Question

I’m writing a .NET wrapper for an unmanaged DLL. The original DLL is C++ with a C shim that basically just replicates the API in C form, so that language bindings aren’t such a pain. I’ve already written a Python binding for it, so I know what I’m trying to do should work.

The DLL has a number of callbacks exported as global members, viz.:

__declspec(dllexport) extern int (*some_callback)(int32_t*, uint32_t);

I need to attach a managed function to this pointer, but I can’t figure out how to get at non-function resources in an unmanaged library. Unless I’m just blind, DllImport only imports functions.

Is there a C# way to do this, or do I need to write a little shim DLL in C that provides registration functions? I hate that approach, because it just feels inelegant, but if I have to, I have to.

Answer 1

You’re right, P/Invoke cannot handle data exports from a DLL. It is however technically possible by obtaining the export directly. This is very ugly and you’ll probably regret it some day, but this worked:

Sample C/C++ DLL:

#include "stdafx.h"

typedef int (__stdcall * pfnCallback)(int*, unsigned*);

extern "C" __declspec(dllexport) 
pfnCallback some_callback;

pfnCallback some_callback;

static int theCallback(int*, unsigned*) {
    return 42;
}

BOOL APIENTRY DllMain( HMODULE hModule, DWORD reason, LPVOID reserved) {
    if (reason == DLL_PROCESS_ATTACH) {
        some_callback = theCallback;
    }
    return TRUE;
}

Test C# code:

using System;
using System.ComponentModel;
using System.Runtime.InteropServices;

class Program {
    unsafe static void Main(string[] args) {
        IntPtr hMod = LoadLibrary("cpptemp10.dll");
        if (hMod == IntPtr.Zero) throw new Win32Exception();
        IntPtr export = GetProcAddress(hMod, "some_callback");
        if (export == IntPtr.Zero) throw new Win32Exception();
        IntPtr callback = Marshal.ReadIntPtr(export);
        some_callback dlg = (some_callback)Marshal.GetDelegateForFunctionPointer(callback, typeof(some_callback));
        int retval = dlg(null, null);
        Console.WriteLine(retval);
        Console.ReadLine();
    }

    [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
    unsafe delegate int some_callback(int* arg1, uint* arg2);

    [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern IntPtr LoadLibrary(string path);
    [DllImport("kernel32.dll", CharSet = CharSet.Ansi, SetLastError = true)]
    private static extern IntPtr GetProcAddress(IntPtr hMod, string name);

}