C++ 封装 DLL 供 C# 调用详细介绍

这篇文章主要介绍了C++ 封装 DLL 供 C# 调用（以C# 调用C++ 二次封装的VLC播放库为介质，支持回调函数的封装）,需要的朋友可以参考下面我文章的具体内容

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

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  #include  typedef SSIZE_T ssize_t; #include "vlc/vlc.h" #include  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 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 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  #include  #include  #include  std::map 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 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 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  libvlc_instance_t *TestVlcVideo::m_instance = nullptr; TestVlcVideo::TestVlcVideo() : m_mediaPlayer(nullptr) , m_durationMS(0) , m_eventCallback(nullptr) { } void TestVlcVideo::init(std::function 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）

DLLVLCfor_jb51.rar

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