C++ 封装 DLL 供 C# 调用详细介绍

目录

• 1、VLC代码封装
• 1.1 QT（C++）工程
• 1.2static 声明 m_instance 优化效率
• 1.3封装 DLL
• 1.4应用程序的导出函数
• 1.5 vlc 简单封装的具体实现
• 2、C# 调用
• 2.1C# 回调函数声明与定义
• 2.2C# 导出普通函数调用

1、VLC代码封装

1.1 QT（C++）工程

首先需要配置可使用 VLC 正常播放的 QT（C++）工程，获取VLC每一帧并渲染到Qwidget

Libvlcapi

public static class LIBVLCAPI
    {

        #region[libvlc.dll 导出函数]

        // 创建一个libvlc实例，它是引用计数的
        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        private static extern IntPtr libvlc_new(int argc, IntPtr argv);

        // 释放libvlc实例
        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        public static extern void libvlc_release(IntPtr libvlc_instance);

        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        public static extern String libvlc_get_version();

        // 从视频来源(例如Url)构建一个libvlc_meida
        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        private static extern IntPtr libvlc_media_new_location(IntPtr libvlc_instance, IntPtr path);

        // 从视频来源(例如Url)抓图
        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        private static extern int libvlc_video_take_snapshot(IntPtr libvlc_mediaplayer, int num, IntPtr filepath, int i_width, int i_height);

        // 从本地文件路径构建一个libvlc_media
        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        private static extern IntPtr libvlc_media_new_path(IntPtr libvlc_instance, IntPtr path);

        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        public static extern void libvlc_media_release(IntPtr libvlc_media_inst);

        // 创建libvlc_media_player(播放核心)
        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        public static extern IntPtr libvlc_media_player_new(IntPtr libvlc_instance);

        // 将视频(libvlc_media)绑定到播放器上
        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        public static extern void libvlc_media_player_set_media(IntPtr libvlc_media_player, IntPtr libvlc_media);

        // 设置图像输出的窗口
        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        public static extern void libvlc_media_player_set_hwnd(IntPtr libvlc_mediaplayer, Int32 drawable);

        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        public static extern void libvlc_media_player_play(IntPtr libvlc_mediaplayer);

        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        public static extern void libvlc_media_player_pause(IntPtr libvlc_mediaplayer);

        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        public static extern void libvlc_media_player_stop(IntPtr libvlc_mediaplayer);

        // 解析视频资源的媒体信息(如时长等)
        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        public static extern void libvlc_media_parse(IntPtr libvlc_media);

        // 返回视频的时长(必须先调用libvlc_media_parse之后，该函数才会生效)
        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        public static extern Int64 libvlc_media_get_duration(IntPtr libvlc_media);

        // 当前播放的时间
        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        public static extern Int64 libvlc_media_player_get_time(IntPtr libvlc_mediaplayer);

        // 设置播放位置(拖动)
        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        public static extern void libvlc_media_player_set_time(IntPtr libvlc_mediaplayer, Int64 time);

        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        public static extern void libvlc_media_player_release(IntPtr libvlc_mediaplayer);

        // 获取和设置音量
        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        public static extern int libvlc_audio_get_volume(IntPtr libvlc_media_player);

        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        public static extern void libvlc_audio_set_volume(IntPtr libvlc_media_player, int volume);

        // 设置全屏
        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        public static extern void libvlc_set_fullscreen(IntPtr libvlc_media_player, int isFullScreen);

        // 设置屏幕因子
        [DllImport("libvlc", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
        [SuppressUnmanagedCodeSecurity]
        public static extern void libvlc_video_set_scale(IntPtr libvlc_media_player, float f_factor);

        #endregion

        #region[VLC方法]

        public struct PointerToArrayOfPointerHelper
        {
            [MarshalAs(UnmanagedType.ByValArray, SizeConst = 11)]
            public IntPtr[] pointers;
        }

        public static IntPtr libvlc_new(string[] arguments)
        {
            PointerToArrayOfPointerHelper argv = new PointerToArrayOfPointerHelper();
            argv.pointers = new IntPtr[11];

            for (int i = 0; i < arguments.Length; i++)
            {
                argv.pointers[i] = Marshal.StringToHGlobalAnsi(arguments[i]);
            }

            IntPtr argvPtr = IntPtr.Zero;
            try
            {
                int size = Marshal.SizeOf(typeof(PointerToArrayOfPointerHelper));
                argvPtr = Marshal.AllocHGlobal(size);
                Marshal.StructureToPtr(argv, argvPtr, false);

                return libvlc_new(arguments.Length, argvPtr);
            }
            finally
            {
                for (int i = 0; i < arguments.Length + 1; i++)
                {
                    if (argv.pointers[i] != IntPtr.Zero)
                    {
                        Marshal.FreeHGlobal(argv.pointers[i]);
                    }
                }
                if (argvPtr != IntPtr.Zero)
                {
                    Marshal.FreeHGlobal(argvPtr);
                }
            }
        }

        public static IntPtr libvlc_media_new_path(IntPtr libvlc_instance, string path)
        {
            IntPtr pMrl = IntPtr.Zero;
            try
            {
                byte[] bytes = Encoding.UTF8.GetBytes(path);
                pMrl = Marshal.AllocHGlobal(bytes.Length + 1);
                Marshal.Copy(bytes, 0, pMrl, bytes.Length);
                Marshal.WriteByte(pMrl, bytes.Length, 0);
                return libvlc_media_new_path(libvlc_instance, pMrl);
            }
            finally
            {
                if (pMrl != IntPtr.Zero)
                {
                    Marshal.FreeHGlobal(pMrl);
                }
            }
        }

        public static int libvlc_video_take_snapshot(IntPtr libvlc_mediaplayer, int num, string path, int width, int height)
        {
            IntPtr pMrl = IntPtr.Zero;
            try
            {
                byte[] bytes = Encoding.UTF8.GetBytes(path);
                pMrl = Marshal.AllocHGlobal(bytes.Length + 1);
                Marshal.Copy(bytes, 0, pMrl, bytes.Length);
                Marshal.WriteByte(pMrl, bytes.Length, 0);
                return libvlc_video_take_snapshot(libvlc_mediaplayer, num, pMrl, width, height);
            }
            finally
            {
                if (pMrl != IntPtr.Zero)
                {
                    Marshal.FreeHGlobal(pMrl);
                }
            }
        }

        public static IntPtr libvlc_media_new_location(IntPtr libvlc_instance, string path)
        {
            IntPtr pMrl = IntPtr.Zero;
            try
            {
                byte[] bytes = Encoding.UTF8.GetBytes(path);
                pMrl = Marshal.AllocHGlobal(bytes.Length + 1);
                Marshal.Copy(bytes, 0, pMrl, bytes.Length);
                Marshal.WriteByte(pMrl, bytes.Length, 0);
                return libvlc_media_new_path(libvlc_instance, pMrl);
            }
            finally
            {
                if (pMrl != IntPtr.Zero)
                {
                    Marshal.FreeHGlobal(pMrl);
                }
            }
        }

        #endregion

    }

VLCPlayer

 public class VLCPlayer
    {
        private IntPtr libvlc_instance_;
        private IntPtr libvlc_media_player_;

        private double duration_;

        public VLCPlayer(string pluginPath, bool is_record)
        {
            if (is_record == true)
            {
                string plugin_arg = "--plugin-path=" + pluginPath;
                string filename = "c:\\" + DateTime.Now.ToString("yyyyMMddhhmmss") + ".mp4";
                string record_paramter = "--sout=#duplicate{dst=display,dst=std{accs=file,mux=ts,dst=" + filename;
                string[] arguments = { "-I", "--fullscreen", "dummy", "--ignore-config", "--no-video-title", "--width=100", "--height=100", plugin_arg, record_paramter };// "--sout=#duplicate{dst=display,dst=std{accs=file,mux=ts,dst=c:\\1.mp4" };
                libvlc_instance_ = LIBVLCAPI.libvlc_new(arguments);
                libvlc_media_player_ = LIBVLCAPI.libvlc_media_player_new(libvlc_instance_);
            }
            else
            {
                string plugin_arg = "--plugin-path=" + pluginPath;
                string[] arguments = { "-I", "--fullscreen", "dummy", "--ignore-config", "--no-video-title", plugin_arg };
                libvlc_instance_ = LIBVLCAPI.libvlc_new(arguments);
                libvlc_media_player_ = LIBVLCAPI.libvlc_media_player_new(libvlc_instance_);
                //float f1=0.1f;
                //LIBVLCAPI.libvlc_video_set_scale(libvlc_media_player_,f1);
            }
        }

        public void Vlc_release()
        {
            if (libvlc_instance_ != IntPtr.Zero)
                LIBVLCAPI.libvlc_release(libvlc_instance_);

            // if (libvlc_media_player_ != IntPtr.Zero)
            // LIBVLCAPI.libvlc_media_release(libvlc_media_player_);

        }
        public void SetRenderWindow(int wndHandle)
        {
            if (libvlc_instance_ != IntPtr.Zero && wndHandle != 0)
            {
                LIBVLCAPI.libvlc_media_player_set_hwnd(libvlc_media_player_, wndHandle);
            }
        }

        public void PlayFile(string filePath)
        {
            IntPtr libvlc_media = LIBVLCAPI.libvlc_media_new_path(libvlc_instance_, filePath);
            if (libvlc_media != IntPtr.Zero)
            {
                LIBVLCAPI.libvlc_media_parse(libvlc_media);
                duration_ = LIBVLCAPI.libvlc_media_get_duration(libvlc_media) / 1000.0;

                LIBVLCAPI.libvlc_media_player_set_media(libvlc_media_player_, libvlc_media);
                LIBVLCAPI.libvlc_media_release(libvlc_media);

                LIBVLCAPI.libvlc_media_player_play(libvlc_media_player_);
            }
        }

        public void PlayFile_rtsp(string filePath)//libvlc_media_new_location
        {
            IntPtr libvlc_media = LIBVLCAPI.libvlc_media_new_location(libvlc_instance_, filePath);
            if (libvlc_media != IntPtr.Zero)
            {
                // LIBVLCAPI.libvlc_media_parse(libvlc_media);
                // duration_ = LIBVLCAPI.libvlc_media_get_duration(libvlc_media) / 1000.0;

                LIBVLCAPI.libvlc_media_player_set_media(libvlc_media_player_, libvlc_media);
                LIBVLCAPI.libvlc_media_release(libvlc_media);

                LIBVLCAPI.libvlc_media_player_play(libvlc_media_player_);
            }
        }

        public void Pause()
        {
            if (libvlc_media_player_ != IntPtr.Zero)
            {
                LIBVLCAPI.libvlc_media_player_pause(libvlc_media_player_);
            }
        }

        public void take_snapshot()
        {
            if (libvlc_media_player_ != IntPtr.Zero)
            {
                string filepath = "c:\\";
                LIBVLCAPI.libvlc_video_take_snapshot(libvlc_media_player_, 0, filepath, 0, 0);
            }
        }

        public void full_screen()
        {
            if (libvlc_media_player_ != IntPtr.Zero)
            {

                LIBVLCAPI.libvlc_set_fullscreen(libvlc_media_player_, 0);
            }
        }

        public void Stop()
        {
            if (libvlc_media_player_ != IntPtr.Zero)
            {
                LIBVLCAPI.libvlc_media_player_stop(libvlc_media_player_);
            }
        }

        public double GetPlayTime()
        {
            return LIBVLCAPI.libvlc_media_player_get_time(libvlc_media_player_) / 1000.0;
        }

        public void SetPlayTime(double seekTime)
        {
            LIBVLCAPI.libvlc_media_player_set_time(libvlc_media_player_, (Int64)(seekTime * 1000));
        }

        public int GetVolume()
        {
            return LIBVLCAPI.libvlc_audio_get_volume(libvlc_media_player_);
        }

        public void SetVolume(int volume)
        {
            LIBVLCAPI.libvlc_audio_set_volume(libvlc_media_player_, volume);
        }

        public void SetFullScreen(bool istrue)
        {
            LIBVLCAPI.libvlc_set_fullscreen(libvlc_media_player_, istrue ? 1 : 0);
        }

        public double Duration()
        {
            return duration_;
        }

        public string Version()
        {
            return LIBVLCAPI.libvlc_get_version();
        }
    }

1.2static 声明 m_instance 优化效率

如下：

#pragma once
#include <memory>
#include <basetsd.h>
typedef SSIZE_T ssize_t;
#include "vlc/vlc.h"
#include <mutex>

struct libvlc_media_track_info_t;
struct libvlc_media_t;
struct libvlc_instance_t;
struct libvlc_media_player_t;
struct libvlc_event_t;

class context;

enum  MediaState {

    NothingSpecial = 0,
    Opening = 1,
    Buffering = 2,
    Playing = 3,
    Paused = 4,
    Stopped = 5,
    Ended = 6,
    Error = 7
};

class  TestVlcVideo
{
public:
    TestVlcVideo();
     void init( std::function<void(int)> eventCallback);
     void setHwnd(const int64_t iHwnd) ;
     bool loadMedia(const char* &url) ;
     int  play() ;
     void pause() ;
     void stop() ;
     void setRatio(const char* &ratio) ;
     int getVolume() ;
     int setVolume(const int volume) ;
     int getMediaState() ;
    libvlc_instance_t * getVlcInstance();
    libvlc_media_player_t * getVlcMediaPlayer();
private:
    static void vlcEvents(const libvlc_event_t *ev, void *param);
    static libvlc_instance_t *m_instance;
    libvlc_media_player_t *m_mediaPlayer = nullptr;
    int64_t m_durationMS;
    std::function<void(int)> m_eventCallback;
    MediaState m_currentMediaState;
};

上面 static 声明的 m_instance 是为了优化效率，不必每次播放视频的时候都新建。

这是第二步工作。

1.3封装 DLL

需要封装真正的 DLL 了，向C#暴露的也是这个类里面的方法。

#pragma once

typedef  int(*CallBackMediaState)(int);

#ifdef DLLVLC_EXPORTS // 用来导出函数
#define DLLVLC_API __declspec(dllexport)
#else // 用来标识为导入函数，对于引用该头文件的外部模块来说dllimport这个标记对编译优化有作用
#define DLLVLC_API __declspec(dllimport)
#endif

#include "Testvlcvideo.h"

namespace TestVLCDLL {

extern "C" {
    /*
     *  @brief   VLC Instance和Player实例初始化
     *  @param   CallBackMediaState callback  回调函数为媒体播放状态
     *  @return  每次vlcInit会返回一个VLC的Player ID，此ID唯一，后面的接口都需要此ID找到对应的Player
     */
    DLLVLC_API int   vlcInit(CallBackMediaState callback);
    /*
     *  @brief  VLC 媒体加载接口
     *  @param  int index   PlayerID
     *  @param  const char *path   媒体路径
     */
    DLLVLC_API bool  vlcLoad(int index, const char *path);
    /*
     *  @brief  设置句柄，如不设置则为默认窗口播放
     *  @param  const int64_t iHwnd windows窗口句柄
     */
    DLLVLC_API bool  vlcSetHwnd(int index,const int64_t iHwnd);
    DLLVLC_API bool  play(int index);
    DLLVLC_API bool  pause(int index);
    DLLVLC_API bool  stop(int index);
    /*
     *  @brief  设置播放窗口比例
     *  @param  形如  16:9  4:3 等字符串
     */
    DLLVLC_API bool  setRatio(int index,const char* ratio);
    /*
     *  @brief  设置媒体播放音量
     */
    DLLVLC_API bool  setVolume(int index, int volume);
    /*
    *  @brief  获取媒体总时长
    */
    DLLVLC_API int64_t  getMediaLength(int index);
    /*
    *  @brief  获取当前播放状态
    */
    DLLVLC_API int   getMediaState(int index);
    /*
    *  @brief  销毁VLC Player
    */
    DLLVLC_API bool  vlcDisponse(int index);

    }
}

首先在最开始定义了 CallBackMediaState 回调函数，对应C++ 层使用函数指针和std::function 都可以。然后使用 DLLVLC_EXPORTS 指示本类为导出类，然后再使用 DLLVLC_API 宏定义导出函数，这些方法都是 dll 暴露给外部调用的方法。

1.4应用程序的导出函数

// DLLVLC.cpp : 定义 DLL 应用程序的导出函数。
#define DLLVLC_EXPORTS
#include "DLLVLC.h"
#include "Testvlcvideo.h"
#include <iostream>
#include <map>
#include <mutex>
#include <atomic>

std::map<int, TestVlcVideo*> g_mapVLC;
std::atomic_int g_iIndex = 0;
std::mutex g_mt;

DLLVLC_API int TestVLCDLL::vlcInit(CallBackMediaState callback)
{
    //如果是初次调用，则初始化instance，否则复用instance
    std::lock_guard<std::mutex> l(g_mt);
    ++g_iIndex;
    TestVlcVideo *vlcVideo = new TestVlcVideo;
    g_mapVLC.emplace(g_iIndex, vlcVideo);
    g_mapVLC.at(g_iIndex)->init(callback);
    return g_iIndex;
}

DLLVLC_API bool  TestVLCDLL::play(int index)
{
    std::lock_guard<std::mutex> l(g_mt);
    TestVlcVideo *vlcVideo = g_mapVLC.at(index);
    if (nullptr == vlcVideo)
    {
        return false;
    }
    vlcVideo->play();
    return true;
}

.......

因为我们采用的是导出接口方法，而不是导出类（导出类比较麻烦，自己测试未能成功），因此在制作 dll 库时，使用静态 map 保存相关实例，使用对应的 init方法和 dispose 方法借助 id 参数创建和销毁对象。

1.5 vlc 简单封装的具体实现

下来再看下我们第一段代码的 cpp 文件，就是 vlc 简单封装的具体实现：

#include "Testvlcvideo.h"
#include <iostream>

libvlc_instance_t *TestVlcVideo::m_instance = nullptr;

TestVlcVideo::TestVlcVideo()
: m_mediaPlayer(nullptr)
, m_durationMS(0)
, m_eventCallback(nullptr)
{

}

void TestVlcVideo::init(std::function<void(int)> eventCallback)
{
    getVlcInstance();
    {
        getVlcMediaPlayer();
        libvlc_event_manager_t *em = libvlc_media_player_event_manager(m_mediaPlayer);
        {
            libvlc_event_attach(em, libvlc_MediaPlayerPlaying, vlcEvents, this);
            libvlc_event_attach(em, libvlc_MediaPlayerPaused, vlcEvents, this);
            libvlc_event_attach(em, libvlc_MediaPlayerStopped, vlcEvents, this);
            libvlc_event_attach(em, libvlc_MediaPlayerNothingSpecial, vlcEvents, this);
            libvlc_event_attach(em, libvlc_MediaPlayerOpening, vlcEvents, this);
            libvlc_event_attach(em, libvlc_MediaPlayerBuffering, vlcEvents, this);
            libvlc_event_attach(em, libvlc_MediaPlayerEndReached, vlcEvents, this);
            libvlc_event_attach(em, libvlc_MediaPlayerPositionChanged, vlcEvents, this);
        }
        m_eventCallback = std::move(eventCallback);
    }
}

void TestVlcVideo::setHwnd(const int64_t iHwnd)
{
    libvlc_media_player_set_hwnd(m_mediaPlayer, (void *)iHwnd);
}

bool TestVlcVideo::loadMedia(const char* &url)
{
    libvlc_media_t *m_media = nullptr;
    std::string url_ = url;
    if (url_.find("://") == std::string::npos)
    {
        m_media = libvlc_media_new_path(getVlcInstance (), url);
    }
    else
    {
        m_media = libvlc_media_new_location(getVlcInstance(), url);
    }
    if (nullptr == m_media)
    {
        m_currentMediaState = MediaState::Error;
        return false;
    }

    libvlc_media_player_set_media(getVlcMediaPlayer (), m_media);
    libvlc_media_parse(m_media);
    m_durationMS = libvlc_media_get_duration(m_media);
    libvlc_media_release(m_media);
    return true;
}

libvlc_instance_t *  TestVlcVideo::getVlcInstance()
{
    if (nullptr == m_instance)
    {
        m_instance = libvlc_new(0, NULL);
    }
    return m_instance;
}

libvlc_media_player_t * TestVlcVideo::getVlcMediaPlayer()
{
    if (nullptr == m_mediaPlayer)
    {
        m_mediaPlayer = libvlc_media_player_new(m_instance);
    }
    return m_mediaPlayer;
}

int TestVlcVideo::play()
{
    return libvlc_media_player_play(m_mediaPlayer);
}

void TestVlcVideo::pause()
{
    if(libvlc_media_player_is_playing(m_mediaPlayer))
    {
        libvlc_media_player_set_pause(m_mediaPlayer, 1);
    }
    else
    {
        libvlc_media_player_set_pause(m_mediaPlayer, 0);
    }
}

到这儿，一般情况下我们还需要配置 def 文件，以避免导出的函数名被增加额外的信息，而不是简短的“play”等。但是可以看到我们在所有的导出函数前增加了“extern C ”标识。意思是这些函数按照 C 标准进行编译，由于C++ 的函数重载，再加上各个编译器的不同，导致编译而出的函数名被（mangled name），且各不相同，但是C不支持重载，因此采用统一的编译规定，同时也可以保证此函数被 C 正确调用，所以我们就无需写 def 文件也可以保证函数名不被破坏。

2、C# 调用

上面简要说完了 C++ 端关于 DLL 的封装，再总结一下大概就是这几点：

• 至少需要两个文件，一个是自己对具体实现的封装类，一个是导出方法文件，本文中我们没有使用类，而是直接导出函数。
• 回调函数像这样 typedef int(*CallBackMediaState)(int); 去定义。
• 导出文件添加宏 dllexport

#ifdef DLLVLC_EXPORTS // 用来导出函数
#define DLLVLC_API __declspec(dllexport)
#else // 用来标识为导入函数，对于引用该头文件的外部模块来说dllimport这个标记对编译优化有作用
#define DLLVLC_API __declspec(dllimport)
#endif

导出函数添加 extern "C" DLLVLC_API 声明。

2.1C# 回调函数声明与定义

[DllImport(@"C:\Users\HiWin10\Desktop\DLLVLC\DLLVLC\DLLVLC\x64\Release\DLLVLC.dll", EntryPoint = "vlcInit",
        SetLastError = true,
        CharSet = CharSet.Ansi,
        ExactSpelling = false,
        CallingConvention = CallingConvention.Cdecl)]
        public extern static int vlcInit(DllcallBack pfun);

        [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
        public delegate int DllcallBack(int MediaState);

C# 的回调函数即为委托，需要提前定义委托 DllcallBack ，然后我们假定它是被 C++ 认可的，作为参数传入 vlcInit。在下面我们需要写此委托函数具体的实现：

public static int CsharpCall(int MediaState)
        {
            Console.WriteLine(MediaState);
            return MediaState;
        }

使用的时候：

static int index;
        static void Main(string[] args)
        {
            DllcallBack mycall;
            mycall = new DllcallBack(Program.CsharpCall);
            index = vlcInit(mycall);
            ......
        }

经过验证，此种方式的回调函数能被 C++ 承认，对应于C++的 std::function。

2.2C# 导出普通函数调用

[DllImport(@"C:\Users\HiWin10\Desktop\DLLVLC\DLLVLC\DLLVLC\x64\Release\DLLVLC.dll", EntryPoint = "vlcLoad",
        CallingConvention = CallingConvention.Cdecl)]
        public extern static bool vlcLoad(int index, string path);

        [DllImport(@"C:\Users\HiWin10\Desktop\DLLVLC\DLLVLC\DLLVLC\x64\Release\DLLVLC.dll", EntryPoint = "vlcSetHwnd",
        CallingConvention = CallingConvention.Cdecl)]
        public extern static bool vlcSetHwnd(int index, int iHwnd);

上面是 C# 关于普通导出函数的加载方法，在 main 函数中直接进行调用即可。

static int index;
        static void Main(string[] args)
        {
            DllcallBack mycall;
            mycall = new DllcallBack(Program.CsharpCall);
            index = vlcInit(mycall);
            Console.WriteLine(vlcLoad(index, @"D:\1.mp4"));
            Console.WriteLine(getMediaLength(index));
            play(index);
            setRatio(index,"16:9");

其实 C# 端的调用还是比较简单的，上面的方式是采用静态加载的方式，需要将C++ 的 dll 放到 C# 工程 bin 目录下，而动态加载的方式笔者未进行尝试。

整个过程就完成了，使用 C++ 封装的方式可以使用一些只支持 C++，只有 C++ API 的强大库，也可以防止反编译，还可以使代码更好的分层等。

下面附上demo链接，有需要的小伙伴可下载运行（VS2015）

download.csdn.net/download/u0…

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