Demonstrates using pinvoke to pass a callback from a .net managed application to a C dll

header.h

#ifndef _HEADER_H
#define _HEADER_H

typedef void (__stdcall *PFN_CALLBACK1)(const wchar_t* str);

extern "C" __declspec(dllexport) int __stdcall dllFunc(PFN_CALLBACK1 callback1 );

#endif

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices;



namespace MyCSharpApp
{
    class Program
    {
        public delegate void MyCallback1(IntPtr str);

        // need to have the charset attribute set
        [DllImport("../../../Debug/MyCDll.dll", CharSet = CharSet.Auto)]
        public static extern void dllFunc(MyCallback1 callback1);

        [DllImport("msvcrt.dll")]
        public static extern int strlen(IntPtr ptr);

        public static string Utf8PtrToString(IntPtr ptr)
        {
            if (ptr == IntPtr.Zero)
                return null;

            int len = strlen(ptr);
            byte[] bytes = new byte[len];
            Marshal.Copy(ptr, bytes, 0, len);
            return System.Text.Encoding.UTF8.GetString(bytes);
        }

        public static void MyFunc1(IntPtr ucs2)
        {
            string text = Marshal.PtrToStringAuto(ucs2);
            Console.WriteLine("I am Function 1... text {0}", text);
        }

        public static void MyFunc2(IntPtr utf8)
        {
            // this no worky
            string text = Utf8PtrToString(utf8);
            Console.WriteLine("I am Function 2; 한국; I got value {0}", text);
        }

        static void Main(string[] args)
        {

            MyCallback1 cb1 = new MyCallback1(MyFunc1);

            dllFunc(cb1);

        }
    }
}

source.cpp

#include "stdio.h"
#include "header.h"

#include <string.h>

extern "C" __declspec(dllexport)
int __stdcall dllFunc(PFN_CALLBACK1 callback1) {
	// the write to console won't handle the unicode
	printf("you are inside the dll, こんにちは\n");
	// these will work
	callback1(L"hello 한국\n");
	callback1(L"and hi hi 中国\n");
	 return 0;
}