Getting IntPtr of a delegate and invoking

Question

To simulate a call from c++, I'm trying the following code

 private delegate void CppFuncDelegate(string message); private static void Main(string[] args) { Console.WriteLine("+++ BEGIN TEST +++"); Action<string> action = str => Console.WriteLine("Received:" + str); IntPtr delegatePtr = action.Method.MethodHandle.GetFunctionPointer(); CppFuncDelegate cppFuncDelegate = (CppFuncDelegate) Marshal.GetDelegateForFunctionPointer(delegatePtr, typeof(CppFuncDelegate)); cppFuncDelegate.Invoke("Hello"); }

but I get

PInvokeStackImbalance was detected. A call to PInvoke function 'Test!Test.Program+CppFuncDelegate::Invoke' has unbalanced the stack. This is likely because the managed PInvoke signature does not match the unmanaged target signature. Check that the calling convention and parameters of the PInvoke signature match the target unmanaged signature.

Can anyone tell me what I'm doing wrong?

NOTE: Please don't tell me to do action.Invoke(); that's not what this exercise is about. I want to get IntPtr handle to the delegate and use GetDelegateForFunctionPointer() and then invoke the returned delegate.

Thanks.

Have you checked if the calling convention of the PInvoke signature matches the target unmanaged signature? — dtb
– dtb, Commented May 23, 2012 at 14:57
@dtb CppFuncDelegate takes a string and returns void and I have the action delegate declared as Action<string>; so I think they match. Is there any other way to check the PInvoke signature? — Professor Chaos
– Professor Chaos, Commented May 23, 2012 at 15:02
I wan't talking about the parameters, I was talking about the calling convention. — dtb
– dtb, Commented May 23, 2012 at 15:02

Ňuf · Accepted Answer · 2012-05-23 16:34:21Z

You cannot use GetDelegateForFunctionPointer() to get delegate of managed function. This is from MSDN:

You cannot pass an invalid function pointer to GetDelegateForFunctionPointer. In addition, you can only use this method for pure unmanaged function pointers. You cannot use this method with function pointers obtained through C++ or from GetFunctionPointer. You cannot use this method to create a delegate from a function pointer to another managed delegate.

I'm not sure why you can't do this, but I guess it's because Microsoft's Common Language Runtime uses FastCall calling convention, but FastCall is not supported for PInvoke.

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