[DllImport("Engine.dll")]
public static extern void DoWork(); 

[UnmanagedFunctionPointer(CallingConvention.StdCall)]
public delegate void ProgressCallback(int value);

typedef void (__stdcall * ProgressCallback)(int);

DLL void DoWork(ProgressCallback progressCallback)

#define DLL __declspec(dllexport)

ProgressCallback callback =(value) =>{Console.WriteLine("Progress = {0}", value);};

DoWork(callback);

#include "Windows.h"#ifdef __cplusplus
extern "C"
{
#endif#define DLL __declspec(dllexport)typedef void (__stdcall * ProgressCallback)(int);typedef char* (__stdcall * GetFilePathCallback)(char* filter);DLL void DoWork(ProgressCallback progressCallback);DLL void ProcessFile(GetFilePathCallback getPath);#ifdef __cplusplus
}
#endif

#include "Main.h"
#include <stdio.h>DLL void DoWork(ProgressCallback progressCallback)
{int counter = 0;for(; counter<=100; counter++){// do the work...if (progressCallback){// send progress update
            progressCallback(counter);}}
}DLL void ProcessFile(GetFilePathCallback getPath)
{if (getPath){// get file path...char* path = getPath("Text Files|*.txt");// open the file for readingFILE *file = fopen(path, "r");// read bufferchar line[1024];// print file info to the screenprintf("File path: %s\n", path ? path : "N/A");printf("File content:\n");while(fgets(line, 1024, file) != NULL){printf("%s", line);}// close the file
        fclose(file);}
}

using System;
using System.Runtime.InteropServices;
using System.Windows.Forms;class Program
{[UnmanagedFunctionPointer(CallingConvention.StdCall)]delegate void ProgressCallback(int value);[UnmanagedFunctionPointer(CallingConvention.StdCall)]delegate string GetFilePathCallback(string filter);[DllImport("Engine.dll")]public static extern void DoWork([MarshalAs(UnmanagedType.FunctionPtr)] ProgressCallback callbackPointer);[DllImport("Engine.dll")]public static extern void ProcessFile([MarshalAs(UnmanagedType.FunctionPtr)] GetFilePathCallback callbackPointer);[STAThread]static void Main(string[] args){// define a progress callback delegateProgressCallback callback =(value) =>{Console.WriteLine("Progress = {0}", value);};Console.WriteLine("Press any key to run DoWork....");Console.ReadKey(true);// call DoWork in C code
        DoWork(callback);Console.WriteLine();Console.WriteLine("Press any key to run ProcessFile....");Console.ReadKey(true);// define a get file path callback delegateGetFilePathCallback getPath =(filter) =>{string path = default(string);OpenFileDialog ofd =new OpenFileDialog(){Filter = filter};if (ofd.ShowDialog() == DialogResult.OK){path = ofd.FileName;}return path;};// call ProcessFile in C code
        ProcessFile(getPath);}
}