FileAss

File associations and single-instance apps for Lazarus.

Two non-visual components for Free Pascal / Lazarus: TFileAssMan registers file associations with the operating system, and TFileAssInstance keeps one copy of an application running. Most setup is done in the Object Inspector, and the application code is the same on Windows and Linux.

📄 File associations

Put TFileAssMan on a form, set the application name and ID, and add extensions and document icons to its FileTypes collection. Register or remove them from your application's install action:

FileAssMan1.RegisterAll;
// When removing the associations:
FileAssMan1.UnregisterAll;

Registration only happens when you call it, and it keeps the user's chosen default applications. Configuration and registration

⭐ One application instance, without writing IPC code

Put TFileAssInstance on the main form, set its ApplicationID (or link it to TFileAssMan) and set Enabled to True. No event handler is needed to keep one instance running.

To open files in the running instance, select fimForwardParameters and handle OnParameters. Requests that arrive during startup are queued and delivered once the application is ready. Forwarding, readiness and events

Demo

Install the package, then open demo/FileAssDemo.lpi. Register the sample types, click a document icon to find its file, and open more files into the running instance. The demo's .lfm shows the component settings.

🛠️ Other uses

Installer exports, application-menu launchers and icons and console applications use the same core. The developer guide covers setup, properties and platform details; the Object Inspector shows a hint for every property.

✅ Tested on

	Linux	Windows	macOS
Tested on	Mint, Cinnamon desktop	Windows 10, and Wine	Sequoia 15, Intel, in a virtual machine
Built with	Lazarus trunk, FPC 3.3.1	the same, cross-compiled	Lazarus 4.8, FPC 3.2.4
Core unit tests	✅	✅	✅
Register and remove file types	✅	✅	not how a Mac does it - see the next row
Installer export	✅ .desktop, MIME, icons	✅ .reg	✅ Info.plist, accepted by Apple's plutil
Document icons show in the file manager	✅	✅	not checked yet
One instance: unit tests	✅	✅	✅
Fifty copies started at once pick one primary and lose nothing	✅	✅	✅
Launches queued while the first copy is still starting	✅	✅	✅
A killed primary releases the instance, and its queue is kept	✅	✅	✅
A document opened by the system is passed to the running copy	✅	✅	✅
The demo, clicked through by hand	✅	✅	not yet

Still wanted: the demo clicked through in Finder on a real Mac (and how the document icons look there), Apple Silicon, Windows 11, and file managers other than Cinnamon's. Test reports are welcome in the issues.

License

0BSD. Copyright (c) 2026 Tony Stone.