How to call a delegate in C# (Windows forms) from an unmanaged C++ dll

Question

I've seen a few examples of how to do this from C++ an unmanaged dll, to a C++ managed CLR dll. I currently have a C++ dll (NetPcapWrap), that references a C dll (npcap.dll). Since the npcap.dll is written in C I cannot compile with CLR so NetPcapWrap is unmanaged. My .Net Forms application references the NetPcapWrap.dll using pInvoke to gets information from the npcap.dll.

Within the npcap.dll is a looping function that writes out data to std::out what I need to do instead call a delegate of the .Net windows Forms app.

I've seen one potential solution https://www.codeproject.com/tips/695387/calling-csharp-net-methods-from-unmanaged-c-cplusp but this would require yet another dll that is compiled with CLR enabled.

Is there a way to do "reverse" pInvoke from an unmanaged C++ dll to a .Net Winforms application?

Sorry, I don't have any code to post because I can't figure out how to write the potion. I can share how I pInvoke the C++ dll but that is not my question.

Thanks

Ahmed AEK · Accepted Answer

you need the C# code to send a delegate to a C function that expects a function pointer, then store it in some global.

and C# needs to also store this delegate in a static variable to prevent the GC from cleaning it.

on C/C++ side.

// C++ file

using myfunctype = void (*)(void);
// replace with typedef in C
// typedef void (*myfunctype)(void);

extern "C" 
__declspec( dllexport )
myfunctype func1;
myfunctype func1 = 0;

extern "C"
__declspec( dllexport )
int SetFunction(myfunctype func_ptr)
{
    func1 = func_ptr;
    return 5;
}

extern "C"
__declspec( dllexport )
int CallFunction()
{
    func1();
    return 5;
}

then on C# side

// csharp file
using System.Runtime.InteropServices;

namespace myspace;

public delegate void CFunc(); 

public class NativeLib
{
    public static void CsharpFunc()
    {
        Console.WriteLine("hello world from Csharp!");
    }
    public static CFunc func_static = CsharpFunc;

    [DllImport("Clib", CallingConvention = CallingConvention.Cdecl)]
    public static extern int SetFunction(CFunc func);

    [DllImport("Clib", CallingConvention = CallingConvention.Cdecl)]
    public static extern int CallFunction();

}

then your main app needs to call them.

// csharp calling application
using myspace;

class Program
{
    public static int Main(String[] args)
    {
        NativeLib.SetFunction(NativeLib.func_static);

        NativeLib.CallFunction();
        return 0;
    }
}

as a bonus, this also works on linux/macos where managed C++ dlls don't exist. (tested with dotnet 6 and 8)

while this small example works, you may need to pin any involved C# object from moving using GCHandle.

note that you cannot allow C++ exceptions to leak into C#, you also cannot allow C# exceptions to leak into C++.

How to call a delegate in C# (Windows forms) from an unmanaged C++ dll

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