extern "C" __declspec(dllexport)

#include "stdafx.h"
#include "stdio.h"
extern "C" __declspec(dllexport) void PrintMsg(const char* msg)
{
    printf("我在C++里编写:PrintMsg.\n下面的字符串是C#传入的:%s\n", msg);
    return;
}

        static void Main(string[] args)
        {
            PrintMsg(@"C#夜未眠");

            Console.ReadKey();
        }

        #region 自定义DLL互操作:

        [DllImport(@"E:\code\ConsoleApplication3\Debug\ConsoleApplication3.dll",
            CallingConvention = CallingConvention.StdCall)]
        static extern void PrintMsg(string msg);

        #endregion

        //直接对标准微软C运行库中的函数进行平台调用
        [DllImport("user32.dll", EntryPoint = "MessageBox")]
        public static extern int MessageBox(int hwnd, string lpText, string lpCaption, int wType);

        [DllImport("msvcrt.dll",CallingConvention = CallingConvention.Cdecl)]
        static extern int puts(string msg);//puts函数将字符串传送到输入流中

        [DllImport("msvcrt.dll",CallingConvention = CallingConvention.Cdecl)]
        static extern int _flushall();//手动清除所有输入流

        public static void SayStdC()
        {
            puts("puts函数调用");
            _flushall();
        }

        public static void SayWinAPI()
        {
            MessageBox(0, "Hello C# Code!", "CSharp互操作", 0);
        }