int maxRow, int maxColPos)
         {
-            if (minColPos >= ColumnCount)
-            {
-                return false;
-            }
-            if (maxColPos >= ColumnCount)
+            lock (SyncRoot)
             {
-                maxColPos = ColumnCount - 1;
-            }
-            var c = GetPosition(col);
-            if(c>=0)
-            {
-                if (c == 0)
+                if (minColPos >= ColumnCount)
                 {
-                    if (col >= maxColPos)
-                    {
-                        return false;
-                    }
-                    if (row == minRow)
-                    {
-                        return false;
-                    }
-                    row--;
-                    col = maxColPos;                    
-                    return PrevCell(ref row, ref col, minRow, minColPos, maxRow, maxColPos);
+                    return false;
                 }
-                else
+                if (maxColPos >= ColumnCount)
                 {
-                    var ret=GetPrevCell(ref row, ref c, minRow, minColPos, maxColPos);
-                    if (ret)
-                    {
-                        col = _columnIndex[c].Index;
-                    }
-                    return ret;
+                    maxColPos = ColumnCount - 1;
                 }
-            }
-            else
-            {
-                c=~c;
-                if (c == 0)
+                var c = GetPosition(col);
+                if(c>=0)
                 {
-                    if (col >= maxColPos || row<=0)
+                    if (c == 0)
                     {
-                        return false;
+                        if (col >= maxColPos)
+                        {
+                            return false;
+                        }
+                        if (row == minRow)
+                        {
+                            return false;
+                        }
+                        row--;
+                        col = maxColPos;
+                        return PrevCell(ref row, ref col, minRow, minColPos, maxRow, maxColPos);
+                    }
+                    else
+                    {
+                        var ret=GetPrevCell(ref row, ref c, minRow, minColPos, maxColPos);
+                        if (ret)
+                        {
+                            col = _columnIndex[c].Index;
+                        }
+                        return ret;
                     }
-                    col = maxColPos;
-                    row--;
-                    return PrevCell(ref row, ref col, minRow, minColPos, maxRow, maxColPos);
                 }
                 else
                 {
-                    var ret = GetPrevCell(ref row, ref c, minRow, minColPos, maxColPos);
-                    if (ret)
+                    c=~c;
+                    if (c == 0)
                     {
-                        col = _columnIndex[c].Index;
+                        if (col >= maxColPos || row<=0)
+                        {
+                            return false;
+                        }
+                        col = maxColPos;
+                        row--;
+                        return PrevCell(ref row, ref col, minRow, minColPos, maxRow, maxColPos);
+                    }
+                    else
+                    {
+                        var ret = GetPrevCell(ref row, ref c, minRow, minColPos, maxColPos);
+                        if (ret)
+                        {
+                            col = _columnIndex[c].Index;
+                        }
+                        return ret;
                     }
-                    return ret;
                 }
             }
         }
         internal bool GetPrevCell(ref int row, ref int colPos, int startRow, int startColPos, int endColPos)
         {
-            if (ColumnCount == 0)
+            lock (SyncRoot)
             {
-                return false;
-            }
-            else
-            {
-                if (--colPos >= startColPos)
-//                if (++colPos < ColumnCount && colPos <= endColPos)
+                if (ColumnCount == 0)
                 {
-                    var r = _columnIndex[colPos].GetNextRow(row);
-                    if (r == row) //Exists next Row
-                    {
-                        return true;
-                    }
-                    else
+                    return false;
+                }
+                else
+                {
+                    if (--colPos >= startColPos)
+    //                if (++colPos < ColumnCount && colPos <= endColPos)
                     {
-                        int minRow, minCol;
-                        if (r > row && r >= startRow)
+                        var r = _columnIndex[colPos].GetNextRow(row);
+                        if (r == row) //Exists next Row
                         {
-                            minRow = r;
-                            minCol = colPos;
+                            return true;
                         }
                         else
                         {
-                            minRow = int.MaxValue;
-                            minCol = 0;
-                        }
+                            int minRow, minCol;
+                            if (r > row && r >= startRow)
+                            {
+                                minRow = r;
+                                minCol = colPos;
+                            }
+                            else
+                            {
+                                minRow = int.MaxValue;
+                                minCol = 0;
+                            }
 
-                        var c = colPos - 1;
-                        if (c >= startColPos)
-                        {
-                            while (c >= startColPos)
+                            var c = colPos - 1;
+                            if (c >= startColPos)
                             {
-                                r = _columnIndex[c].GetNextRow(row);
-                                if (r == row) //Exists next Row
-                                {
-                                    colPos = c;
-                                    return true;
-                                }
-                                if (r > row && r < minRow && r >= startRow)
+                                while (c >= startColPos)
                                 {
-                                    minRow = r;
-                                    minCol = c;
+                                    r = _columnIndex[c].GetNextRow(row);
+                                    if (r == row) //Exists next Row
+                                    {
+                                        colPos = c;
+                                        return true;
+                                    }
+                                    if (r > row && r < minRow && r >= startRow)
+                                    {
+                                        minRow = r;
+                                        minCol = c;
+                                    }
+                                    c--;
                                 }
-                                c--;
                             }
-                        }
-                        if (row > startRow)
-                        {
-                            c = endColPos;
-                            row--;
-                            while (c > colPos)
+                            if (row > startRow)
                             {
-                                r = _columnIndex[c].GetNextRow(row);
-                                if (r == row) //Exists next Row
+                                c = endColPos;
+                                row--;
+                                while (c > colPos)
                                 {
-                                    colPos = c;
-                                    return true;
-                                }
-                                if (r > row && r < minRow && r >= startRow)
-                                {
-                                    minRow = r;
-                                    minCol = c;
+                                    r = _columnIndex[c].GetNextRow(row);
+                                    if (r == row) //Exists next Row
+                                    {
+                                        colPos = c;
+                                        return true;
+                                    }
+                                    if (r > row && r < minRow && r >= startRow)
+                                    {
+                                        minRow = r;
+                                        minCol = c;
+                                    }
+                                    c--;
                                 }
-                                c--;
+                            }
+                            if (minRow == int.MaxValue || startRow < minRow)
+                            {
+                                return false;
+                            }
+                            else
+                            {
+                                row = minRow;
+                                colPos = minCol;
+                                return true;
                             }
                         }
-                        if (minRow == int.MaxValue || startRow < minRow)
+                    }
+                    else
+                    {
+                        colPos = ColumnCount;
+                        row--;
+                        if (row < startRow)
                         {
+                            Reset();
                             return false;
                         }
                         else
                         {
-                            row = minRow;
-                            colPos = minCol;
-                            return true;
+                            return GetPrevCell(ref colPos, ref row, startRow, startColPos, endColPos);
                         }
                     }
                 }
-                else
-                {
-                    colPos = ColumnCount;
-                    row--;
-                    if (row < startRow)
-                    {
-                        Reset();
-                        return false;
-                    }
-                    else
-                    {
-                        return GetPrevCell(ref colPos, ref row, startRow, startColPos, endColPos);
-                    }
-                }
             }
         }
         public void Reset()
@@ -2020,23 +2065,26 @@ public CellsStoreEnumerator(CellStore<T> cellStore, int StartRow, int StartCol,
 
         internal void Init()
         {
-            minRow = _startRow;
-            maxRow = _endRow;
+            lock (_cellStore.SyncRoot)
+            {
+                minRow = _startRow;
+                maxRow = _endRow;
 
-            minColPos = _cellStore.GetPosition(_startCol);
-            if (minColPos < 0) minColPos = ~minColPos;
-            maxColPos = _cellStore.GetPosition(_endCol);
-            if (maxColPos < 0) maxColPos = ~maxColPos-1;
-            row = minRow;
-            colPos = minColPos - 1;
+                minColPos = _cellStore.GetPosition(_startCol);
+                if (minColPos < 0) minColPos = ~minColPos;
+                maxColPos = _cellStore.GetPosition(_endCol);
+                if (maxColPos < 0) maxColPos = ~maxColPos-1;
+                row = minRow;
+                colPos = minColPos - 1;
 
-            var cols = maxColPos - minColPos + 1;
-            pagePos = new int[cols];
-            cellPos = new int[cols];
-            for (int i = 0; i < cols; i++)
-            {
-                pagePos[i] = -1;
-                cellPos[i] = -1;
+                var cols = maxColPos - minColPos + 1;
+                pagePos = new int[cols];
+                cellPos = new int[cols];
+                for (int i = 0; i < cols; i++)
+                {
+                    pagePos[i] = -1;
+                    cellPos[i] = -1;
+                }
             }
         }
         internal int Row 
@@ -2050,23 +2098,26 @@ internal int Column
         {
             get
             {
-                if (colPos == -1) MoveNext();
-                if (colPos == -1) return 0;
-                return _cellStore._columnIndex[colPos].Index;
+                lock (_cellStore.SyncRoot)
+                {
+                    if (colPos == -1) MoveNext();
+                    if (colPos == -1) return 0;
+                    return _cellStore._columnIndex[colPos].Index;
+                }
             }
         }
         internal T Value
         {
             get
             {
-                lock (_cellStore)
+                lock (_cellStore.SyncRoot)
                 {
                     return _cellStore.GetValue(row, Column);
                 }
             }
             set
             {
-                lock (_cellStore)
+                lock (_cellStore.SyncRoot)
                 {
                     _cellStore.SetValue(row, Column, value);
                 }
@@ -2079,7 +2130,7 @@ internal bool Next()
         }
         internal bool Previous()
         {
-            lock (_cellStore)
+            lock (_cellStore.SyncRoot)
             {
                 return _cellStore.GetPrevCell(ref row, ref colPos, minRow, minColPos, maxColPos);
             }
@@ -2141,14 +2192,17 @@ internal class FlagCellStore : CellStore<byte>
     {
         internal void SetFlagValue(int Row, int Col, bool value, CellFlags cellFlags)
         {
-            CellFlags currentValue = (CellFlags) GetValue(Row, Col);
-            if (value)
-            {
-                SetValue(Row, Col, (byte)(currentValue | cellFlags)); // add the CellFlag bit
-            }
-            else
+            lock (SyncRoot)
             {
-                SetValue(Row, Col, (byte)(currentValue & ~cellFlags)); // remove the CellFlag bit
+                CellFlags currentValue = (CellFlags) GetValue(Row, Col);
+                if (value)
+                {
+                    SetValue(Row, Col, (byte)(currentValue | cellFlags)); // add the CellFlag bit
+                }
+                else
+                {
+                    SetValue(Row, Col, (byte)(currentValue & ~cellFlags)); // remove the CellFlag bit
+                }
             }
         }
         internal bool GetFlagValue(int Row, int Col, CellFlags cellFlags)
diff --git a/Epplus-FixCalcsEdition/EPPlus/ExcelPackage.cs b/Epplus-FixCalcsEdition/EPPlus/ExcelPackage.cs
index 6b82c55..52aef7c 100644
--- a/Epplus-FixCalcsEdition/EPPlus/ExcelPackage.cs
+++ b/Epplus-FixCalcsEdition/EPPlus/ExcelPackage.cs
@@ -241,6 +241,8 @@ internal class ImageInfo
         private Packaging.ZipPackage _package;
         private ExcelPackageLoadLimits _loadLimits;
 		internal ExcelWorkbook _workbook;
+        // One reentrant monitor coordinates workbook services and package lifetime.
+        internal readonly object SyncRoot = new object();
         /// <summary>
         /// Maximum number of columns in a worksheet (16384). 
         /// </summary>
@@ -589,7 +591,7 @@ private Uri GetNewUri(Packaging.ZipPackage package, string sUri)
             Uri uri;
             do
             {
-                uri = new Uri(string.Format(sUri, _id++), UriKind.Relative);
+                uri = new Uri(string.Format(sUri, System.Threading.Interlocked.Increment(ref _id) - 1), UriKind.Relative);
             }
             while (package.PartExists(uri));
             return uri;
@@ -772,17 +774,20 @@ public ExcelWorkbook Workbook
 		{
 			get
 			{
-                if (_workbook == null)
+                lock (SyncRoot)
                 {
-                    var nsm = CreateDefaultNSM();
+                    if (_workbook == null)
+                    {
+                        var nsm = CreateDefaultNSM();
 
-                    _workbook = new ExcelWorkbook(this, nsm);
+                        _workbook = new ExcelWorkbook(this, nsm);
 
-                    _workbook.GetExternalReferences();
-                    _workbook.GetDefinedNames();
+                        _workbook.GetExternalReferences();
+                        _workbook.GetDefinedNames();
 
+                    }
+                    return (_workbook);
                 }
-                return (_workbook);
 			}
 		}
         /// <summary>
@@ -884,24 +889,27 @@ internal void SaveWorkbook(Uri uri, XmlDocument xmlDoc)
 		/// </summary>
 		public void Dispose()
 		{
-            if(_package != null)
+            lock (SyncRoot)
             {
-		        if (_isExternalStream==false && _stream != null && (_stream.CanRead || _stream.CanWrite))
+                if(_package != null)
                 {
-                    CloseStream();
-                }
-                _package.Close();
-                if(_workbook != null)
-                {
-                    _workbook.Dispose();
+                    if (_isExternalStream==false && _stream != null && (_stream.CanRead || _stream.CanWrite))
+                    {
+                        CloseStream();
+                    }
+                    _package.Close();
+                    if(_workbook != null)
+                    {
+                        _workbook.Dispose();
+                    }
+                    _package = null;
+                    _images = null;
+                    _file = null;
+                    _workbook = null;
+                    _stream = null;
+                    _workbook = null;
+                    GC.Collect();
                 }
-                _package = null;
-                _images = null;
-                _file = null;
-                _workbook = null;
-                _stream = null;
-                _workbook = null;
-                GC.Collect();
             }
 		}
         #endregion
@@ -915,87 +923,90 @@ public void Dispose()
         /// </summary>
         public void Save()
         {
-            try
+            lock (SyncRoot)
             {
-                if (_stream is MemoryStream && _stream.Length > 0)
-                {
-                    //Close any open memorystream and "renew" then. This can occure if the package is saved twice. 
-                    //The stream is left open on save to enable the user to read the stream-property.
-                    //Non-memorystream streams will leave the closing to the user before saving a second time.
-                    CloseStream();
-                }
-
-                Workbook.Save();
-                if (File == null)
+                try
                 {
-                    if(Encryption.IsEncrypted)
-                    {
-                        var ms = new MemoryStream();
-                        _package.Save(ms);
-                        byte[] file = ms.ToArray(); 
-                        EncryptedPackageHandler eph = new EncryptedPackageHandler();
-                        var msEnc = eph.EncryptPackage(file, Encryption);
-                        CopyStream(msEnc, ref _stream);
-                    }   
-                    else
+                    if (_stream is MemoryStream && _stream.Length > 0)
                     {
-                        _package.Save(_stream);
+                        //Close any open memorystream and "renew" then. This can occure if the package is saved twice.
+                        //The stream is left open on save to enable the user to read the stream-property.
+                        //Non-memorystream streams will leave the closing to the user before saving a second time.
+                        CloseStream();
                     }
-                    _stream.Flush();
-                    _package.Close();
-                }
-                else
-                {
-                    if (System.IO.File.Exists(File.FullName))
+
+                    Workbook.Save();
+                    if (File == null)
                     {
-                        try
+                        if(Encryption.IsEncrypted)
                         {
-                            System.IO.File.Delete(File.FullName);
+                            var ms = new MemoryStream();
+                            _package.Save(ms);
+                            byte[] file = ms.ToArray();
+                            EncryptedPackageHandler eph = new EncryptedPackageHandler();
+                            var msEnc = eph.EncryptPackage(file, Encryption);
+                            CopyStream(msEnc, ref _stream);
                         }
-                        catch (Exception ex)
+                        else
                         {
-                            throw (new Exception(string.Format("Error overwriting file {0}", File.FullName), ex));
+                            _package.Save(_stream);
                         }
+                        _stream.Flush();
+                        _package.Close();
                     }
-
-                    _package.Save(_stream);
-                    _package.Close();
-                    if (Stream is MemoryStream)
+                    else
                     {
-                        var fi = new FileStream(File.FullName, FileMode.Create);
-                        //EncryptPackage
-                        if (Encryption.IsEncrypted)
+                        if (System.IO.File.Exists(File.FullName))
                         {
-                            byte[] file = ((MemoryStream)Stream).ToArray();
-                            EncryptedPackageHandler eph = new EncryptedPackageHandler();
-                            var ms = eph.EncryptPackage(file, Encryption);
-                             
-                            fi.Write(ms.ToArray(), 0, (int)ms.Length);
+                            try
+                            {
+                                System.IO.File.Delete(File.FullName);
+                            }
+                            catch (Exception ex)
+                            {
+                                throw (new Exception(string.Format("Error overwriting file {0}", File.FullName), ex));
+                            }
+                        }
+
+                        _package.Save(_stream);
+                        _package.Close();
+                        if (Stream is MemoryStream)
+                        {
+                            var fi = new FileStream(File.FullName, FileMode.Create);
+                            //EncryptPackage
+                            if (Encryption.IsEncrypted)
+                            {
+                                byte[] file = ((MemoryStream)Stream).ToArray();
+                                EncryptedPackageHandler eph = new EncryptedPackageHandler();
+                                var ms = eph.EncryptPackage(file, Encryption);
+
+                                fi.Write(ms.ToArray(), 0, (int)ms.Length);
+                            }
+                            else
+                            {
+                                fi.Write(((MemoryStream)Stream).ToArray(), 0, (int)Stream.Length);
+                            }
+                            fi.Close();
+                            fi.Dispose();
                         }
                         else
-                        {                            
-                            fi.Write(((MemoryStream)Stream).ToArray(), 0, (int)Stream.Length);
+                        {
+                            System.IO.File.WriteAllBytes(File.FullName, GetAsByteArray(false));
                         }
-                        fi.Close();
-                        fi.Dispose();
+                    }
+                }
+                catch (Exception ex)
+                {
+                    if (File == null)
+                    {
+                        throw;
                     }
                     else
                     {
-                        System.IO.File.WriteAllBytes(File.FullName, GetAsByteArray(false));
+                        throw (new InvalidOperationException(string.Format("Error saving file {0}", File.FullName), ex));
                     }
                 }
             }
-            catch (Exception ex)
-            {
-                if (File == null)
-                {
-                    throw;
-                }
-                else
-                {
-                    throw (new InvalidOperationException(string.Format("Error saving file {0}", File.FullName), ex));
-                }
-            }
         }
         /// <summary>
         /// Saves all the components back into the package.
@@ -1006,8 +1017,11 @@ public void Save()
         /// <param name="password">This parameter overrides the Workbook.Encryption.Password.</param>
         public void Save(string password)
 		{
-            Encryption.Password = password;
-            Save();
+            lock (SyncRoot)
+            {
+                Encryption.Password = password;
+                Save();
+            }
         }
         /// <summary>
         /// Saves the workbook to a new file
@@ -1016,8 +1030,11 @@ public void Save(string password)
         /// <param name="file">The file location</param>
         public void SaveAs(FileInfo file)
         {
-            File = file;
-            Save();
+            lock (SyncRoot)
+            {
+                File = file;
+                Save();
+            }
         }
         /// <summary>
         /// Saves the workbook to a new file
@@ -1028,9 +1045,12 @@ public void SaveAs(FileInfo file)
         /// This parameter overrides the Encryption.Password.</param>
         public void SaveAs(FileInfo file, string password)
         {
-            File = file;
-            Encryption.Password = password;
-            Save();
+            lock (SyncRoot)
+            {
+                File = file;
+                Encryption.Password = password;
+                Save();
+            }
         }
         /// <summary>
         /// Copies the Package to the Outstream
@@ -1039,12 +1059,15 @@ public void SaveAs(FileInfo file, string password)
         /// <param name="OutputStream">The stream to copy the package to</param>
         public void SaveAs(Stream OutputStream)
         {
-            File = null;
-            Save();
-
-            if (OutputStream != _stream)
+            lock (SyncRoot)
             {
-                CopyStream(_stream, ref OutputStream);
+                File = null;
+                Save();
+
+                if (OutputStream != _stream)
+                {
+                    CopyStream(_stream, ref OutputStream);
+                }
             }
         }
         /// <summary>
@@ -1056,8 +1079,11 @@ public void SaveAs(Stream OutputStream)
         /// This parameter overrides the Encryption.Password.</param>
         public void SaveAs(Stream OutputStream, string password)
         {
-            Encryption.Password = password;
-            SaveAs(OutputStream);
+            lock (SyncRoot)
+            {
+                Encryption.Password = password;
+                SaveAs(OutputStream);
+            }
         }
         FileInfo _file = null;
 
@@ -1185,36 +1211,42 @@ public byte[] GetAsByteArray()
         /// <returns></returns>
         public byte[] GetAsByteArray(string password)
         {
-            if (password != null)
+            lock (SyncRoot)
             {
-                Encryption.Password = password;
+                if (password != null)
+                {
+                    Encryption.Password = password;
+                }
+                return GetAsByteArray(true);
             }
-            return GetAsByteArray(true);
         }
         internal byte[] GetAsByteArray(bool save)
         {
-            if (save)
+            lock (SyncRoot)
             {
-                Workbook.Save();
-                _package.Close();
-                _package.Save(_stream);
-            }
-            Byte[] byRet = new byte[Stream.Length];
-            long pos = Stream.Position;            
-            Stream.Seek(0, SeekOrigin.Begin);
-            Stream.Read(byRet, 0, (int)Stream.Length);
+                if (save)
+                {
+                    Workbook.Save();
+                    _package.Close();
+                    _package.Save(_stream);
+                }
+                Byte[] byRet = new byte[Stream.Length];
+                long pos = Stream.Position;
+                Stream.Seek(0, SeekOrigin.Begin);
+                Stream.Read(byRet, 0, (int)Stream.Length);
 
-            //Encrypt Workbook?
-            if (Encryption.IsEncrypted)
-            {
-                EncryptedPackageHandler eph=new EncryptedPackageHandler();
-                var ms = eph.EncryptPackage(byRet, Encryption);
-                byRet = ms.ToArray();
-            }
+                //Encrypt Workbook?
+                if (Encryption.IsEncrypted)
+                {
+                    EncryptedPackageHandler eph=new EncryptedPackageHandler();
+                    var ms = eph.EncryptPackage(byRet, Encryption);
+                    byRet = ms.ToArray();
+                }
 
-            Stream.Seek(pos, SeekOrigin.Begin);
-            Stream.Close();
-            return byRet;
+                Stream.Seek(pos, SeekOrigin.Begin);
+                Stream.Close();
+                return byRet;
+            }
         }
         /// <summary>
         /// Loads the specified package data from a stream.
diff --git a/Epplus-FixCalcsEdition/EPPlus/ExcelStyleCollection.cs b/Epplus-FixCalcsEdition/EPPlus/ExcelStyleCollection.cs
index 7827127..d2d0ad5 100644
--- a/Epplus-FixCalcsEdition/EPPlus/ExcelStyleCollection.cs
+++ b/Epplus-FixCalcsEdition/EPPlus/ExcelStyleCollection.cs
@@ -53,6 +53,7 @@ public ExcelStyleCollection(bool SetNextIdManual)
             _setNextIdManual = SetNextIdManual;
         }
         public XmlNode TopNode { get; set; }
+        private readonly object _syncRoot = new object();
         internal List<T> _list = new List<T>();
         Dictionary<string, int> _dic = new Dictionary<string, int>(StringComparer.OrdinalIgnoreCase);
         internal int NextId=0;
@@ -60,7 +61,10 @@ public ExcelStyleCollection(bool SetNextIdManual)
 
         public IEnumerator<T> GetEnumerator()
         {
-            return _list.GetEnumerator();
+            lock (_syncRoot)
+            {
+                return new List<T>(_list).GetEnumerator();
+            }
         }
 
         #endregion
@@ -68,21 +72,30 @@ public IEnumerator<T> GetEnumerator()
 
         System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
         {
-            return _list.GetEnumerator();
+            lock (_syncRoot)
+            {
+                return new List<T>(_list).GetEnumerator();
+            }
         }
         #endregion
         public T this[int PositionID]
         {
             get
             {
-                return _list[PositionID];
+                lock (_syncRoot)
+                {
+                    return _list[PositionID];
+                }
             }
         }
         public int Count
         {
             get
             {
-                return _list.Count;
+                lock (_syncRoot)
+                {
+                    return _list.Count;
+                }
             }
         }
         //internal int Add(T item)
@@ -93,10 +106,13 @@ public int Count
         //}
         internal int Add(string key, T item)
         {
-            _list.Add(item);
-            if (!_dic.ContainsKey(key.ToLower(CultureInfo.InvariantCulture))) _dic.Add(key.ToLower(CultureInfo.InvariantCulture), _list.Count - 1);
-            if (_setNextIdManual) NextId++;
-            return _list.Count-1;
+            lock (_syncRoot)
+            {
+                _list.Add(item);
+                if (!_dic.ContainsKey(key.ToLower(CultureInfo.InvariantCulture))) _dic.Add(key.ToLower(CultureInfo.InvariantCulture), _list.Count - 1);
+                if (_setNextIdManual) NextId++;
+                return _list.Count-1;
+            }
         }
         /// <summary>
         /// Finds the key 
@@ -106,14 +122,17 @@ internal int Add(string key, T item)
         /// <returns>True if found</returns>
         internal bool FindByID(string key, ref T obj)
         {
-            if (_dic.ContainsKey(key))
-            {
-                obj = _list[_dic[key.ToLower(CultureInfo.InvariantCulture)]];
-                return true;
-            }
-            else
+            lock (_syncRoot)
             {
-                return false;
+                if (_dic.ContainsKey(key))
+                {
+                    obj = _list[_dic[key.ToLower(CultureInfo.InvariantCulture)]];
+                    return true;
+                }
+                else
+                {
+                    return false;
+                }
             }
         }
         /// <summary>
@@ -123,22 +142,31 @@ internal bool FindByID(string key, ref T obj)
         /// <returns></returns>
         internal int FindIndexByID(string key)
         {
-            if (_dic.ContainsKey(key))
-            {
-                return _dic[key];
-            }
-            else
+            lock (_syncRoot)
             {
-                return int.MinValue;
+                if (_dic.ContainsKey(key))
+                {
+                    return _dic[key];
+                }
+                else
+                {
+                    return int.MinValue;
+                }
             }
         }
         internal bool ExistsKey(string key)
         {
-            return _dic.ContainsKey(key);
+            lock (_syncRoot)
+            {
+                return _dic.ContainsKey(key);
+            }
         }
         internal void Sort(Comparison<T> c)
         {
-            _list.Sort(c);
+            lock (_syncRoot)
+            {
+                _list.Sort(c);
+            }
         }
     }
 }
diff --git a/Epplus-FixCalcsEdition/EPPlus/ExcelStyles.cs b/Epplus-FixCalcsEdition/EPPlus/ExcelStyles.cs
index da4c70d..351fc1b 100644
--- a/Epplus-FixCalcsEdition/EPPlus/ExcelStyles.cs
+++ b/Epplus-FixCalcsEdition/EPPlus/ExcelStyles.cs
@@ -173,12 +173,13 @@ internal ExcelStyle GetStyleObject(int Id,int PositionID, string Address)
         /// <returns></returns>
         internal int PropertyChange(StyleBase sender, Style.StyleChangeEventArgs e)
         {
-            var address = new ExcelAddressBase(e.Address);
-            var ws = _wb.Worksheets[e.PositionID];
-            Dictionary<int, int> styleCashe = new Dictionary<int, int>();
-            //Set single address
-            lock (ws._values)
+            lock (_wb.SyncRoot)
             {
+                _wb.ThrowIfDisposed();
+                var address = new ExcelAddressBase(e.Address);
+                var ws = _wb.Worksheets[e.PositionID];
+                Dictionary<int, int> styleCashe = new Dictionary<int, int>();
+                //Set single address
                 SetStyleAddress(sender, e, address, ws, ref styleCashe);
                 if (address.Addresses != null)
                 {
@@ -544,25 +545,27 @@ internal int GetStyleId(ExcelWorksheet ws, int row, int col)
         /// <returns></returns>
         internal int NamedStylePropertyChange(StyleBase sender, Style.StyleChangeEventArgs e)
         {
-
-            int index = NamedStyles.FindIndexByID(e.Address);
-            if (index >= 0)
+            lock (_wb.SyncRoot)
             {
-                int newId = CellStyleXfs[NamedStyles[index].StyleXfId].GetNewID(CellStyleXfs, sender, e.StyleClass, e.StyleProperty, e.Value);
-                int prevIx=NamedStyles[index].StyleXfId;
-                NamedStyles[index].StyleXfId = newId;
-                NamedStyles[index].Style.Index = newId;
-
-                NamedStyles[index].XfId = int.MinValue;
-                foreach (var style in CellXfs)
+                int index = NamedStyles.FindIndexByID(e.Address);
+                if (index >= 0)
                 {
-                    if (style.XfId == prevIx)
+                    int newId = CellStyleXfs[NamedStyles[index].StyleXfId].GetNewID(CellStyleXfs, sender, e.StyleClass, e.StyleProperty, e.Value);
+                    int prevIx=NamedStyles[index].StyleXfId;
+                    NamedStyles[index].StyleXfId = newId;
+                    NamedStyles[index].Style.Index = newId;
+
+                    NamedStyles[index].XfId = int.MinValue;
+                    foreach (var style in CellXfs)
                     {
-                        style.XfId = newId;
+                        if (style.XfId == prevIx)
+                        {
+                            style.XfId = newId;
+                        }
                     }
                 }
+                return 0;
             }
-            return 0;
         }
         public ExcelStyleCollection<ExcelNumberFormatXml> NumberFormats = new ExcelStyleCollection<ExcelNumberFormatXml>();
         public ExcelStyleCollection<ExcelFontXml> Fonts = new ExcelStyleCollection<ExcelFontXml>();
@@ -584,230 +587,236 @@ public ExcelNamedStyleXml CreateNamedStyle(string name)
         }
         public ExcelNamedStyleXml CreateNamedStyle(string name, ExcelStyle Template)
         {
-            if (_wb.Styles.NamedStyles.ExistsKey(name))
+            lock (_wb.SyncRoot)
             {
-                throw new Exception(string.Format("Key {0} already exists in collection", name));
-            }
+                if (_wb.Styles.NamedStyles.ExistsKey(name))
+                {
+                    throw new Exception(string.Format("Key {0} already exists in collection", name));
+                }
 
-            ExcelNamedStyleXml style;
-            style = new ExcelNamedStyleXml(NameSpaceManager, this);
-            int xfIdCopy, positionID;
-            ExcelStyles styles;
-            if (Template == null)
-            {
-//                style.Style = new ExcelStyle(this, NamedStylePropertyChange, -1, name, 0);
-                xfIdCopy = 0;
-                positionID = -1;
-                styles = this;
-            }
-            else
-            {
-                if (Template.PositionID < 0 && Template.Styles==this)
+                ExcelNamedStyleXml style;
+                style = new ExcelNamedStyleXml(NameSpaceManager, this);
+                int xfIdCopy, positionID;
+                ExcelStyles styles;
+                if (Template == null)
                 {
-                    xfIdCopy = Template.Index;
-                    
-                    positionID=Template.PositionID;
+    //                style.Style = new ExcelStyle(this, NamedStylePropertyChange, -1, name, 0);
+                    xfIdCopy = 0;
+                    positionID = -1;
                     styles = this;
-                    //style.Style = new ExcelStyle(this, NamedStylePropertyChange, Template.PositionID, name, Template.Index);
-                    //style.StyleXfId = Template.Index;
                 }
                 else
                 {
-                    xfIdCopy = Template.XfId;
-                    positionID = -1;
-                    styles = Template.Styles;
+                    if (Template.PositionID < 0 && Template.Styles==this)
+                    {
+                        xfIdCopy = Template.Index;
+
+                        positionID=Template.PositionID;
+                        styles = this;
+                        //style.Style = new ExcelStyle(this, NamedStylePropertyChange, Template.PositionID, name, Template.Index);
+                        //style.StyleXfId = Template.Index;
+                    }
+                    else
+                    {
+                        xfIdCopy = Template.XfId;
+                        positionID = -1;
+                        styles = Template.Styles;
+                    }
                 }
+                //Clone namedstyle
+                int styleXfId = CloneStyle(styles, xfIdCopy, true);
+                //Close cells style
+                CellStyleXfs[styleXfId].XfId = CellStyleXfs.Count-1;
+                int xfid = CloneStyle(styles, xfIdCopy, true, true); //Always add a new style (We create a new named style here)
+                CellXfs[xfid].XfId = styleXfId;
+                style.Style = new ExcelStyle(this, NamedStylePropertyChange, positionID, name, styleXfId);
+                style.StyleXfId = styleXfId;
+
+                style.Name = name;
+                int ix =_wb.Styles.NamedStyles.Add(style.Name, style);
+                style.Style.SetIndex(ix);
+                //style.Style.XfId = ix;
+                return style;
             }
-            //Clone namedstyle
-            int styleXfId = CloneStyle(styles, xfIdCopy, true);
-            //Close cells style
-            CellStyleXfs[styleXfId].XfId = CellStyleXfs.Count-1;
-            int xfid = CloneStyle(styles, xfIdCopy, true, true); //Always add a new style (We create a new named style here)
-            CellXfs[xfid].XfId = styleXfId;
-            style.Style = new ExcelStyle(this, NamedStylePropertyChange, positionID, name, styleXfId);
-            style.StyleXfId = styleXfId;
-            
-            style.Name = name;
-            int ix =_wb.Styles.NamedStyles.Add(style.Name, style);
-            style.Style.SetIndex(ix);
-            //style.Style.XfId = ix;
-            return style;
         }
         public void UpdateXml()
         {
-            RemoveUnusedStyles();
-
-            //NumberFormat
-            XmlNode nfNode=_styleXml.SelectSingleNode(NumberFormatsPath, _nameSpaceManager);
-            if (nfNode == null)
-            {
-                CreateNode(NumberFormatsPath, true);
-                nfNode = _styleXml.SelectSingleNode(NumberFormatsPath, _nameSpaceManager);
-            }
-            else
+            lock (_wb.SyncRoot)
             {
-                nfNode.RemoveAll();                
-            }
+                RemoveUnusedStyles();
 
-            int count = 0;
-            int normalIx = NamedStyles.FindIndexByID("Normal");
-            if (NamedStyles.Count > 0 && normalIx>=0 && NamedStyles[normalIx].Style.Numberformat.NumFmtID >= 164)
-            {
-                ExcelNumberFormatXml nf = NumberFormats[NumberFormats.FindIndexByID(NamedStyles[normalIx].Style.Numberformat.Id)];
-                nfNode.AppendChild(nf.CreateXmlNode(_styleXml.CreateElement("numFmt", ExcelPackage.schemaMain)));
-                nf.newID = count++;
-            }
-            foreach (ExcelNumberFormatXml nf in NumberFormats)
-            {
-                if(!nf.BuildIn /*&& nf.newID<0*/) //Buildin formats are not updated.
+                //NumberFormat
+                XmlNode nfNode=_styleXml.SelectSingleNode(NumberFormatsPath, _nameSpaceManager);
+                if (nfNode == null)
                 {
-                    nfNode.AppendChild(nf.CreateXmlNode(_styleXml.CreateElement("numFmt", ExcelPackage.schemaMain)));
-                    nf.newID = count;
-                    count++;
+                    CreateNode(NumberFormatsPath, true);
+                    nfNode = _styleXml.SelectSingleNode(NumberFormatsPath, _nameSpaceManager);
+                }
+                else
+                {
+                    nfNode.RemoveAll();
                 }
-            }
-            (nfNode as XmlElement).SetAttribute("count", count.ToString());
 
-            //Font
-            count=0;
-            XmlNode fntNode = _styleXml.SelectSingleNode(FontsPath, _nameSpaceManager);
-            fntNode.RemoveAll();
+                int count = 0;
+                int normalIx = NamedStyles.FindIndexByID("Normal");
+                if (NamedStyles.Count > 0 && normalIx>=0 && NamedStyles[normalIx].Style.Numberformat.NumFmtID >= 164)
+                {
+                    ExcelNumberFormatXml nf = NumberFormats[NumberFormats.FindIndexByID(NamedStyles[normalIx].Style.Numberformat.Id)];
+                    nfNode.AppendChild(nf.CreateXmlNode(_styleXml.CreateElement("numFmt", ExcelPackage.schemaMain)));
+                    nf.newID = count++;
+                }
+                foreach (ExcelNumberFormatXml nf in NumberFormats)
+                {
+                    if(!nf.BuildIn /*&& nf.newID<0*/) //Buildin formats are not updated.
+                    {
+                        nfNode.AppendChild(nf.CreateXmlNode(_styleXml.CreateElement("numFmt", ExcelPackage.schemaMain)));
+                        nf.newID = count;
+                        count++;
+                    }
+                }
+                (nfNode as XmlElement).SetAttribute("count", count.ToString());
 
-            //Normal should be first in the collection
-            if (NamedStyles.Count > 0 && normalIx >= 0 && NamedStyles[normalIx].Style.Font.Index > 0)
-            {
-                ExcelFontXml fnt = Fonts[NamedStyles[normalIx].Style.Font.Index];
-                fntNode.AppendChild(fnt.CreateXmlNode(_styleXml.CreateElement("font", ExcelPackage.schemaMain)));
-                fnt.newID = count++;
-            }
+                //Font
+                count=0;
+                XmlNode fntNode = _styleXml.SelectSingleNode(FontsPath, _nameSpaceManager);
+                fntNode.RemoveAll();
 
-            foreach (ExcelFontXml fnt in Fonts)
-            {
-                if (fnt.useCnt > 0/* && fnt.newID<0*/)
+                //Normal should be first in the collection
+                if (NamedStyles.Count > 0 && normalIx >= 0 && NamedStyles[normalIx].Style.Font.Index > 0)
                 {
+                    ExcelFontXml fnt = Fonts[NamedStyles[normalIx].Style.Font.Index];
                     fntNode.AppendChild(fnt.CreateXmlNode(_styleXml.CreateElement("font", ExcelPackage.schemaMain)));
-                    fnt.newID = count;
-                    count++;
+                    fnt.newID = count++;
                 }
-            }
-            (fntNode as XmlElement).SetAttribute("count", count.ToString());
 
+                foreach (ExcelFontXml fnt in Fonts)
+                {
+                    if (fnt.useCnt > 0/* && fnt.newID<0*/)
+                    {
+                        fntNode.AppendChild(fnt.CreateXmlNode(_styleXml.CreateElement("font", ExcelPackage.schemaMain)));
+                        fnt.newID = count;
+                        count++;
+                    }
+                }
+                (fntNode as XmlElement).SetAttribute("count", count.ToString());
 
-            //Fills
-            count = 0;
-            XmlNode fillsNode = _styleXml.SelectSingleNode(FillsPath, _nameSpaceManager);
-            fillsNode.RemoveAll();
-            Fills[0].useCnt = 1;    //Must exist (none);  
-            Fills[1].useCnt = 1;    //Must exist (gray125);
-            foreach (ExcelFillXml fill in Fills)
-            {
-                if (fill.useCnt > 0)
+
+                //Fills
+                count = 0;
+                XmlNode fillsNode = _styleXml.SelectSingleNode(FillsPath, _nameSpaceManager);
+                fillsNode.RemoveAll();
+                Fills[0].useCnt = 1;    //Must exist (none);
+                Fills[1].useCnt = 1;    //Must exist (gray125);
+                foreach (ExcelFillXml fill in Fills)
                 {
-                    fillsNode.AppendChild(fill.CreateXmlNode(_styleXml.CreateElement("fill", ExcelPackage.schemaMain)));
-                    fill.newID = count;
-                    count++;
+                    if (fill.useCnt > 0)
+                    {
+                        fillsNode.AppendChild(fill.CreateXmlNode(_styleXml.CreateElement("fill", ExcelPackage.schemaMain)));
+                        fill.newID = count;
+                        count++;
+                    }
                 }
-            }
 
-            (fillsNode as XmlElement).SetAttribute("count", count.ToString());
+                (fillsNode as XmlElement).SetAttribute("count", count.ToString());
 
-            //Borders
-            count = 0;
-            XmlNode bordersNode = _styleXml.SelectSingleNode(BordersPath, _nameSpaceManager);
-            bordersNode.RemoveAll();
-            Borders[0].useCnt = 1;    //Must exist blank;
-            foreach (ExcelBorderXml border in Borders)
-            {
-                if (border.useCnt > 0)
+                //Borders
+                count = 0;
+                XmlNode bordersNode = _styleXml.SelectSingleNode(BordersPath, _nameSpaceManager);
+                bordersNode.RemoveAll();
+                Borders[0].useCnt = 1;    //Must exist blank;
+                foreach (ExcelBorderXml border in Borders)
                 {
-                    bordersNode.AppendChild(border.CreateXmlNode(_styleXml.CreateElement("border", ExcelPackage.schemaMain)));
-                    border.newID = count;
-                    count++;
+                    if (border.useCnt > 0)
+                    {
+                        bordersNode.AppendChild(border.CreateXmlNode(_styleXml.CreateElement("border", ExcelPackage.schemaMain)));
+                        border.newID = count;
+                        count++;
+                    }
                 }
-            }
-            (bordersNode as XmlElement).SetAttribute("count", count.ToString());
+                (bordersNode as XmlElement).SetAttribute("count", count.ToString());
 
-            XmlNode styleXfsNode = _styleXml.SelectSingleNode(CellStyleXfsPath, _nameSpaceManager);
-            if (styleXfsNode == null && NamedStyles.Count > 0)
-            {
-                CreateNode(CellStyleXfsPath);
-                styleXfsNode = _styleXml.SelectSingleNode(CellStyleXfsPath, _nameSpaceManager);
-            }
-            if (NamedStyles.Count > 0)
-            {
-                styleXfsNode.RemoveAll();
-            }
-            //NamedStyles
-            count = normalIx > -1 ? 1 : 0;  //If we have a normal style, we make sure it's added first.
+                XmlNode styleXfsNode = _styleXml.SelectSingleNode(CellStyleXfsPath, _nameSpaceManager);
+                if (styleXfsNode == null && NamedStyles.Count > 0)
+                {
+                    CreateNode(CellStyleXfsPath);
+                    styleXfsNode = _styleXml.SelectSingleNode(CellStyleXfsPath, _nameSpaceManager);
+                }
+                if (NamedStyles.Count > 0)
+                {
+                    styleXfsNode.RemoveAll();
+                }
+                //NamedStyles
+                count = normalIx > -1 ? 1 : 0;  //If we have a normal style, we make sure it's added first.
 
-            XmlNode cellStyleNode = _styleXml.SelectSingleNode(CellStylesPath, _nameSpaceManager);
-            if(cellStyleNode!=null)
-            {
-                cellStyleNode.RemoveAll();
-            }
-            XmlNode cellXfsNode = _styleXml.SelectSingleNode(CellXfsPath, _nameSpaceManager);
-            cellXfsNode.RemoveAll();
+                XmlNode cellStyleNode = _styleXml.SelectSingleNode(CellStylesPath, _nameSpaceManager);
+                if(cellStyleNode!=null)
+                {
+                    cellStyleNode.RemoveAll();
+                }
+                XmlNode cellXfsNode = _styleXml.SelectSingleNode(CellXfsPath, _nameSpaceManager);
+                cellXfsNode.RemoveAll();
 
-            if (NamedStyles.Count > 0 && normalIx >= 0)
-            {
-                NamedStyles[normalIx].newID = 0;
-                AddNamedStyle(0, styleXfsNode, cellXfsNode, NamedStyles[normalIx]);
-            }
-            foreach (ExcelNamedStyleXml style in NamedStyles)
-            {
-                if (!style.Name.Equals("normal", StringComparison.OrdinalIgnoreCase))
+                if (NamedStyles.Count > 0 && normalIx >= 0)
                 {
-                    AddNamedStyle(count++, styleXfsNode, cellXfsNode, style);
+                    NamedStyles[normalIx].newID = 0;
+                    AddNamedStyle(0, styleXfsNode, cellXfsNode, NamedStyles[normalIx]);
                 }
-                else
+                foreach (ExcelNamedStyleXml style in NamedStyles)
                 {
-                    style.newID = 0;
+                    if (!style.Name.Equals("normal", StringComparison.OrdinalIgnoreCase))
+                    {
+                        AddNamedStyle(count++, styleXfsNode, cellXfsNode, style);
+                    }
+                    else
+                    {
+                        style.newID = 0;
+                    }
+                    cellStyleNode.AppendChild(style.CreateXmlNode(_styleXml.CreateElement("cellStyle", ExcelPackage.schemaMain)));
                 }
-                cellStyleNode.AppendChild(style.CreateXmlNode(_styleXml.CreateElement("cellStyle", ExcelPackage.schemaMain)));
-            }
-            if (cellStyleNode!=null) (cellStyleNode as XmlElement).SetAttribute("count", count.ToString());
-            if (styleXfsNode != null) (styleXfsNode as XmlElement).SetAttribute("count", count.ToString());
+                if (cellStyleNode!=null) (cellStyleNode as XmlElement).SetAttribute("count", count.ToString());
+                if (styleXfsNode != null) (styleXfsNode as XmlElement).SetAttribute("count", count.ToString());
 
-            //CellStyle
-            int xfix = 0;
-            foreach (ExcelXfs xf in CellXfs)
-            {
-                if (xf.useCnt > 0 && !(normalIx >= 0 && NamedStyles[normalIx].StyleXfId == xfix))
+                //CellStyle
+                int xfix = 0;
+                foreach (ExcelXfs xf in CellXfs)
                 {
-                    cellXfsNode.AppendChild(xf.CreateXmlNode(_styleXml.CreateElement("xf", ExcelPackage.schemaMain)));
-                    xf.newID = count;
-                    count++;
+                    if (xf.useCnt > 0 && !(normalIx >= 0 && NamedStyles[normalIx].StyleXfId == xfix))
+                    {
+                        cellXfsNode.AppendChild(xf.CreateXmlNode(_styleXml.CreateElement("xf", ExcelPackage.schemaMain)));
+                        xf.newID = count;
+                        count++;
+                    }
+                    xfix++;
                 }
-                xfix++;
-            }
-            (cellXfsNode as XmlElement).SetAttribute("count", count.ToString());
+                (cellXfsNode as XmlElement).SetAttribute("count", count.ToString());
 
-            //Set dxf styling for conditional Formatting
-            XmlNode dxfsNode = _styleXml.SelectSingleNode(dxfsPath, _nameSpaceManager);
-            foreach (var ws in _wb.Worksheets)
-            {
-                if (ws is ExcelChartsheet) continue;
-                foreach (var cf in ws.ConditionalFormatting)
+                //Set dxf styling for conditional Formatting
+                XmlNode dxfsNode = _styleXml.SelectSingleNode(dxfsPath, _nameSpaceManager);
+                foreach (var ws in _wb.Worksheets)
                 {
-                    if (cf.Style.HasValue)
+                    if (ws is ExcelChartsheet) continue;
+                    foreach (var cf in ws.ConditionalFormatting)
                     {
-                        int ix = Dxfs.FindIndexByID(cf.Style.Id);
-                        if (ix < 0)
-                        {
-                            ((ExcelConditionalFormattingRule)cf).DxfId = Dxfs.Count;
-                            Dxfs.Add(cf.Style.Id, cf.Style);
-                            var elem = ((XmlDocument)TopNode).CreateElement("d", "dxf", ExcelPackage.schemaMain);
-                            cf.Style.CreateNodes(new XmlHelperInstance(NameSpaceManager, elem), "");
-                            dxfsNode.AppendChild(elem);
-                        }
-                        else
+                        if (cf.Style.HasValue)
                         {
-                            ((ExcelConditionalFormattingRule)cf).DxfId = ix;
+                            int ix = Dxfs.FindIndexByID(cf.Style.Id);
+                            if (ix < 0)
+                            {
+                                ((ExcelConditionalFormattingRule)cf).DxfId = Dxfs.Count;
+                                Dxfs.Add(cf.Style.Id, cf.Style);
+                                var elem = ((XmlDocument)TopNode).CreateElement("d", "dxf", ExcelPackage.schemaMain);
+                                cf.Style.CreateNodes(new XmlHelperInstance(NameSpaceManager, elem), "");
+                                dxfsNode.AppendChild(elem);
+                            }
+                            else
+                            {
+                                ((ExcelConditionalFormattingRule)cf).DxfId = ix;
+                            }
                         }
                     }
                 }
+                if (dxfsNode != null) (dxfsNode as XmlElement).SetAttribute("count", Dxfs.Count.ToString());
             }
-            if (dxfsNode != null) (dxfsNode as XmlElement).SetAttribute("count", Dxfs.Count.ToString());
         }
 
         private void AddNamedStyle(int id, XmlNode styleXfsNode,XmlNode cellXfsNode, ExcelNamedStyleXml style)
@@ -877,28 +886,31 @@ private void RemoveUnusedStyles()
         }
         internal int GetStyleIdFromName(string Name)
         {
-            int i = NamedStyles.FindIndexByID(Name);
-            if (i >= 0)
+            lock (_wb.SyncRoot)
             {
-                int id = NamedStyles[i].XfId;
-                if (id < 0)
+                int i = NamedStyles.FindIndexByID(Name);
+                if (i >= 0)
                 {
-                    int styleXfId=NamedStyles[i].StyleXfId;
-                    ExcelXfs newStyle = CellStyleXfs[styleXfId].Copy();
-                    newStyle.XfId = styleXfId;
-                    id = CellXfs.FindIndexByID(newStyle.Id);
+                    int id = NamedStyles[i].XfId;
                     if (id < 0)
                     {
-                        id = CellXfs.Add(newStyle.Id, newStyle);
+                        int styleXfId=NamedStyles[i].StyleXfId;
+                        ExcelXfs newStyle = CellStyleXfs[styleXfId].Copy();
+                        newStyle.XfId = styleXfId;
+                        id = CellXfs.FindIndexByID(newStyle.Id);
+                        if (id < 0)
+                        {
+                            id = CellXfs.Add(newStyle.Id, newStyle);
+                        }
+                        NamedStyles[i].XfId=id;
                     }
-                    NamedStyles[i].XfId=id;
+                    return id;
+                }
+                else
+                {
+                    return 0;
+                    //throw(new Exception("Named style does not exist"));
                 }
-                return id;
-            }
-            else
-            {
-                return 0;
-                //throw(new Exception("Named style does not exist"));        	         
             }
         }
    #region XmlHelpFunctions
@@ -941,139 +953,142 @@ internal int CloneStyle(ExcelStyles style, int styleID, bool isNamedStyle)
         }
         internal int CloneStyle(ExcelStyles style, int styleID, bool isNamedStyle, bool allwaysAddCellXfs)
         {
-            ExcelXfs xfs;
-            lock (style)
+            lock (_wb.SyncRoot)
             {
-                if (isNamedStyle)
-                {
-                    xfs = style.CellStyleXfs[styleID];
-                }
-                else
+                ExcelXfs xfs;
+                lock (style)
                 {
-                    xfs = style.CellXfs[styleID];
-                }
-                ExcelXfs newXfs = xfs.Copy(this);
-                //Numberformat
-                if (xfs.NumberFormatId > 0)
-                {
-                    //rake36: Two problems here...
-                    //rake36:  1. the first time through when format stays equal to String.Empty, it adds a string.empty to the list of Number Formats
-                    //rake36:  2. when adding a second sheet, if the numberformatid == 164, it finds the 164 added by previous sheets but was using the array index
-                    //rake36:      for the numberformatid
-
-                    string format = string.Empty;
-                    foreach (var fmt in style.NumberFormats)
+                    if (isNamedStyle)
                     {
-                        if (fmt.NumFmtId == xfs.NumberFormatId)
-                        {
-                            format = fmt.Format;
-                            break;
-                        }
+                        xfs = style.CellStyleXfs[styleID];
                     }
-                    //rake36: Don't add another format if it's blank
-                    if (!String.IsNullOrEmpty(format))
+                    else
                     {
-                        int ix = NumberFormats.FindIndexByID(format);
-                        if (ix < 0)
+                        xfs = style.CellXfs[styleID];
+                    }
+                    ExcelXfs newXfs = xfs.Copy(this);
+                    //Numberformat
+                    if (xfs.NumberFormatId > 0)
+                    {
+                        //rake36: Two problems here...
+                        //rake36:  1. the first time through when format stays equal to String.Empty, it adds a string.empty to the list of Number Formats
+                        //rake36:  2. when adding a second sheet, if the numberformatid == 164, it finds the 164 added by previous sheets but was using the array index
+                        //rake36:      for the numberformatid
+
+                        string format = string.Empty;
+                        foreach (var fmt in style.NumberFormats)
                         {
-                            var item = new ExcelNumberFormatXml(NameSpaceManager) { Format = format, NumFmtId = NumberFormats.NextId++ };
-                            NumberFormats.Add(format, item);
-                            //rake36: Use the just added format id
-                            newXfs.NumberFormatId = item.NumFmtId;
+                            if (fmt.NumFmtId == xfs.NumberFormatId)
+                            {
+                                format = fmt.Format;
+                                break;
+                            }
                         }
-                        else
+                        //rake36: Don't add another format if it's blank
+                        if (!String.IsNullOrEmpty(format))
                         {
-                            //rake36: Use the format id defined by the index... not the index itself
-                            newXfs.NumberFormatId = NumberFormats[ix].NumFmtId;
+                            int ix = NumberFormats.FindIndexByID(format);
+                            if (ix < 0)
+                            {
+                                var item = new ExcelNumberFormatXml(NameSpaceManager) { Format = format, NumFmtId = NumberFormats.NextId++ };
+                                NumberFormats.Add(format, item);
+                                //rake36: Use the just added format id
+                                newXfs.NumberFormatId = item.NumFmtId;
+                            }
+                            else
+                            {
+                                //rake36: Use the format id defined by the index... not the index itself
+                                newXfs.NumberFormatId = NumberFormats[ix].NumFmtId;
+                            }
                         }
                     }
-                }
 
-                //Font
-                if (xfs.FontId > -1)
-                {
-                    int ix = Fonts.FindIndexByID(xfs.Font.Id);
-                    if (ix < 0)
+                    //Font
+                    if (xfs.FontId > -1)
                     {
-                        ExcelFontXml item = style.Fonts[xfs.FontId].Copy();
-                        ix = Fonts.Add(xfs.Font.Id, item);
+                        int ix = Fonts.FindIndexByID(xfs.Font.Id);
+                        if (ix < 0)
+                        {
+                            ExcelFontXml item = style.Fonts[xfs.FontId].Copy();
+                            ix = Fonts.Add(xfs.Font.Id, item);
+                        }
+                        newXfs.FontId = ix;
                     }
-                    newXfs.FontId = ix;
-                }
 
-                //Border
-                if (xfs.BorderId > -1)
-                {
-                    int ix = Borders.FindIndexByID(xfs.Border.Id);
-                    if (ix < 0)
+                    //Border
+                    if (xfs.BorderId > -1)
                     {
-                        ExcelBorderXml item = style.Borders[xfs.BorderId].Copy();
-                        ix = Borders.Add(xfs.Border.Id, item);
+                        int ix = Borders.FindIndexByID(xfs.Border.Id);
+                        if (ix < 0)
+                        {
+                            ExcelBorderXml item = style.Borders[xfs.BorderId].Copy();
+                            ix = Borders.Add(xfs.Border.Id, item);
+                        }
+                        newXfs.BorderId = ix;
                     }
-                    newXfs.BorderId = ix;
-                }
 
-                //Fill
-                if (xfs.FillId > -1)
-                {
-                    int ix = Fills.FindIndexByID(xfs.Fill.Id);
-                    if (ix < 0)
+                    //Fill
+                    if (xfs.FillId > -1)
                     {
-                        var item = style.Fills[xfs.FillId].Copy();
-                        ix = Fills.Add(xfs.Fill.Id, item);
+                        int ix = Fills.FindIndexByID(xfs.Fill.Id);
+                        if (ix < 0)
+                        {
+                            var item = style.Fills[xfs.FillId].Copy();
+                            ix = Fills.Add(xfs.Fill.Id, item);
+                        }
+                        newXfs.FillId = ix;
                     }
-                    newXfs.FillId = ix;
-                }
 
-                //Named style reference
-                if (xfs.XfId > 0)
-                {
-                    var id = style.CellStyleXfs[xfs.XfId].Id;
-                    var newId = CellStyleXfs.FindIndexByID(id);
-                    if (newId >= 0)
+                    //Named style reference
+                    if (xfs.XfId > 0)
                     {
-                        newXfs.XfId = newId;
-                    }
-                    else if(style._wb!=_wb && allwaysAddCellXfs==false) //Not the same workbook, copy the namedstyle to the workbook or match the id
-                    {
-                        var nsFind = style.NamedStyles.ToDictionary(d => (d.StyleXfId));
-                        if (nsFind.ContainsKey(xfs.XfId))
+                        var id = style.CellStyleXfs[xfs.XfId].Id;
+                        var newId = CellStyleXfs.FindIndexByID(id);
+                        if (newId >= 0)
                         {
-                            var st = nsFind[xfs.XfId];
-                            if (NamedStyles.ExistsKey(st.Name))
-                            {
-                                newXfs.XfId = NamedStyles.FindIndexByID(st.Name);
-                            }
-                            else
+                            newXfs.XfId = newId;
+                        }
+                        else if(style._wb!=_wb && allwaysAddCellXfs==false) //Not the same workbook, copy the namedstyle to the workbook or match the id
+                        {
+                            var nsFind = style.NamedStyles.ToDictionary(d => (d.StyleXfId));
+                            if (nsFind.ContainsKey(xfs.XfId))
                             {
-                                var ns = CreateNamedStyle(st.Name, st.Style);
-                                newXfs.XfId = NamedStyles.Count - 1;
+                                var st = nsFind[xfs.XfId];
+                                if (NamedStyles.ExistsKey(st.Name))
+                                {
+                                    newXfs.XfId = NamedStyles.FindIndexByID(st.Name);
+                                }
+                                else
+                                {
+                                    var ns = CreateNamedStyle(st.Name, st.Style);
+                                    newXfs.XfId = NamedStyles.Count - 1;
+                                }
                             }
                         }
                     }
-                }
 
-                int index;
-                if (isNamedStyle && allwaysAddCellXfs==false)
-                {
-                    index = CellStyleXfs.Add(newXfs.Id, newXfs);
-                }
-                else
-                {
-                    if (allwaysAddCellXfs)
+                    int index;
+                    if (isNamedStyle && allwaysAddCellXfs==false)
                     {
-                        index = CellXfs.Add(newXfs.Id, newXfs);
+                        index = CellStyleXfs.Add(newXfs.Id, newXfs);
                     }
                     else
                     {
-                        index = CellXfs.FindIndexByID(newXfs.Id);
-                        if (index < 0)
+                        if (allwaysAddCellXfs)
                         {
                             index = CellXfs.Add(newXfs.Id, newXfs);
                         }
+                        else
+                        {
+                            index = CellXfs.FindIndexByID(newXfs.Id);
+                            if (index < 0)
+                            {
+                                index = CellXfs.Add(newXfs.Id, newXfs);
+                            }
+                        }
                     }
+                    return index;
                 }
-                return index;
             }
         }
     }
diff --git a/Epplus-FixCalcsEdition/EPPlus/ExcelWorkbook.cs b/Epplus-FixCalcsEdition/EPPlus/ExcelWorkbook.cs
index e748f50..45e33a0 100644
--- a/Epplus-FixCalcsEdition/EPPlus/ExcelWorkbook.cs
+++ b/Epplus-FixCalcsEdition/EPPlus/ExcelWorkbook.cs
@@ -94,6 +94,15 @@ internal class SharedStringItem
 		private OfficeProperties _properties;
 
 		private ExcelStyles _styles;
+        internal readonly object SyncRoot;
+
+        internal void ThrowIfDisposed()
+        {
+            if (_package == null)
+            {
+                throw new ObjectDisposedException(nameof(ExcelWorkbook));
+            }
+        }
 		#endregion
 
 		#region ExcelWorkbook Constructor
@@ -105,6 +114,7 @@ internal class SharedStringItem
 		internal ExcelWorkbook(ExcelPackage package, XmlNamespaceManager namespaceManager) :
 			base(namespaceManager)
 		{
+            SyncRoot = package.SyncRoot;
 			_package = package;
 			WorkbookUri = new Uri("/xl/workbook.xml", UriKind.Relative);
 			SharedStringsUri = new Uri("/xl/sharedStrings.xml", UriKind.Relative);
@@ -280,6 +290,8 @@ public ExcelWorksheets Worksheets
 		{
 			get
 			{
+                lock (SyncRoot)
+                {
 				if (_worksheets == null)
 				{
 					var sheetsNode = _workbookXml.DocumentElement.SelectSingleNode("d:sheets", _namespaceManager);
@@ -291,6 +303,7 @@ public ExcelWorksheets Worksheets
 					_worksheets = new ExcelWorksheets(_package, _namespaceManager, sheetsNode);
 				}
 				return (_worksheets);
+                }
 			}
 		}
 		#endregion
@@ -312,11 +325,14 @@ internal FormulaParser FormulaParser
         {
             get
             {
-                if (_formulaParser == null)
+                lock (SyncRoot)
                 {
-                    _formulaParser = new FormulaParser(new EpplusExcelDataProvider(_package));
+                    if (_formulaParser == null)
+                    {
+                        _formulaParser = new FormulaParser(new EpplusExcelDataProvider(_package));
+                    }
+                    return _formulaParser;
                 }
-                return _formulaParser;
             }
         }
 
@@ -324,11 +340,14 @@ public FormulaParserManager FormulaParserManager
 	    {
 	        get
 	        {
-	            if (_parserManager == null)
+                lock (SyncRoot)
 	            {
-	                _parserManager = new FormulaParserManager(FormulaParser);
+                    if (_parserManager == null)
+                    {
+                        _parserManager = new FormulaParserManager(FormulaParser, SyncRoot);
+                    }
+                    return _parserManager;
 	            }
-	            return _parserManager;
 	        }
 	    }
         /// <summary>
@@ -656,43 +675,46 @@ public XmlDocument StylesXml
 		{
 			get
 			{
-				if (_stylesXml == null)
-				{
-					if (_package.Package.PartExists(StylesUri))
-						_stylesXml = _package.GetXmlFromUri(StylesUri);
-					else
-					{
-						// create a new styles part and add to the package
-						Packaging.ZipPackagePart part = _package.Package.CreatePart(StylesUri, @"application/vnd.openxmlformats-officedocument.spreadsheetml.styles+xml", _package.Compression);
-						// create the style sheet
+                lock (SyncRoot)
+                {
+                    if (_stylesXml == null)
+                    {
+                        if (_package.Package.PartExists(StylesUri))
+                            _stylesXml = _package.GetXmlFromUri(StylesUri);
+                        else
+                        {
+                            // create a new styles part and add to the package
+                            Packaging.ZipPackagePart part = _package.Package.CreatePart(StylesUri, @"application/vnd.openxmlformats-officedocument.spreadsheetml.styles+xml", _package.Compression);
+                            // create the style sheet
 
-						StringBuilder xml = new StringBuilder("<styleSheet xmlns=\"http://schemas.openxmlformats.org/spreadsheetml/2006/main\">");
-						xml.Append("<numFmts />");
-						xml.Append("<fonts count=\"1\"><font><sz val=\"11\" /><name val=\"Calibri\" /></font></fonts>");
-						xml.Append("<fills><fill><patternFill patternType=\"none\" /></fill><fill><patternFill patternType=\"gray125\" /></fill></fills>");
-						xml.Append("<borders><border><left /><right /><top /><bottom /><diagonal /></border></borders>");
-						xml.Append("<cellStyleXfs count=\"1\"><xf numFmtId=\"0\" fontId=\"0\" /></cellStyleXfs>");
-						xml.Append("<cellXfs count=\"1\"><xf numFmtId=\"0\" fontId=\"0\" xfId=\"0\" /></cellXfs>");
-						xml.Append("<cellStyles><cellStyle name=\"Normal\" xfId=\"0\" builtinId=\"0\" /></cellStyles>");
-                        xml.Append("<dxfs count=\"0\" />");
-                        xml.Append("</styleSheet>");
+                            StringBuilder xml = new StringBuilder("<styleSheet xmlns=\"http://schemas.openxmlformats.org/spreadsheetml/2006/main\">");
+                            xml.Append("<numFmts />");
+                            xml.Append("<fonts count=\"1\"><font><sz val=\"11\" /><name val=\"Calibri\" /></font></fonts>");
+                            xml.Append("<fills><fill><patternFill patternType=\"none\" /></fill><fill><patternFill patternType=\"gray125\" /></fill></fills>");
+                            xml.Append("<borders><border><left /><right /><top /><bottom /><diagonal /></border></borders>");
+                            xml.Append("<cellStyleXfs count=\"1\"><xf numFmtId=\"0\" fontId=\"0\" /></cellStyleXfs>");
+                            xml.Append("<cellXfs count=\"1\"><xf numFmtId=\"0\" fontId=\"0\" xfId=\"0\" /></cellXfs>");
+                            xml.Append("<cellStyles><cellStyle name=\"Normal\" xfId=\"0\" builtinId=\"0\" /></cellStyles>");
+                            xml.Append("<dxfs count=\"0\" />");
+                            xml.Append("</styleSheet>");
 						
-						_stylesXml = new XmlDocument();
-						_stylesXml.LoadXml(xml.ToString());
+                            _stylesXml = new XmlDocument();
+                            _stylesXml.LoadXml(xml.ToString());
 						
-						//Save it to the package
-						StreamWriter stream = new StreamWriter(part.GetStream(FileMode.Create, FileAccess.Write));
+                            //Save it to the package
+                            StreamWriter stream = new StreamWriter(part.GetStream(FileMode.Create, FileAccess.Write));
 
-						_stylesXml.Save(stream);
-						//stream.Close();
-						_package.Package.Flush();
+                            _stylesXml.Save(stream);
+                            //stream.Close();
+                            _package.Package.Flush();
 
-						// create the relationship between the workbook and the new shared strings part
-						_package.Workbook.Part.CreateRelationship(UriHelper.GetRelativeUri(WorkbookUri, StylesUri), Packaging.TargetMode.Internal, ExcelPackage.schemaRelationships + "/styles");
-						_package.Package.Flush();
-					}
-				}
-				return (_stylesXml);
+                            // create the relationship between the workbook and the new shared strings part
+                            _package.Workbook.Part.CreateRelationship(UriHelper.GetRelativeUri(WorkbookUri, StylesUri), Packaging.TargetMode.Internal, ExcelPackage.schemaRelationships + "/styles");
+                            _package.Package.Flush();
+                        }
+                    }
+                    return (_stylesXml);
+                }
 			}
 			set
 			{
@@ -706,11 +728,14 @@ public ExcelStyles Styles
 		{
 			get
 			{
-				if (_styles == null)
-				{
-					_styles = new ExcelStyles(NameSpaceManager, StylesXml, this);
-				}
-				return _styles;
+                lock (SyncRoot)
+                {
+                    if (_styles == null)
+                    {
+                        _styles = new ExcelStyles(NameSpaceManager, StylesXml, this);
+                    }
+                    return _styles;
+                }
 			}
 		}
 		#endregion
@@ -800,80 +825,82 @@ public bool FullCalcOnLoad
 		/// </summary>
 		internal void Save()  // Workbook Save
 		{
-			if (Worksheets.Count == 0)
-				throw new InvalidOperationException("The workbook must contain at least one worksheet");
+            lock (SyncRoot)
+            {
+                if (Worksheets.Count == 0)
+                    throw new InvalidOperationException("The workbook must contain at least one worksheet");
 
-			DeleteCalcChain();
+                DeleteCalcChain();
 
-            if (_vba == null && !_package.Package.PartExists(new Uri(ExcelVbaProject.PartUri, UriKind.Relative)))
-            {
-                if (Part.ContentType != ExcelPackage.contentTypeWorkbookDefault)
+                if (_vba == null && !_package.Package.PartExists(new Uri(ExcelVbaProject.PartUri, UriKind.Relative)))
                 {
-                    Part.ContentType = ExcelPackage.contentTypeWorkbookDefault;
+                    if (Part.ContentType != ExcelPackage.contentTypeWorkbookDefault)
+                    {
+                        Part.ContentType = ExcelPackage.contentTypeWorkbookDefault;
+                    }
                 }
-            }
-            else
-            {
-                if (Part.ContentType != ExcelPackage.contentTypeWorkbookMacroEnabled)
+                else
                 {
-                    Part.ContentType = ExcelPackage.contentTypeWorkbookMacroEnabled;
+                    if (Part.ContentType != ExcelPackage.contentTypeWorkbookMacroEnabled)
+                    {
+                        Part.ContentType = ExcelPackage.contentTypeWorkbookMacroEnabled;
+                    }
                 }
-            }
-			
-            UpdateDefinedNamesXml();
 
-			// save the workbook
-			if (_workbookXml != null)
-			{
-				_package.SavePart(WorkbookUri, _workbookXml);
-			}
+                UpdateDefinedNamesXml();
 
-			// save the properties of the workbook
-			if (_properties != null)
-			{
-				_properties.Save();
-			}
+                // save the workbook
+                if (_workbookXml != null)
+                {
+                    _package.SavePart(WorkbookUri, _workbookXml);
+                }
 
-			// save the style sheet
-			Styles.UpdateXml();
-			_package.SavePart(StylesUri, _stylesXml);
+                // save the properties of the workbook
+                if (_properties != null)
+                {
+                    _properties.Save();
+                }
 
-			// save all the open worksheets
-			var isProtected = Protection.LockWindows || Protection.LockStructure;
-			foreach (ExcelWorksheet worksheet in Worksheets)
-			{
-				if (isProtected && Protection.LockWindows)
-				{
-					worksheet.View.WindowProtection = true;
-				}
-				worksheet.Save();
-                worksheet.Part.SaveHandler = worksheet.SaveHandler;
-			}
+                // save the style sheet
+                Styles.UpdateXml();
+                _package.SavePart(StylesUri, _stylesXml);
 
-            // Issue 15252: save SharedStrings only once
-            Packaging.ZipPackagePart part;
-            if (_package.Package.PartExists(SharedStringsUri))
-            {
-                part = _package.Package.GetPart(SharedStringsUri);
-            }
-            else
-            {
-                part = _package.Package.CreatePart(SharedStringsUri, @"application/vnd.openxmlformats-officedocument.spreadsheetml.sharedStrings+xml", _package.Compression);
-                Part.CreateRelationship(UriHelper.GetRelativeUri(WorkbookUri, SharedStringsUri), Packaging.TargetMode.Internal, ExcelPackage.schemaRelationships + "/sharedStrings");
-            }
+                // save all the open worksheets
+                var isProtected = Protection.LockWindows || Protection.LockStructure;
+                foreach (ExcelWorksheet worksheet in Worksheets)
+                {
+                    if (isProtected && Protection.LockWindows)
+                    {
+                        worksheet.View.WindowProtection = true;
+                    }
+                    worksheet.Save();
+                    worksheet.Part.SaveHandler = worksheet.SaveHandler;
+                }
 
-            part.SaveHandler = SaveSharedStringHandler;
-            //UpdateSharedStringsXml();
-			
-			// Data validation
-			ValidateDataValidations();
+                // Issue 15252: save SharedStrings only once
+                Packaging.ZipPackagePart part;
+                if (_package.Package.PartExists(SharedStringsUri))
+                {
+                    part = _package.Package.GetPart(SharedStringsUri);
+                }
+                else
+                {
+                    part = _package.Package.CreatePart(SharedStringsUri, @"application/vnd.openxmlformats-officedocument.spreadsheetml.sharedStrings+xml", _package.Compression);
+                    Part.CreateRelationship(UriHelper.GetRelativeUri(WorkbookUri, SharedStringsUri), Packaging.TargetMode.Internal, ExcelPackage.schemaRelationships + "/sharedStrings");
+                }
 
-            //VBA
-            if (_vba!=null)
-            {
-                VbaProject.Save();
-            }
+                part.SaveHandler = SaveSharedStringHandler;
+                //UpdateSharedStringsXml();
+
+                // Data validation
+                ValidateDataValidations();
 
+                //VBA
+                if (_vba!=null)
+                {
+                    VbaProject.Save();
+                }
+            }
 		}
 		private void DeleteCalcChain()
 		{
@@ -1134,29 +1161,32 @@ internal void GetExternalReferences()
 
         public void Dispose()
         {
-            if (_sharedStrings != null)
+            lock (SyncRoot)
             {
-                _sharedStrings.Clear();
-                _sharedStrings = null;
-            }
-            if (_sharedStringsList != null)
-            {
-                _sharedStringsList.Clear();
-                _sharedStringsList = null;
-            }
-            _vba = null;
-            if (_worksheets != null)
-            {
-                _worksheets.Dispose();
-                _worksheets = null;
+                if (_sharedStrings != null)
+                {
+                    _sharedStrings.Clear();
+                    _sharedStrings = null;
+                }
+                if (_sharedStringsList != null)
+                {
+                    _sharedStringsList.Clear();
+                    _sharedStringsList = null;
+                }
+                _vba = null;
+                if (_worksheets != null)
+                {
+                    _worksheets.Dispose();
+                    _worksheets = null;
+                }
+                _package = null;
+                _properties = null;
+                if (_formulaParser != null)
+                {
+                    _formulaParser.Dispose();
+                    _formulaParser = null;
+                }
             }
-            _package = null;
-            _properties = null;
-            if (_formulaParser != null)
-            {
-                _formulaParser.Dispose();
-                _formulaParser = null;
-            }   
         }
 
         internal void ReadAllTables()
diff --git a/Epplus-FixCalcsEdition/EPPlus/FormulaParsing/CalculateExtentions.cs b/Epplus-FixCalcsEdition/EPPlus/FormulaParsing/CalculateExtentions.cs
index f43f8bd..a7856f6 100644
--- a/Epplus-FixCalcsEdition/EPPlus/FormulaParsing/CalculateExtentions.cs
+++ b/Epplus-FixCalcsEdition/EPPlus/FormulaParsing/CalculateExtentions.cs
@@ -49,37 +49,40 @@ public static void Calculate(this ExcelWorkbook workbook)
         }
         public static void Calculate(this ExcelWorkbook workbook, ExcelCalculationOption options)
         {
-            Init(workbook);
-
-            var dc = DependencyChainFactory.Create(workbook, options);
-            workbook.FormulaParser.InitNewCalc();
-            if (workbook.FormulaParser.Logger != null)
+            lock (workbook.SyncRoot)
             {
-                var msg = string.Format("Starting... number of cells to parse: {0}", dc.list.Count);
-                workbook.FormulaParser.Logger.Log(msg);
-            }
+                Init(workbook);
 
-            //TODO: Remove when tests are done. Outputs the dc to a text file. 
-            //var fileDc = new System.IO.StreamWriter("c:\\temp\\dc.txt");
-                        
-            //for (int i = 0; i < dc.list.Count; i++)
-            //{
-            //    fileDc.WriteLine(i.ToString() + "," + dc.list[i].Column.ToString() + "," + dc.list[i].Row.ToString() + "," + (dc.list[i].ws==null ? "" : dc.list[i].ws.Name) + "," + dc.list[i].Formula);
-            //}
-            //fileDc.Close();
-            //fileDc = new System.IO.StreamWriter("c:\\temp\\dcorder.txt");
-            //for (int i = 0; i < dc.CalcOrder.Count; i++)
-            //{
-            //    fileDc.WriteLine(dc.CalcOrder[i].ToString());
-            //}
-            //fileDc.Close();
-            //fileDc = null;
+                var dc = DependencyChainFactory.Create(workbook, options);
+                workbook.FormulaParser.InitNewCalc();
+                if (workbook.FormulaParser.Logger != null)
+                {
+                    var msg = string.Format("Starting... number of cells to parse: {0}", dc.list.Count);
+                    workbook.FormulaParser.Logger.Log(msg);
+                }
 
-            //TODO: Add calculation here
+                //TODO: Remove when tests are done. Outputs the dc to a text file.
+                //var fileDc = new System.IO.StreamWriter("c:\\temp\\dc.txt");
 
-            CalcChain(workbook, workbook.FormulaParser, dc);
+                //for (int i = 0; i < dc.list.Count; i++)
+                //{
+                //    fileDc.WriteLine(i.ToString() + "," + dc.list[i].Column.ToString() + "," + dc.list[i].Row.ToString() + "," + (dc.list[i].ws==null ? "" : dc.list[i].ws.Name) + "," + dc.list[i].Formula);
+                //}
+                //fileDc.Close();
+                //fileDc = new System.IO.StreamWriter("c:\\temp\\dcorder.txt");
+                //for (int i = 0; i < dc.CalcOrder.Count; i++)
+                //{
+                //    fileDc.WriteLine(dc.CalcOrder[i].ToString());
+                //}
+                //fileDc.Close();
+                //fileDc = null;
 
-            //workbook._isCalculated = true;
+                //TODO: Add calculation here
+
+                CalcChain(workbook, workbook.FormulaParser, dc);
+
+                //workbook._isCalculated = true;
+            }
         }
         public static void Calculate(this ExcelWorksheet worksheet)
         {
@@ -87,17 +90,20 @@ public static void Calculate(this ExcelWorksheet worksheet)
         }
         public static void Calculate(this ExcelWorksheet worksheet, ExcelCalculationOption options)
         {
-            Init(worksheet.Workbook);
-            //worksheet.Workbook._formulaParser = null; TODO:Cant reset. Don't work with userdefined or overrided worksheet functions            
-            var dc = DependencyChainFactory.Create(worksheet, options);
-            var parser = worksheet.Workbook.FormulaParser;
-            parser.InitNewCalc();
-            if (parser.Logger != null)
+            lock (worksheet.Workbook.SyncRoot)
             {
-                var msg = string.Format("Starting... number of cells to parse: {0}", dc.list.Count);
-                parser.Logger.Log(msg);
+                Init(worksheet.Workbook);
+                //worksheet.Workbook._formulaParser = null; TODO:Cant reset. Don't work with userdefined or overrided worksheet functions
+                var dc = DependencyChainFactory.Create(worksheet, options);
+                var parser = worksheet.Workbook.FormulaParser;
+                parser.InitNewCalc();
+                if (parser.Logger != null)
+                {
+                    var msg = string.Format("Starting... number of cells to parse: {0}", dc.list.Count);
+                    parser.Logger.Log(msg);
+                }
+                CalcChain(worksheet.Workbook, parser, dc);
             }
-            CalcChain(worksheet.Workbook, parser, dc);
         }
         public static void Calculate(this ExcelRangeBase range)
         {
@@ -105,11 +111,14 @@ public static void Calculate(this ExcelRangeBase range)
         }
         public static void Calculate(this ExcelRangeBase range, ExcelCalculationOption options)
         {
-            Init(range._workbook);
-            var parser = range._workbook.FormulaParser;
-            parser.InitNewCalc();
-            var dc = DependencyChainFactory.Create(range, options);
-            CalcChain(range._workbook, parser, dc);
+            lock (range._workbook.SyncRoot)
+            {
+                Init(range._workbook);
+                var parser = range._workbook.FormulaParser;
+                parser.InitNewCalc();
+                var dc = DependencyChainFactory.Create(range, options);
+                CalcChain(range._workbook, parser, dc);
+            }
         }
         public static object Calculate(this ExcelWorksheet worksheet, string Formula)
         {
@@ -117,25 +126,28 @@ public static object Calculate(this ExcelWorksheet worksheet, string Formula)
         }
         public static object Calculate(this ExcelWorksheet worksheet, string Formula, ExcelCalculationOption options)
         {
-            try
+            lock (worksheet.Workbook.SyncRoot)
             {
-                worksheet.CheckSheetType();
-                if(string.IsNullOrEmpty(Formula.Trim())) return null;
-                Init(worksheet.Workbook);
-                var parser = worksheet.Workbook.FormulaParser;
-                parser.InitNewCalc();
-                if (Formula[0] == '=') Formula = Formula.Substring(1); //Remove any starting equal sign
-                var dc = DependencyChainFactory.Create(worksheet, Formula, options);
-                var f = dc.list[0];
-                dc.CalcOrder.RemoveAt(dc.CalcOrder.Count - 1);
+                try
+                {
+                    worksheet.CheckSheetType();
+                    if(string.IsNullOrEmpty(Formula.Trim())) return null;
+                    Init(worksheet.Workbook);
+                    var parser = worksheet.Workbook.FormulaParser;
+                    parser.InitNewCalc();
+                    if (Formula[0] == '=') Formula = Formula.Substring(1); //Remove any starting equal sign
+                    var dc = DependencyChainFactory.Create(worksheet, Formula, options);
+                    var f = dc.list[0];
+                    dc.CalcOrder.RemoveAt(dc.CalcOrder.Count - 1);
 
-                CalcChain(worksheet.Workbook, parser, dc);
+                    CalcChain(worksheet.Workbook, parser, dc);
 
-                return parser.ParseCell(f.Tokens, worksheet.Name, -1, -1);
-            }
-            catch (Exception ex)
-            {
-                return new ExcelErrorValueException(ex.Message, ExcelErrorValue.Create(eErrorType.Value));
+                    return parser.ParseCell(f.Tokens, worksheet.Name, -1, -1);
+                }
+                catch (Exception ex)
+                {
+                    return new ExcelErrorValueException(ex.Message, ExcelErrorValue.Create(eErrorType.Value));
+                }
             }
         }
         private static void CalcChain(ExcelWorkbook wb, FormulaParser parser, DependencyChain dc)
@@ -168,6 +180,7 @@ private static void CalcChain(ExcelWorkbook wb, FormulaParser parser, Dependency
         }
         private static void Init(ExcelWorkbook workbook)
         {
+            workbook.ThrowIfDisposed();
             workbook._formulaTokens = new CellStore<List<Token>>();;
             foreach (var ws in workbook.Worksheets)
             {
diff --git a/Epplus-FixCalcsEdition/EPPlus/FormulaParsing/Excel/Operators/OperatorsDict.cs b/Epplus-FixCalcsEdition/EPPlus/FormulaParsing/Excel/Operators/OperatorsDict.cs
index 916f362..f45984c 100644
--- a/Epplus-FixCalcsEdition/EPPlus/FormulaParsing/Excel/Operators/OperatorsDict.cs
+++ b/Epplus-FixCalcsEdition/EPPlus/FormulaParsing/Excel/Operators/OperatorsDict.cs
@@ -53,17 +53,14 @@ public OperatorsDict()
             Add("&", Operator.Concat);
         }
 
-        private static IDictionary<string, IOperator> _instance;
+        private static readonly Lazy<IDictionary<string, IOperator>> _instance =
+            new Lazy<IDictionary<string, IOperator>>(() => new OperatorsDict());
 
         public static IDictionary<string, IOperator> Instance
         {
             get 
             {
-                if (_instance == null)
-                {
-                    _instance = new OperatorsDict();
-                }
-                return _instance;
+                return _instance.Value;
             }
         }
     }
diff --git a/Epplus-FixCalcsEdition/EPPlus/FormulaParsing/ExcelAddressCache.cs b/Epplus-FixCalcsEdition/EPPlus/FormulaParsing/ExcelAddressCache.cs
index ec77e3d..8b5d34c 100644
--- a/Epplus-FixCalcsEdition/EPPlus/FormulaParsing/ExcelAddressCache.cs
+++ b/Epplus-FixCalcsEdition/EPPlus/FormulaParsing/ExcelAddressCache.cs
@@ -69,7 +69,13 @@ public bool Add(int id, string address)
         /// </summary>
         public int Count
         {
-            get { return _addressCache.Count; }
+            get
+            {
+                lock (_myLock)
+                {
+                    return _addressCache.Count;
+                }
+            }
         }
 
         /// <summary>
@@ -79,8 +85,11 @@ public int Count
         /// <returns></returns>
         public string Get(int id)
         {
-            if (!_addressCache.ContainsKey(id)) return string.Empty;
-            return _addressCache[id];
+            lock (_myLock)
+            {
+                string address;
+                return _addressCache.TryGetValue(id, out address) ? address : string.Empty;
+            }
         }
 
         /// <summary>
diff --git a/Epplus-FixCalcsEdition/EPPlus/FormulaParsing/FormulaParserManager.cs b/Epplus-FixCalcsEdition/EPPlus/FormulaParsing/FormulaParserManager.cs
index ad5a96b..9e68811 100644
--- a/Epplus-FixCalcsEdition/EPPlus/FormulaParsing/FormulaParserManager.cs
+++ b/Epplus-FixCalcsEdition/EPPlus/FormulaParsing/FormulaParserManager.cs
@@ -45,9 +45,15 @@ namespace CompuMaster.Epplus4.FormulaParsing
     public class FormulaParserManager
     {
         private readonly FormulaParser _parser;
+        private readonly object _syncRoot;
 
-        internal FormulaParserManager(FormulaParser parser)
+        internal FormulaParserManager(FormulaParser parser) : this(parser, new object())
         {
+        }
+
+        internal FormulaParserManager(FormulaParser parser, object syncRoot)
+        {
+            _syncRoot = syncRoot;
             Require.That(parser).Named("parser").IsNotNull();
             _parser = parser;
         }
@@ -60,7 +66,10 @@ internal FormulaParserManager(FormulaParser parser)
         /// <param name="module">A <see cref="IFunctionModule"/> containing <see cref="ExcelFunction"/>s.</param>
         public void LoadFunctionModule(IFunctionModule module)
         {
-            _parser.Configure(x => x.FunctionRepository.LoadModule(module));
+            lock (_syncRoot)
+            {
+                _parser.Configure(x => x.FunctionRepository.LoadModule(module));
+            }
         }
 
         /// <summary>
@@ -106,15 +115,18 @@ public IEnumerable<string> GetImplementedFunctionNames()
         /// <returns>An enumeration of <see cref="KeyValuePair{String,ExcelFunction}"/>, where the key is the function name</returns>
         public IEnumerable<KeyValuePair<string, ExcelFunction>> GetImplementedFunctions()
         {
-            var functions = new List<KeyValuePair<string, ExcelFunction>>();
-            _parser.Configure(parsingConfiguration =>
+            lock (_syncRoot)
             {
-                foreach (var name in parsingConfiguration.FunctionRepository.FunctionNames)
+                var functions = new List<KeyValuePair<string, ExcelFunction>>();
+                _parser.Configure(parsingConfiguration =>
                 {
-                    functions.Add(new KeyValuePair<string, ExcelFunction>(name, parsingConfiguration.FunctionRepository.GetFunction(name)));
-                }
-            });
-            return functions;
+                    foreach (var name in parsingConfiguration.FunctionRepository.FunctionNames)
+                    {
+                        functions.Add(new KeyValuePair<string, ExcelFunction>(name, parsingConfiguration.FunctionRepository.GetFunction(name)));
+                    }
+                });
+                return functions;
+            }
         } 
 
         /// <summary>
@@ -124,7 +136,10 @@ public IEnumerable<KeyValuePair<string, ExcelFunction>> GetImplementedFunctions(
         /// <returns></returns>
         public object Parse(string formula)
         {
-            return _parser.Parse(formula);
+            lock (_syncRoot)
+            {
+                return _parser.Parse(formula);
+            }
         }
 
         /// <summary>
@@ -134,7 +149,10 @@ public object Parse(string formula)
         /// <see cref="CompuMaster.Epplus4.FormulaParsing.Logging.LoggerFactory"/>
         public void AttachLogger(IFormulaParserLogger logger)
         {
-            _parser.Configure(c => c.AttachLogger(logger));
+            lock (_syncRoot)
+            {
+                _parser.Configure(c => c.AttachLogger(logger));
+            }
         }
 
         /// <summary>
@@ -143,14 +161,20 @@ public void AttachLogger(IFormulaParserLogger logger)
         /// <param name="logfile"></param>
         public void AttachLogger(FileInfo logfile)
         {
-            _parser.Configure(c => c.AttachLogger(LoggerFactory.CreateTextFileLogger(logfile)));
+            lock (_syncRoot)
+            {
+                _parser.Configure(c => c.AttachLogger(LoggerFactory.CreateTextFileLogger(logfile)));
+            }
         }
         /// <summary>
         /// Detaches any attached logger from the formula parser.
         /// </summary>
         public void DetachLogger()
         {
-            _parser.Configure(c => c.DetachLogger());
+            lock (_syncRoot)
+            {
+                _parser.Configure(c => c.DetachLogger());
+            }
         }
     }
 }
diff --git a/Epplus-FixCalcsEdition/EPPlus/README.md b/Epplus-FixCalcsEdition/EPPlus/README.md
index 17b0968..33bfa8a 100644
--- a/Epplus-FixCalcsEdition/EPPlus/README.md
+++ b/Epplus-FixCalcsEdition/EPPlus/README.md
@@ -5,12 +5,30 @@ This package is based on upstream EPPlus 4.5.3.3; CompuMaster NuGet releases use
 ## Special CompuMaster Edition
 
 * Resets internal calculation caches so Microsoft Excel recalculates dependent formulas when reopening a workbook.
+* Fixes internal races in cell storage, style updates, and calculation/lifetime coordination; general multithreaded workbook access remains unsupported.
 * Uses zero-based worksheet indexing by default on every target framework; the existing compatibility setting can switch to one-based indexing.
 * Uses `System.IO.Compression.ZipArchive` for XLSX packaging, with compatibility handling for older encrypted workbooks.
 * Limits ZIP/XML expansion and encrypted-input work when loading XLSX files; callers can adjust the defaults for trusted files.
 
 These limits do not make arbitrary untrusted workbooks safe to parse. See the [security guidance](https://github.com/CompuMasterGmbH/CompuMaster.Excel/blob/main/CM.Data.EpplusFixCalcsEdition/README.md#security-and-untrusted-workbooks).
 
+## Known Issues
+
+**Warning: Epplus4 (including its fork CompuMaster.EPPlus4) is not thread-safe. Multithreaded use of the library is not supported. Concurrent access can cause exceptions and incorrect or lost cell values without an exception.**
+
+Use each `ExcelPackage` in one thread throughout its lifetime, including access to its workbook, worksheets and ranges, formula calculation, saving, and disposal. This follows [Jan Källman's recommendation to avoid multiple threads on a single workbook](https://github.com/EPPlusSoftware/EPPlus/issues/894#issuecomment-1578082491). Different worksheets in one package are not independent concurrency boundaries. If a package must be shared, serialize all access with one stable external lock per package/workbook, including complete select/read/modify/write sequences; locks per worksheet or cell are insufficient.
+
+The CompuMaster edition fixes the reproduced internal races in cell-store growth and access, workbook-wide style updates, and concurrent formula calculations. Calculation, parser-manager operations, saving, and disposal now share a package-local monitor. Regression tests cover these cases and independently owned packages. These internal fixes do not establish general thread safety or replace caller coordination. See [issue #26](https://github.com/CompuMasterGmbH/CompuMaster.Excel/issues/26) for the original failures, implementation scope, and remaining investigation items.
+
+The following known limitations remain:
+
+1. **A shared, cached `ExcelRange` can target the wrong cell.** Its indexers change the range object's address and return the same object. Another thread can change that address between selection and use. This behavior is retained for compatibility; do not share cached range objects between threads.
+2. **Compound operations and arbitrary concurrent workbook access remain unsupported.** Internal cell-store locks do not make selection followed by reading/writing atomic, provide snapshot cell enumeration, or coordinate every cell/structure change with calculation, saving, loading, or disposal. Concurrent edits can still invalidate a traversal or produce inconsistent output. Coordinate complete operations externally, including reads; direct XML/collection mutation and copying styles between workbooks also require caller coordination.
+3. **Callbacks must not reenter workbook services during calculation.** Custom functions and loggers run while the package monitor is held. Recursively calculating/parsing, saving, or disposing the same package can interfere with active parser state or workbook lifetime. Waiting for another thread that needs the same package can deadlock. The monitor permits internal nested calls but does not make such callback behavior supported.
+4. **Some stream copies are globally serialized.** The inherited `CopyStream` lock can limit throughput even for independent packages. It does not make sharing a stream safe; each package must own its stream exclusively.
+
+Use separately owned packages and streams for parallel jobs, with one thread accessing each package at a time. The independent-package regression tests cover selected operations, not every feature. Other lazy initialization paths, the shared `RAND()` seed, and publicly mutable operator/validation objects still require investigation; these code observations are not all demonstrated runtime defects.
+
 ## Announcement: This is the last version of EPPlus under the LGPL License
 EPPlus will from version 5 be licensed under the [Polyform Noncommercial 1.0.0]( https://polyformproject.org/licenses/noncommercial/1.0.0/) license.  
 With the new license EPPlus is still free to use in some cases, but will require a commercial license to be used in a commercial business.  
@@ -163,6 +181,7 @@ New features 4.0
 
 Replaced Packaging API with DotNetZip
 * This will remove any problems with Isolated Storage and enable multithreading
+* This historical upstream note does not establish thread safety; see Known Issues above.
 
 
 New Cell store
diff --git a/Epplus-FixCalcsEdition/README.md b/Epplus-FixCalcsEdition/README.md
index ded0996..4242928 100644
--- a/Epplus-FixCalcsEdition/README.md
+++ b/Epplus-FixCalcsEdition/README.md
@@ -1,4 +1,28 @@
 # EPPlus
+
+## Special CompuMaster Edition
+
+This folder contains the maintained `CompuMaster.EPPlus4` fork of EPPlus 4.5.3.3. It adds calculation-cache reset behavior and other changes described in the [package README](EPPlus/README.md), which is also included in the NuGet package.
+
+## Known Issues
+
+**Warning: Epplus4 (including its fork CompuMaster.EPPlus4) is not thread-safe. Multithreaded use of the library is not supported. Concurrent access can cause exceptions and incorrect or lost cell values without an exception.**
+
+Use each `ExcelPackage` in one thread throughout its lifetime, including access to its workbook, worksheets and ranges, formula calculation, saving, and disposal. This follows [Jan Källman's recommendation to avoid multiple threads on a single workbook](https://github.com/EPPlusSoftware/EPPlus/issues/894#issuecomment-1578082491). Different worksheets in one package are not independent concurrency boundaries. If a package must be shared, serialize all access with one stable external lock per package/workbook, including complete select/read/modify/write sequences; locks per worksheet or cell are insufficient.
+
+The CompuMaster edition fixes the reproduced internal races in cell-store growth and access, workbook-wide style updates, and concurrent formula calculations. Calculation, parser-manager operations, saving, and disposal now share a package-local monitor. Regression tests cover these cases and independently owned packages. These internal fixes do not establish general thread safety or replace caller coordination. See [issue #26](https://github.com/CompuMasterGmbH/CompuMaster.Excel/issues/26) for the original failures, implementation scope, and remaining investigation items.
+
+The following known limitations remain:
+
+1. **A shared, cached `ExcelRange` can target the wrong cell.** Its indexers change the range object's address and return the same object. Another thread can change that address between selection and use. This behavior is retained for compatibility; do not share cached range objects between threads.
+2. **Compound operations and arbitrary concurrent workbook access remain unsupported.** Internal cell-store locks do not make selection followed by reading/writing atomic, provide snapshot cell enumeration, or coordinate every cell/structure change with calculation, saving, loading, or disposal. Concurrent edits can still invalidate a traversal or produce inconsistent output. Coordinate complete operations externally, including reads; direct XML/collection mutation and copying styles between workbooks also require caller coordination.
+3. **Callbacks must not reenter workbook services during calculation.** Custom functions and loggers run while the package monitor is held. Recursively calculating/parsing, saving, or disposing the same package can interfere with active parser state or workbook lifetime. Waiting for another thread that needs the same package can deadlock. The monitor permits internal nested calls but does not make such callback behavior supported.
+4. **Some stream copies are globally serialized.** The inherited `CopyStream` lock can limit throughput even for independent packages. It does not make sharing a stream safe; each package must own its stream exclusively.
+
+Use separately owned packages and streams for parallel jobs, with one thread accessing each package at a time. The independent-package regression tests cover selected operations, not every feature. Other lazy initialization paths, the shared `RAND()` seed, and publicly mutable operator/validation objects still require investigation; these code observations are not all demonstrated runtime defects.
+
+## Upstream EPPlus Archive
+
 **This repository has moved to https://github.com/EPPlusSoftware/EPPlus.** 
 
 **The code in this archive represents the final version of EPPlus under LGPL. There will be no more activity here.**  
diff --git a/ExcelOpsTest/Epplus4SynchronizationTest.vb b/ExcelOpsTest/Epplus4SynchronizationTest.vb
new file mode 100644
index 0000000..8e1545a
--- /dev/null
+++ b/ExcelOpsTest/Epplus4SynchronizationTest.vb
@@ -0,0 +1,311 @@
+Imports System.IO
+Imports System.Reflection
+Imports System.Threading
+Imports System.Threading.Tasks
+Imports CompuMaster.Epplus4
+Imports CompuMaster.Epplus4.FormulaParsing
+Imports CompuMaster.Epplus4.FormulaParsing.Excel.Functions
+Imports CompuMaster.Epplus4.FormulaParsing.ExpressionGraph
+Imports NUnit.Framework
+
+''' <summary>
+''' Verifies internal synchronization without implying general workbook thread safety.
+''' </summary>
+<TestFixture>
+Public Class Epplus4SynchronizationTest
+
+    ' Fresh ranges isolate store synchronization from the mutable cached-range contract.
+    <TestCase(False), TestCase(True)>
+    Public Sub CellWritesSurviveColumnGrowth(parallel As Boolean)
+        Using package As New ExcelPackage()
+            Dim sheet = package.Workbook.Worksheets.Add("Values")
+            RunWorkers(parallel, 64,
+                Sub(column)
+                    For row = 1 To 300
+                        Dim value = column * 1000 + row
+                        sheet.Cells(row, column + 1).Value = value
+                        Assert.That(sheet.Cells(row, column + 1).Value, [Is].EqualTo(value))
+                    Next
+                End Sub)
+            For column = 0 To 63
+                For row = 1 To 300
+                    Assert.That(sheet.Cells(row, column + 1).Value, [Is].EqualTo(column * 1000 + row))
+                Next
+            Next
+        End Using
+    End Sub
+
+    ' Every worksheet adds distinct workbook-wide format IDs; validate saved IDs as well.
+    <TestCase(False), TestCase(True)>
+    Public Sub StylesAcrossWorksheetsRoundTrip(parallel As Boolean)
+        Using package As New ExcelPackage()
+            Dim sheets = Enumerable.Range(0, 8).Select(Function(i) package.Workbook.Worksheets.Add("Sheet" & i)).ToArray()
+            RunWorkers(parallel, sheets.Length,
+                Sub(i)
+                    For row = 1 To 150
+                        sheets(i).Cells(row, 1).Value = row
+                        sheets(i).Cells(row, 1).Style.Numberformat.Format = FormatFor(i, row)
+                    Next
+                End Sub)
+            Using reopened As New ExcelPackage(New MemoryStream(package.GetAsByteArray()))
+                For i = 0 To sheets.Length - 1
+                    For row = 1 To 150
+                        Assert.That(reopened.Workbook.Worksheets(i).Cells(row, 1).Style.Numberformat.Format, [Is].EqualTo(FormatFor(i, row)))
+                    Next
+                Next
+            End Using
+        End Using
+    End Sub
+
+    <TestCase(False), TestCase(True)>
+    Public Sub CalculationsAcrossWorksheetsKeepResults(parallel As Boolean)
+        Using package As New ExcelPackage()
+            Dim sheets = Enumerable.Range(0, 8).Select(Function(i) package.Workbook.Worksheets.Add("Sheet" & i)).ToArray()
+            For i = 0 To sheets.Length - 1
+                For row = 1 To 150
+                    sheets(i).Cells(row, 1).Value = i + row
+                    sheets(i).Cells(row, 2).Formula = "A" & row & "*2+1"
+                Next
+            Next
+            RunWorkers(parallel, sheets.Length, Sub(i) CalculationExtension.Calculate(sheets(i)))
+            For i = 0 To sheets.Length - 1
+                For row = 1 To 150
+                    Assert.That(sheets(i).Cells(row, 2).Value, [Is].EqualTo(CDbl((i + row) * 2 + 1)))
+                Next
+            Next
+        End Using
+    End Sub
+
+    <Test>
+    Public Sub IndependentPackagesRoundTripInParallel()
+        RunWorkers(True, 32,
+            Sub(i)
+                Using package As New ExcelPackage()
+                    Dim sheet = package.Workbook.Worksheets.Add("Sheet")
+                    sheet.Cells(1, 1).Value = i
+                    sheet.Cells(1, 2).Formula = "A1*2+1"
+                    sheet.Cells(1, 1).Style.Numberformat.Format = "0.00"
+                    CalculationExtension.Calculate(sheet)
+                    Using reopened As New ExcelPackage(New MemoryStream(package.GetAsByteArray()))
+                        Assert.That(reopened.Workbook.Worksheets(0).Cells(1, 2).Value, [Is].EqualTo(CDbl(i * 2 + 1)))
+                    End Using
+                End Using
+            End Sub)
+    End Sub
+
+    <Test>
+    Public Sub FirstUsePublishesOneWorkbookAndStyles()
+        Using package As New ExcelPackage()
+            Dim workbooks(31) As ExcelWorkbook
+            Dim styles(31) As ExcelStyles
+            Dim managers(31) As FormulaParserManager
+            Dim worksheets(31) As ExcelWorksheets
+            RunWorkers(True, workbooks.Length,
+                Sub(i)
+                    workbooks(i) = package.Workbook
+                    styles(i) = workbooks(i).Styles
+                    managers(i) = workbooks(i).FormulaParserManager
+                    worksheets(i) = workbooks(i).Worksheets
+                End Sub)
+            For i = 1 To workbooks.Length - 1
+                Assert.That(workbooks(i), [Is].SameAs(workbooks(0)))
+                Assert.That(styles(i), [Is].SameAs(styles(0)))
+                Assert.That(managers(i), [Is].SameAs(managers(0)))
+                Assert.That(worksheets(i), [Is].SameAs(worksheets(0)))
+            Next
+        End Using
+    End Sub
+
+    <Test>
+    Public Sub AddressCacheSupportsConcurrentClearAndLookup()
+        Dim cache As New ExcelAddressCache()
+        RunWorkers(True, 8,
+            Sub(worker)
+                For i = 1 To 3000
+                    If worker = 0 Then
+                        cache.Clear()
+                    Else
+                        cache.Add(i, "A" & i)
+                        Dim address = cache.Get(i)
+                        Assert.That(address = String.Empty OrElse address = "A" & i, [Is].True)
+                        Assert.That(cache.Count, [Is].GreaterThanOrEqualTo(0))
+                    End If
+                Next
+            End Sub)
+    End Sub
+
+    ' The shared range remains mutable for compatibility; callers must coordinate selection and use.
+    <Test>
+    Public Sub CachedRangeKeepsExistingSelectionSemantics()
+        Using package As New ExcelPackage()
+            Dim sheet = package.Workbook.Worksheets.Add("Sheet")
+            Dim cells = sheet.Cells
+            Dim first = cells(1, 1)
+            Dim second = cells(1, 2)
+            Assert.That(first, [Is].SameAs(second))
+            first.Value = "selected last"
+            Assert.That(sheet.Cells(1, 1).Value, [Is].Null)
+            Assert.That(sheet.Cells(1, 2).Value, [Is].EqualTo("selected last"))
+        End Using
+    End Sub
+
+    ' Growing beyond 32 columns used to replace the monitor while a write held it.
+    <Test>
+    Public Sub CellStoreMonitorSurvivesColumnIndexGrowth()
+        Using package As New ExcelPackage()
+            Dim sheet = package.Workbook.Worksheets.Add("Sheet")
+            Dim store = GetType(ExcelWorksheet).GetField("_values", BindingFlags.Instance Or BindingFlags.NonPublic).GetValue(sheet)
+            Dim rootField = store.GetType().GetField("SyncRoot", BindingFlags.Instance Or BindingFlags.NonPublic)
+            Dim columnsField = store.GetType().GetField("_columnIndex", BindingFlags.Instance Or BindingFlags.NonPublic)
+            Dim monitorBeforeGrowth = If(rootField Is Nothing, columnsField.GetValue(store), rootField.GetValue(store))
+            For col = 1 To 70
+                sheet.Cells(1, col).Value = col
+            Next
+            Dim monitorAfterGrowth = If(rootField Is Nothing, columnsField.GetValue(store), rootField.GetValue(store))
+            Assert.That(monitorAfterGrowth, [Is].SameAs(monitorBeforeGrowth))
+            Assert.That(sheet.Cells(1, 70).Value, [Is].EqualTo(70))
+        End Using
+    End Sub
+
+    ' Pause inside the parser before starting a competing public operation.
+    <TestCase("workbook"), TestCase("worksheet"), TestCase("range"), TestCase("formula"), TestCase("parse"), TestCase("save"), TestCase("dispose")>
+    Public Sub CompetingOperationsWaitForActiveCalculation(operation As String)
+        Using package As New ExcelPackage(), entered As New ManualResetEventSlim(), release As New ManualResetEventSlim(), attempting As New ManualResetEventSlim(), finished As New ManualResetEventSlim()
+            Dim sheet = package.Workbook.Worksheets.Add("First")
+            Dim other = package.Workbook.Worksheets.Add("Second")
+            package.Workbook.FormulaParserManager.LoadFunctionModule(New CallbackModule(
+                Sub()
+                    entered.Set()
+                    If Not release.Wait(TimeSpan.FromSeconds(10)) Then Throw New TimeoutException()
+                End Sub))
+            sheet.Cells(1, 1).Formula = "BLOCKING()"
+            other.Cells(1, 1).Formula = "2+3"
+            Dim manager = package.Workbook.FormulaParserManager
+            Dim first = Task.Factory.StartNew(Sub() CalculationExtension.Calculate(sheet), CancellationToken.None, TaskCreationOptions.LongRunning, TaskScheduler.Default)
+            Dim second As Task = Nothing
+            Dim blocked As Boolean = False
+            Try
+                Assert.That(entered.Wait(TimeSpan.FromSeconds(10)), [Is].True)
+                second = Task.Factory.StartNew(
+                    Sub()
+                        attempting.Set()
+                        Try
+                            Select Case operation
+                                Case "workbook"
+                                    CalculationExtension.Calculate(package.Workbook)
+                                Case "worksheet"
+                                    CalculationExtension.Calculate(other)
+                                Case "range"
+                                    CalculationExtension.Calculate(other.Cells(1, 1))
+                                Case "formula"
+                                    Assert.That(CalculationExtension.Calculate(other, "2+3"), [Is].EqualTo(5.0))
+                                Case "parse"
+                                    Assert.That(manager.Parse("2+3"), [Is].EqualTo(5.0))
+                                Case "save"
+                                    Dim bytes = package.GetAsByteArray()
+                                    Using reopened As New ExcelPackage(New MemoryStream(bytes))
+                                        Assert.That(reopened.Workbook.Worksheets(0).Cells(1, 1).Value, [Is].EqualTo(7.0))
+                                    End Using
+                                Case "dispose"
+                                    package.Dispose()
+                            End Select
+                        Finally
+                            finished.Set()
+                        End Try
+                    End Sub, CancellationToken.None, TaskCreationOptions.LongRunning, TaskScheduler.Default)
+                Assert.That(attempting.Wait(TimeSpan.FromSeconds(10)), [Is].True)
+                blocked = Not finished.Wait(TimeSpan.FromMilliseconds(250))
+            Finally
+                release.Set()
+                Dim tasks = If(second Is Nothing, New Task() {first}, New Task() {first, second})
+                Assert.That(Task.WaitAll(tasks, TimeSpan.FromSeconds(10)), [Is].True)
+            End Try
+            Assert.That(blocked, [Is].True, "The competing " & operation & " operation ran during calculation.")
+            If operation <> "dispose" Then
+                Assert.That(sheet.Cells(1, 1).Value, [Is].EqualTo(7.0))
+            End If
+        End Using
+    End Sub
+
+    ' Independent parsers must be able to enter custom functions at the same time.
+    <Test>
+    Public Sub IndependentWorkbookCalculationsCanOverlap()
+        Using firstPackage As New ExcelPackage(), secondPackage As New ExcelPackage(), barrier As New Barrier(2)
+            Dim first = firstPackage.Workbook.Worksheets.Add("First")
+            Dim second = secondPackage.Workbook.Worksheets.Add("Second")
+            Dim callback As Action =
+                Sub()
+                    If Not barrier.SignalAndWait(TimeSpan.FromSeconds(10)) Then Throw New TimeoutException("Independent calculations were serialized.")
+                End Sub
+            firstPackage.Workbook.FormulaParserManager.LoadFunctionModule(New CallbackModule(callback))
+            secondPackage.Workbook.FormulaParserManager.LoadFunctionModule(New CallbackModule(callback))
+            first.Cells(1, 1).Formula = "BLOCKING()"
+            second.Cells(1, 1).Formula = "BLOCKING()"
+            Dim tasks = New Task() {
+                Task.Factory.StartNew(Sub() CalculationExtension.Calculate(first), CancellationToken.None, TaskCreationOptions.LongRunning, TaskScheduler.Default),
+                Task.Factory.StartNew(Sub() CalculationExtension.Calculate(second), CancellationToken.None, TaskCreationOptions.LongRunning, TaskScheduler.Default)
+            }
+            Assert.That(Task.WaitAll(tasks, TimeSpan.FromSeconds(15)), [Is].True)
+            Assert.That(first.Cells(1, 1).Value, [Is].EqualTo(7.0))
+            Assert.That(second.Cells(1, 1).Value, [Is].EqualTo(7.0))
+        End Using
+    End Sub
+
+    ' Enumeration captures collection membership before a competing style update.
+    <Test>
+    Public Sub StyleEnumerationKeepsItsSnapshot()
+        Using package As New ExcelPackage()
+            Dim sheet = package.Workbook.Worksheets.Add("Sheet")
+            sheet.Cells(1, 1).Style.Numberformat.Format = "0.00""snapshot-a"""
+            Dim formats = package.Workbook.Styles.NumberFormats
+            Dim count = formats.Count
+            Using iterator = formats.GetEnumerator()
+                Dim writer = Task.Run(Sub() sheet.Cells(1, 2).Style.Numberformat.Format = "0.00""snapshot-b""")
+                Assert.That(writer.Wait(TimeSpan.FromSeconds(10)), [Is].True)
+                Dim enumerated = 0
+                While iterator.MoveNext()
+                    enumerated += 1
+                End While
+                Assert.That(enumerated, [Is].EqualTo(count))
+                Assert.That(formats.Count, [Is].EqualTo(count + 1))
+            End Using
+        End Using
+    End Sub
+
+    Private NotInheritable Class CallbackModule
+        Inherits FunctionsModule
+
+        Public Sub New(callback As Action)
+            Functions.Add("blocking", New CallbackFunction(callback))
+        End Sub
+    End Class
+
+    Private NotInheritable Class CallbackFunction
+        Inherits ExcelFunction
+
+        Private ReadOnly _callback As Action
+
+        Public Sub New(callback As Action)
+            _callback = callback
+        End Sub
+
+        Public Overrides Function Execute(arguments As IEnumerable(Of FunctionArgument), context As ParsingContext) As CompileResult
+            _callback()
+            Return New CompileResult(7.0, DataType.Decimal)
+        End Function
+    End Class
+
+    Private Shared Function FormatFor(sheet As Integer, row As Integer) As String
+        Return "0.00""s" & sheet & "r" & row & """"
+    End Function
+
+    Private Shared Sub RunWorkers(parallel As Boolean, count As Integer, action As Action(Of Integer))
+        If parallel Then
+            System.Threading.Tasks.Parallel.For(0, count, New ParallelOptions With {.MaxDegreeOfParallelism = 8}, action)
+        Else
+            For i = 0 To count - 1
+                action(i)
+            Next
+        End If
+    End Sub
+End Class
diff --git a/README.md b/README.md
index 3960c89..66e6434 100644
--- a/README.md
+++ b/README.md
@@ -57,6 +57,27 @@ PLEASE NOTE:
     * Allow COM interop at clients without using Microsoft.Office.Excel.Interop assemblies for light-weight deployments of applications to customers/clients, regardless of the installed version of Microsoft Office
     * Limitation: supports only a very tiny, but often-used feature set, e.g. print, export to PDF, run VBA code
 
+## Special CompuMaster Edition
+
+`CompuMaster.EPPlus4` is this repository's maintained fork of EPPlus 4.5.3.3. Its calculation-cache changes force Microsoft Excel to recalculate dependent formulas when reopening a workbook. See the [package README](Epplus-FixCalcsEdition/EPPlus/README.md) for the complete CompuMaster feature list and package notes.
+
+## Known Issues
+
+**Warning: Epplus4 (including its fork CompuMaster.EPPlus4) is not thread-safe. Multithreaded use of the library is not supported. Concurrent access can cause exceptions and incorrect or lost cell values without an exception.**
+
+Use each `ExcelPackage` in one thread throughout its lifetime, including access to its workbook, worksheets and ranges, formula calculation, saving, and disposal. This follows [Jan Källman's recommendation to avoid multiple threads on a single workbook](https://github.com/EPPlusSoftware/EPPlus/issues/894#issuecomment-1578082491). Different worksheets in one package are not independent concurrency boundaries. If a package must be shared, serialize all access with one stable external lock per package/workbook, including complete select/read/modify/write sequences; locks per worksheet or cell are insufficient.
+
+The CompuMaster edition fixes the reproduced internal races in cell-store growth and access, workbook-wide style updates, and concurrent formula calculations. Calculation, parser-manager operations, saving, and disposal now share a package-local monitor. Regression tests cover these cases and independently owned packages. These internal fixes do not establish general thread safety or replace caller coordination. See [issue #26](https://github.com/CompuMasterGmbH/CompuMaster.Excel/issues/26) for the original failures, implementation scope, and remaining investigation items.
+
+The following known limitations remain:
+
+1. **A shared, cached `ExcelRange` can target the wrong cell.** Its indexers change the range object's address and return the same object. Another thread can change that address between selection and use. This behavior is retained for compatibility; do not share cached range objects between threads.
+2. **Compound operations and arbitrary concurrent workbook access remain unsupported.** Internal cell-store locks do not make selection followed by reading/writing atomic, provide snapshot cell enumeration, or coordinate every cell/structure change with calculation, saving, loading, or disposal. Concurrent edits can still invalidate a traversal or produce inconsistent output. Coordinate complete operations externally, including reads; direct XML/collection mutation and copying styles between workbooks also require caller coordination.
+3. **Callbacks must not reenter workbook services during calculation.** Custom functions and loggers run while the package monitor is held. Recursively calculating/parsing, saving, or disposing the same package can interfere with active parser state or workbook lifetime. Waiting for another thread that needs the same package can deadlock. The monitor permits internal nested calls but does not make such callback behavior supported.
+4. **Some stream copies are globally serialized.** The inherited `CopyStream` lock can limit throughput even for independent packages. It does not make sharing a stream safe; each package must own its stream exclusively.
+
+Use separately owned packages and streams for parallel jobs, with one thread accessing each package at a time. The independent-package regression tests cover selected operations, not every feature. Other lazy initialization paths, the shared `RAND()` seed, and publicly mutable operator/validation objects still require investigation; these code observations are not all demonstrated runtime defects.
+
 ## Licensing
 
   * Please see license type for every sub project within its directory