fsiSession.Run() is crashing the hosting app.

I am hosting FCS in another app. (Rhino 3D) When I create an FSI session and call Run() it crashes the App. I could not catch an exception. Are these comments here explaining why? The hosting app has already other active threads: https://github.com/dotnet/fsharp/blob/main/src/Compiler/Interactive/fsi.fs#L3759

member x.Run() =
        progress <- condition "FSHARP_INTERACTIVE_PROGRESS"

        // Explanation: When Run is called we do a bunch of processing. For fsi.exe
        // and fsiAnyCpu.exe there are no other active threads at this point, so we can assume this is the
        // unique compilation thread.  For other users of FsiEvaluationSession it is reasonable to assume that
        // the object is not accessed concurrently during startup preparation.
        //
        // We later switch to doing interaction-by-interaction processing on the "event loop" thread

        ...

Skipping this call to Run() and just using the session for evaluations seems to be mostly OK. But I have some other not so critical issues like line numbers missing in exceptions despite using --multiemit. see https://github.com/dotnet/fsharp/discussions/13293#discussioncomment-2949022

What am I missing in my fsi sessions if I don’t call the Run() method at the beginning ?

[KevinRansom]

@goswinr If your app is already pumping messages ‘Application.Run() starts that I believe’, then yes you probably don’t need fsi.Run().

Are you creating a desktop or a dotnet WPF App.

Just for background, we haven’t done any work to enable FSI and dotnet WPF or WinForms projects to work from fcs, although back in the day some work was done to ensure that fsi.exe does not block message pumping. So, from a particularly narrow view of the world the scenarios are currently unsupported. However, they look useful and interesting, I’m sure we can try to help you work through any blockers that may exist.

Please supply the complete project file for your sample, and I will take a look over the holidays if I get a chance.

[goswinr]

@KevinRansom

“we haven’t done any work to enable FSI and dotnet WPF or WinForms”

For now I assume I don’t need to call fsiSession.Run() . Do you have an idea why it crashes the WPF app? It actually works well without since a few years now. I started with the Tsunami Editor as seen here, now I am using AvalonEdit. I am not using fsi.exe just the FCS nuget. Soon I want to make my Scripting Editor public too.

“Are you creating a desktop or a dotnet WPF App.”

For hosting in Rhino3D I use net48, for standalone scripting I use net7.

“I’m sure we can try to help you work through any blockers that may exist.”

Thanks, that’s very kind of you. Only when watching the source of fsharp I see how much work you are actually putting into it. The biggest blocker was https://github.com/dotnet/fsharp/issues/11770 , but now I have a workaround.

• I am still missing line numbers on exceptions when hosted
• can’t get __SOURCE_DIRECTORY__ to work yet.
• and can’t cancel on net7

but I will try again and if unsuccessful create separate issues and samples for those.