C# delegate for C++ callback

Question

I think I have basically understood how to write c# delegates for callbacks, but this one is confusing me. The c++ definition is as follows:

typedef int (__stdcall* Callback)(
long lCode,
long lParamSize,
void* pParam 
);

and my c# approach would be:

 unsafe delegate int CallbackDelegate (int lCode, int lParamSize, IntPtr pParam);

Although this seems to be incorrect, because I get a PInvokeStackInbalance error, which means my definition of the delegate is wrong.

The rest of the parameters of the function are strings or ints, which means they cannot cause the error, and if I just pass a IntPtr.Zero instead of the delegate (which would mean I'm pointing to a non-existent callback function) I get an AccessViolation error, which makes sense aswell.

What am I doing wrong?

EDIT:

The full c++ function is:

int
__stdcall
_Initialize (
const char*         FileName,
Callback            cbFunction,
int                 Code,
const char*         Name,
unsigned int        Option,
unsigned int        Option2
);

My c# version is:

[DllImport("MyDll.dll", CallingConvention = CallingConvention.StdCall)]
public static extern int _Initialize (string FileName, CallbackDelegate cbFunction, int Code, string Name, uint Options, uint Options2);

The function is (for testing) just called inside of the main routine of a console application:

static void Main(string[] args)
{
    CallbackDelegate del = new CallbackDelegate(onCallback);

    Console.Write(_Initialize("SomeFile.dat", del, 1000, "", 0, 4));
    Console.Read();
}

where onCallback is this:

static int onCallback(int lCode, int lParamSize, IntPtr pParam)
{
    return 0;
}

I get the PInvokeStackInbalance error on the line where I call _Initialize, if I pass a IntPtr.Zero instead of the delegate, and change the definition of the function to IntPtr instead of CallbackDelegate then I get a AccessViolationException.

Answer 1

I added your code to my current project where I'm doing a lot of C#/C++ interop in VS2012. And while I hate to use the "it worked on my machine", it worked fine for me. The code as I ran it is listed out below just to illustrate that I didn't make and fundamental changes.

My suggestion is that you build a new native dll with a stub function for _Initialize such as below and see if it works when you can control both sides of the interface. If that works but the real dll doesn't, it comes down to either compiler settings on the native side or their is a bug on the native side and it is stomping on the stack.

extern "C" {

    typedef int (__stdcall* Callback)(long lCode,long lParamSize,void* pParam );

    TRADITIONALDLL_API int __stdcall _Initialize (const char* FileName,Callback cbFunction, int                 Code,
                                const char*         Name,unsigned int        Option,unsigned int        Option2)
    {
        cbFunction(0, 0, nullptr);
        return 0;
    }
}

On the C# side, I added the declarations to my interface class:

 public delegate int CallbackDelegate(int lCode, int lParamSize, IntPtr pParam);

 [DllImport("XXX.dll", CallingConvention = CallingConvention.StdCall)]
 public static extern int _Initialize(string FileName, CallbackDelegate cbFunction, int Code, string Name, uint Options, uint Options2);

And then in my main:

private int onCallback(int lCode, int lParamSize, IntPtr pParam)
{
       return 0;
}
XXXInterface.CallbackDelegate del = new XXXInterface.CallbackDelegate(onCallback);

Console.Write(XXXInterface._Initialize("SomeFile.dat", del, 1000, "", 0, 4));

Answer 2

A .NET long is 64bits. A C++ long may be just 32bits. Check with the C++ compiler which compiled that definition as to what size it's longs are.

Answer 3

[UnmanagedFunctionPointerAttribute(CallingConvention.StdCall)]
unsafe delegate int CallbackDelegate (int lCode, int lParamSize, IntPtr pParam);

If .NET assumes cdecl instead of stdcall, your stack will most assuredly be hosed.