Question

I have written an unmanaged c++ dll that works good (IPinvoke))) There is one resource-consuming function in it – there is a loop with complex time consuming logic. What is the best way to calculate percentage of this loop progress and sending break to this loop – using callbacks or may be passing parameters?
If callbacks is the most good variant – could anyone provide sample?

in dll:

extern "C" _declspec(dllexport) uint8* resourceConsumingFunction(uint8* dataBufer)
{
  //there is a loop with many math here
  return dataBuffer;
}

in c#

[DllImport("MyLib.DLL", CallingConvention = CallingConvention.Cdecl)]
public unsafe static extern byte* resourceConsumingFunction(byte* dataBuf);
//.....
byte* bufbuf = resourceConsumingFunction(data);//there I need to break this function and to get //percentage

Answer 1

Sure, a callback can work. You’ll need a function pointer in the C++ code, something like this:

typedef void (__stdcall * pfnCallback)(int progress, int* cancel);

extern "C" _declspec(dllexport) 
uint8* resourceConsumingFunction(uint8* dataBuffer, pfnCallback callback)
{
    for (int progress = 0;;) {
        int cancel = 0;
        callback(progress, &cancel);
        if (cancel) return null;
        // More code
        //...
    }
    return dataBuffer;
}

And the equivalent C# code would be:

private void delegate pfnCallback(int progress, out bool cancel);

private void makeCall() {
    var callback = new pfnCallback(showProgress);
    var bufptr = resourceConsumingFunction(somebuf, callback);
    GC.KeepAlive(callback);
    // etc...
}

private void showProgress(int progress, out bool cancel) {
   // etc...
}

Using __stdcall for the callback helps keep the delegate declaration simple. The GC.KeepAlive() call is necessary to stop the garbage collector from collecting the delegate object too soon.