Question

I would like to use this C function in C#:

typedef void (*WRITE_CALLBACK)(int hMountEnv, unsigned __int64 NumBytesWritten, 
               void* pContext);

How would I go about defining it so I can call it?

Do I need to do anything else to make this work?

Answer 1

Take a look at http://msdn.microsoft.com/en-us/library/ektebyzx(v=vs.80).aspx for how to marshal function pointers. There is an example on the bottom of that page.

I’ll do the C# for you, and since I don’t know which direction the pointer is going, I’ll show both. There are some gotchas which could be problematic for you involving calling conventions. Marshal.GetDelegateForFunctionPointer and Marshal.GetFunctionPointerForDelegate assume the function pointer will be StdCall so if you don’t have access to the unmanaged library to make sure the function pointer is standard call (I think C defaults to cdecl unfortunately), you’d have to create an unmanaged shim library to change the calling convention, unless there is some other way I don’t know about.

This would be the header of a C DLL I named “UnmanagedLib.dll”.

typedef void ( __stdcall *WRITE_CALLBACK)(int hMountEnv, unsigned __int64 NumBytesWritten, void* pContext);

extern "C" {
__declspec(dllexport) WRITE_CALLBACK __stdcall FunctionProducingFunctionPointer(void);
__declspec(dllexport) void __stdcall FunctionConsumingFunctionPointer(WRITE_CALLBACK callback);
}

This would be the CPP file of the DLL.

#include "UnmanagedLib.h"

void __stdcall SampleFunction(int hMountEnv, unsigned __int64 NumBytesWritten, void* pContext)
{
}

WRITE_CALLBACK __stdcall FunctionProducingFunctionPointer(void)
{
    return &SampleFunction;
}

void __stdcall FunctionConsumingFunctionPointer(WRITE_CALLBACK callback)
{
    // sample call
    (*callback)(0,0,NULL);
}

And lastly, this is a C# program to make use of the DLL.

class Program
{
    public delegate void WRITE_CALLBACK(int hMountEnv, ulong NumBytesWritten, IntPtr pContext);

    [DllImport("UnmanagedLib.dll")]
    public static extern IntPtr FunctionProducingFunctionPointer();

    [DllImport("UnmanagedLib.dll")]
    public static extern void FunctionConsumingFunctionPointer(IntPtr functionPointer);

    public static void SampleFunction(int hMountEnv, ulong NumBytesWritten, IntPtr pContext)
    {
    }

    static void Main(string[] args)
    {
        var functionDelegateToManagedSampleFunction = new WRITE_CALLBACK(SampleFunction);
        var functionDelegateToUnmanagedSampleFunction = Marshal.GetDelegateForFunctionPointer(FunctionProducingFunctionPointer(), typeof(WRITE_CALLBACK));

        // call the unmanaged sample function via its pointer
        functionDelegateToUnmanagedSampleFunction.DynamicInvoke(new object[] {0,0ul,null});

        // pass the managed sample function to the unmanaged code
        FunctionConsumingFunctionPointer(Marshal.GetFunctionPointerForDelegate(functionDelegateToManagedSampleFunction));
    }
}