Interop problem - unmanaged code fails only while in background thread

Hi all,

I have a 3rd party DLL (non-COM) that I am accessing from managed code via PInvoke. The third party function, reads a file, crunches data, and generates a results file. I have wrapped the dll's functions with static function calls from a managed .NET singleton dll. This routine works fine as long as I am running in the foreground thread. As soon as I call this function from a background thread, however it crashes with the following exception:

System.NullReferenceException: Object reference not set to an instance of an object.

I have checked the marshalling with CLRSpy and everything looks to be in order. Before I post any code, I was wondering what general issues could be causing this problem to show up in background worker threads only. I get a results file created but it has 0 bytes. Any suggestions for further debugging efforts would be appreciated as well.

Regards,
Zam

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Interop problem - unmanaged code fails only while in background thread

Jeff Miller
Hi all,

I have a 3rd party DLL (non-COM) that I am accessing from managed code via PInvoke. The third party function, reads a file, crunches data, and generates a results file. I have wrapped the dll's functions with static function calls from a managed .NET singleton dll. This routine works fine as long as I am running in the foreground thread. As soon as I call this function from a background thread, however it crashes with the following exception:

System.NullReferenceException: Object reference not set to an instance of an object.

This routine does not do any UI and is set up by feeding it an input filename via a previous call. I have checked the marshalling with CLRSpy and everything looks to be in order. Before I post any code, I was wondering what general issues could be causing this problem to show up in background worker threads only. I get a results file created but it has 0 bytes.

Any suggestions for further debugging efforts would be appreciated as well.

Regards,
Zam
nicbkw
Great to hear you found CLRSpy useful!

I don't know of any general issues with background thread versus foreground threads that result in NullReferenceException.

One possibility [probable, even] is that the 3rd party dll is crashing(AV) when called from the background thread.
You can find a bit more info on the Exception mapping (e.g. AV->NullReferenceException) on Christopher Brumme's excellent CLR architecture blog - specifically Exceptions (Chris is one of the CLR architects).

Have you tried running the code (especially the 3rd party DLL) under a native debugger (such as WinDBG) - breaking on first chance exceptions

Thanks!
Stephen
http://blogs.msdn.com/stfisher
LapDragon
It can be name conflict or it can be that you or 3d party dll creates some kind of com object that needs to be accessed only in the thread that it was created.
You can name other cases but check first abovementioned.
Interop problem - unmanaged code fails only while in background thread

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