C#调用C++类库的几种方式

1、  直接调用C++类库中的公共方法

使用DllImport特性对方法进行调用，比如一个C++类库SampleCppWrapper.dll中的公共方法：

extern "C" __declspec(dllexport) int __stdcall Add(int n1, int n2);

__stdcall表示调用约定：参数都是从右向左通过堆栈传递， 函数调用在返回前要由被调用者清理堆栈。

在C#中，调用如下：

[DllImport("SampleCppWrapper.dll")]
private static extern int Add(int n1, int n2);

注意参数的类型，之后，可直接在C#编程中使用这个方法。

2、  调用C++类库中的类的方法

C#不能直接调用C++类库中的类，需要一种变通的解决方式，通过再做一个C++类库把要调用的类成员方法暴露出来，比如下面这个C++类：

SampleCppClass.h

#pragma once

class __declspec(dllexport) SampleCppClass
{
public:
    SampleCppClass(void);
    ~SampleCppClass(void);
    
    int Add(int n1, int n2);
    int Sub(int n1, int n2);
};

SampleCppClass.cpp

#include "SampleCppClass.h"

SampleCppClass::SampleCppClass(void)
{
}

SampleCppClass::~SampleCppClass(void)
{
}

int SampleCppClass::Add(int n1, int n2)
{
    return n1 + n2;
}

int SampleCppClass::Sub(int n1, int n2)
{
    return n1 - n2;
}

我们要调用SampleCppClass中的Add和Sub两个方法，所以我们再写一个C++类库，通过公共方法间接调用类成员方法：

SampleCppWrapper.h

#pragma once

#include "..SampleCppClassSampleCppClass.h"

namespace SampleCppWrapper
{
    extern "C" __declspec(dllexport) int __stdcall Add(int n1, int n2);
    extern "C" __declspec(dllexport) int __stdcall Sub(int n1, int n2);
}

SampleCppWrapper.cpp

#include "SampleCppWrapper.h"

namespace SampleCppWrapper
{
    SampleCppClass* g_pObj = new SampleCppClass();

    int __stdcall Add(int n1, int n2)
    {
        return g_pObj->Add(n1, n2);
    }

    int __stdcall Sub(int n1, int n2)
    {
        return g_pObj->Sub(n1, n2);
    }
}

在C#中，再调用SampleCppWrapper.dll中的公共方法：

[DllImport("SampleCppWrapper.dll")]
private static extern int Add(int n1, int n2);
[DllImport("SampleCppWrapper.dll")]
private static extern int Sub(int n1, int n2);

3、  使用C++类库中的回调函数

C++的回调函数是一种事件响应机制，和C#的委托相似，比如一个C++类中的回调函数：

SampleCppClass.h

#pragma once

typedef void (*LoopCallback)(void* pContext);

class __declspec(dllexport) SampleCppClass
{
public:
    SampleCppClass(void);
    ~SampleCppClass(void);
    
    void SetCallbackFunc(LoopCallback callback);
    void SetCallbackContext(void* pContext);
    void Loop();
private:
    LoopCallback m_callback;
    void* m_pContext;
};

SampleCppClass.cpp

#include "SampleCppClass.h"

SampleCppClass::SampleCppClass(void)
{
}

SampleCppClass::~SampleCppClass(void)
{
}

void SampleCppClass::SetCallbackFunc(LoopCallback callback)
{
    m_callback = callback;
}

void SampleCppClass::SetCallbackContext(void* pContext)
{
    m_pContext = pContext;
}

void SampleCppClass::Loop()
{
    for (int i=0; i<10; i++)
    {
        if (m_callback != NULL)
        {
            m_callback(m_pContext);
        }
    }
}

我们通过C++再写一个类库进行封装，把类中的方法暴露出来：

SampleCppWrapper.h

#pragma once

#include "..SampleCppClassSampleCppClass.h"

namespace SampleCppWrapper
{
    typedef void (__stdcall *LoopCallbackWrapper)(void* pContext);

    extern "C" __declspec(dllexport) void __stdcall SetCallbackFunc(LoopCallbackWrapper callback);
    extern "C" __declspec(dllexport) void __stdcall SetCallbackContext(void* pContext);
    extern "C" __declspec(dllexport) void __stdcall Loop();
}

SampleCppWrapper.cpp

#include "SampleCppWrapper.h"

namespace SampleCppWrapper
{
    LoopCallbackWrapper g_callbackWrapper;
    SampleCppClass* g_pObj = new SampleCppClass();

    void LoopCallbackFunc(void* pContext);

    void __stdcall SetCallbackFunc(LoopCallbackWrapper callback)
    {
        g_callbackWrapper = callback;
        g_pObj->SetCallbackFunc(LoopCallbackFunc);
    }

    void __stdcall SetCallbackContext(void* pContext)
    {    
        g_pObj->SetCallbackContext(pContext);
    }

    void __stdcall Loop()
    {
        g_pObj->Loop();
    }

    void LoopCallbackFunc(void* pContext)
    {
        if (g_callbackWrapper != NULL)
        {
            g_callbackWrapper(pContext);
        }
    }
}

然后，在C#中进行调用：

using System;
using System.Runtime.InteropServices;
using System.Windows.Forms;

namespace SampleCsTest
{
    public partial class Form1 : Form
    {
        [StructLayout(LayoutKind.Sequential)]
        private class Context
        {
            public Form1 Form { get; set; }
        }

        private delegate void LoopCallbackHandler(IntPtr pContext);
        private static LoopCallbackHandler callback = LoopCallback;

        [DllImport("SampleCppWrapper.dll")]
        private static extern void SetCallbackFunc(LoopCallbackHandler callback);
        [DllImport("SampleCppWrapper.dll")]
        private static extern void SetCallbackContext(IntPtr pContext);
        [DllImport("SampleCppWrapper.dll")]
        private static extern void Loop();

        private Context ctx = new Context();

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            SetCallbackFunc(callback);
            ctx.Form = this;
            IntPtr ptr = Marshal.AllocHGlobal(Marshal.SizeOf(ctx));
            Marshal.StructureToPtr(ctx, ptr, false);
            SetCallbackContext(ptr);
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Loop();
        }

        private static void LoopCallback(IntPtr pContext)
        {
            Context ctx = (Context)Marshal.PtrToStructure(pContext, typeof(Context));
            ctx.Form.textBox1.Text += "callback" + Environment.NewLine;
        }
    }
}