Callback from C# to C++

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C++

C#

.NET

COM

I've developed a small C++ application that calls a managed .NET C# COM object using the guide lines in this article:

Calling Managed .NET C# COM Objects from Unmanaged C++ Code[^]

Now I would like to establish a callback function in my C++ application that is called from my .NET application. I want to create a function on the .NET side - RegisterCppCallback that is called from the C++ app. and takes some kind of function pointer as argument.

My idea is that is should look something like this on the .NET side:

public delegate void CppCallbackFn();
CppCallbackFn myCallbackFunc;
public void RegisterCppCallback(IntPtr pfn)
        {
            myCallbackFunc = (CppCallbackFn) (Marshal.GetDelegateForFunctionPointer(pfn, typeof(CppCallbackFn))) as CppCallbackFn;
            myCallbackFunc(); // test it
        }

and on the CPP side:

typedef int (__stdcall* CallbackFunc)();

CallbackFunc * mInstance;

int CallbackHandler()
    {
        // some code

        return 0;
    }

// call the RegisterCppCallback function in .NET
mInstance = new CallbackFunc((CallbackFunc) CallbackHandler);
pDotNetCOMPtr->RegisterCppCallback((long) mInstance);

My problem is that the callback function in CPP isn't called.
What am I doing wrong ? Any help is highly appreciated