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F# and COM Controls
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I've been playing around with embedding a webkit browser in F# and ran into some weird problems.

I found an open-source project that takes the webkit build and wraps it in C#. I downloaded this project and complied it.

If I create a new project, I can put the C# control inside there and it works great. But if I create a F# project, it's like F# loses track of the interface signatures. It gives me an error in the C# wrapper code that says it can't find the method in the underlying webkit dll.

So C# can take a C# wrapper around a dll and use the control just fine. But F#, using the same C# wrapper, causes the C# wrapper to fail, complaining about not being able to cast a foo as a bar, or something like that.

Anybody had a similar problem? Isn't F# implementing the COM interfaces in a GUI app the same as C#? Wonder why there would be a difference?

I checked the compilation settings and it all looks good -- no difference between the C# test project and the F# test project. I even tried making a new C# control that wrapped the first wrapper, but it still fails when called from F#.

BTW, is there any way to wrap a regular DLL from F#? If I could do that, I could just reproduce the original C# wrapper code in F#. Any help would be greatly appreciated!

.

To answer your last question, interop via F# is quite similar to interop via other .NET languages. If you're importing COM interfaces, you can use exactly the same attributes you would use in C#.

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[<ComImport; Guid(...); InterfaceType(ComInterfaceType.InterfaceIsIUnknown)>]
type MyComInterface =
  abstract MyMethod : string -> unit
  ...

If you want to do PInvoke, you use "external functions", which unfortunately don't appear to be well documented anywhere. However, the basic idea is that you define external functions within a module using syntax like C/C++ (see [link:msdn.microsoft.com] for slightly more info, along with an example).

By on 9/13/2010 2:30 PM ()

Thanks! While I knew about decorating with COM types, I had forgotten about PInvoke. Even so, looks like a hard row to hoe. Does anybody know if there are any good books/resources on low-level F# integration like this? Things like writing a shell extension, or using a third-party dll. I used to do a bit of this in C++, but the data structure translation alone sounds painful, not to mention all the GUID management.

Thanks again for the quick answer.

By on 9/13/2010 4:46 PM ()

UPDATE: I got my problem fixed.

The "real" problem was that F# is by default MTA threaded and the C# control was STA. I think this is because of the way the webkit.dll is coded but I don't have time to dive down further. It could just be the way the C# wrapper was done.

The error -- can't load library or can't cast x to y -- looked like a COM error, since it was the COM libraries that were being called by the wrapper.

Adding a STAThread attribute to the main loop fixed the problem. Although since I'm writing a highly-threaded app, it probably caused all sorts of other problems, but for now the error is gone and the control is working correctly

The issue here is somewhat related to this thread: [link:cs.hubfs.net]

By on 9/14/2010 8:22 AM ()

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