Encapsulating C Callback function within a C# function

Question

I need to make a DLL written in C accessible to C# programs. I wrote a wrapper DLL in C# that wraps every C interface function. I came out with a code which is functional but eventually crashes with the following message:

Managed Debugging Assistant 'CallbackOnCollectedDelegate' has detected a problem in 'D:\WrapperTest\bin\x64\Debug\WrapperTest.exe'.

Additional information: A callback was made on a garbage collected delegate of type 'vJoyInterfaceWrap!vJoyInterfaceWrap.vJoy+WrapFfbCbFunc::Invoke'.

The part that is responsible to the crash is related to a callback function that the C# code registers with the DLL. The crash occurs when the DLL calls (or returns) from the registered callback function.

C DLL Interface:

// Definition of callback function prototype
// Parameter 1: Pointer to a data structure to be used inside the CB function
// Parameter 2: Pointer to user-defined data passed during registration
typedef void (CALLBACK *FfbGenCB)(PVOID, PVOID);

// Registration of CB function.
// Parameter cb is a pointer to the callback function.
// Parameter data is a pointer to user-defined data
VOID        __cdecl FfbRegisterGenCB(FfbGenCB cb, PVOID data);

C# Interface:

I'd like the user-defined data to be an object (rather than an IntPtr). To do that I encapsulate the user's callback function with a predefined callback function that converts IntPtr to object. Earlier during registration, I register the predefined callback function and convert the user-defined data (a C# object) to IntPtr.

// The user-defined CB function is of type FfbCbFunc
//  FfbCbFunc is defined:
public delegate void FfbCbFunc(IntPtr data,  object userData);

// The registration of the predefined callback function _WrapFfbCbFunc is done by calling
// function FfbRegisterGenCB:
       public void FfbRegisterGenCB(FfbCbFunc cb, object data)
        {
            // Convert object to pointer
            GCHandle handle1 = GCHandle.Alloc(data);

            // Apply the user-defined CB function
            _g_FfbCbFunc = new FfbCbFunc(cb);
            WrapFfbCbFunc wf = new WrapFfbCbFunc(_WrapFfbCbFunc);

            _FfbRegisterGenCB(wf, (IntPtr)handle1);
        }

Here are additional definitions and declaration:

// Placeholder for user defined callback function
private static FfbCbFunc _g_FfbCbFunc;

// predefined callback function that encapsulates the user-defined callback function
public delegate void WrapFfbCbFunc(IntPtr data, IntPtr userData);
public void _WrapFfbCbFunc(IntPtr data, IntPtr userData)
        {
            // Convert userData from pointer to object
            GCHandle handle2 = (GCHandle)userData;
            object obj = handle2.Target as object;

            // Call user-defined CB function
            _g_FfbCbFunc(data, obj);
        }

Wouldn't it be better to have the wrapper written in C and linked together with the rest of the C code into a single DLL? That way you can present the interface just as you want it to the C# application. — Lundin
– Lundin, Commented Mar 2, 2015 at 9:26
A wrapper written in C will not expose native C# interface such as 'object'. If I am mistaken please supply example or reference. — Shaul
– Shaul, Commented Mar 2, 2015 at 9:36

Shaul · Accepted Answer · 2015-03-03 09:39:34Z

1

The wrapper callback function and its reference had to be static:

// Make wf global and static
private static WrapFfbCbFunc wf;

// Make the wrapping callback function static
public static void  _WrapFfbCbFunc(IntPtr data, IntPtr userData)
{

    object obj = null;

    if (userData != IntPtr.Zero)
    {
        // Convert userData from pointer to object
        GCHandle handle2 = (GCHandle)userData;
        obj = handle2.Target as object;
    }

    // Call user-defined CB function
    _g_FfbCbFunc(data, obj);
}

P.S.

I don't think this was such a bad question to be rewarded by a -1 vote. Personally, I just ignore questions that are not interesting.

answered Mar 3, 2015 at 9:39

Shaul

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