C#: problem loading C++ DLL

Question

In my code, I can load "MessageBoxA" from user32.dll and use it, but if I try to load and use a function from my DLL, I get a crash.

My C# code:

[DllImport("SimpleDLL.dll")]
static extern int mymean(int a, int b, int c);

[DllImport("user32.dll")]
static extern int MessageBoxA(int hWnd,
                              string msg,
                              string caption,
                              int type);

[...]

this works

MessageBoxA(0, "Hello, World!", "This is called from a C# app!",  0);

this crashes

int mean = mymean(12, 14, 16);

And my C++ DLL code: SimpleDLL.cpp:

extern "C" _declspec(dllexport) int mymean(int x, int y, int z)
{
    return (x + y + z) / 3;
}

SimpleDLL.def:
LIBRARY "SimpleDLL"
mymean

SimpleDLL.dll is copied to the same folder as the .exe I compile from C# code. Using dependency walker, I can see that all necessary DLLs to load SimpleDLL.dll are present.

Answer 1

C# uses "stdcall" calling convention by default. You've specified "C". You need to either specify

[DllImport("SimpleDLL.dll",CallingConvention=CallingConvention.Cdecl)]

or change your c code to:

int _declspec(dllexport) stdcall mymean(int x, int y, int z)

Answer 2

Turns out my C# app was 64-bit (which is C# visual studio default) and my C++ DLL was 32-bit (which is C++ visual studio default).

Thanks for the tip to check the exception type, it was a badimageformatexception.

Sorry - total C# newbie!