从 C++ 代码向 C# 代码发送回调时访问冲突读取位置 0x00000004

Question

我正在开发一个 Windows 8 手机应用程序，其中我需要将回调事件消息从 C++ 代码发送到 C# 中的 UI 代码。下面是我在其中创建回调函数并将其指针发送到 C++ 代码的 C# 代码。 C++ 代码可以使用该函数指针向 UI 发送异步事件消息。

C# 代码

public partial class MainPage
{
        public MainPage()
        {
            InitializeComponent();
        }
        public delegate void CallBack([In][MarshalAs(UnmanagedType.LPStr)] string strParam);
        [DllImport("Test.dll", CallingConvention = CallingConvention.StdCall)]
        public static extern void PerformActionWithCallBack(int x);

        void DemonstrateCallBack()
        {
            int x;
            CallBack callback_delegate = new CallBack(CallBackFunction);
            // Converting callback_delegate into a function pointer that can be
            // used in unmanaged code.
            IntPtr intptr_delegate = Marshal.GetFunctionPointerForDelegate(callback_delegate);

            //Getting the pointer into integer value and sending it to C++

            x = (int)intptr_delegate;

            testApp.PerformActionWithCallBack(x);
        }

        //CallBack function in which event messages will be received
        void CallBackFunction([In][MarshalAs(UnmanagedType.LPStr)] string strParam)
        {
            if (Dispatcher.CheckAccess() == false)
            {
                //Updating the text block with the received message
                Dispatcher.BeginInvoke(() => MyTextBlock.Text = strParam);
            }
            else
            {
                MyTextBlock.Text = "Update directly";
            }
        }
}

以下是向 C# 发送事件消息的 C++ 代码

//Callback function prototype
typedef void(__stdcall *PCallBack)(LPSTR s);
LPSTR cbMsg = NULL;

//Global instance callback function
PCallBack gUICallback;

void TestApp::PerformActionWithCallBack(int x)
{
    //Assigning the received calllback function pointer from C# to global function pointer
    gUICallback = (PCallBack)x;
}

//This function will send event messages to C#
void SendCallbackMsg()
{
    while(1)
    {
         cbMsg = "Hello";
         Sleep(100);
         gUICallback(cbMsg);
         cbMsg = "Hi";
         Sleep(100);
         gUICallback(cbMsg);
    }
 }

使用此代码，我能够成功地在 C# 中获取事件消息，但是在发送 650-700 个回调后，我的应用程序给出了访问冲突异常，之后没有任何效果。我怀疑我将函数指针从 C# 传递到 C++ 但无法解决它的方式。

Answer 1

你的问题是，callback_delegate 被垃圾回收了。

MSDN 说

您必须手动阻止垃圾收集器从托管代码中收集委托。垃圾收集器不跟踪对非托管代码的引用。

试试这个

public partial class MainPage
{
    private CallBack _callBackDelegate = null;
    public MainPage()
    {
        InitializeComponent();
    }
    public delegate void CallBack([In][MarshalAs(UnmanagedType.LPStr)] string strParam);
    [DllImport("Test.dll", CallingConvention = CallingConvention.StdCall)]
    public static extern void PerformActionWithCallBack(int x);

    void DemonstrateCallBack()
    {
        _callBackDelegate = new CallBack(CallBackFunction);
        int x;

        // Converting _callBackDelegate  into a function pointer that can be
        // used in unmanaged code.
        IntPtr intptr_delegate = Marshal.GetFunctionPointerForDelegate(_callBackDelegate );

        //Getting the pointer into integer value and sending it to C++

        x = (int)intptr_delegate;

        testApp.PerformActionWithCallBack(x);
    }

    //CallBack function in which event messages will be received
    void CallBackFunction([In][MarshalAs(UnmanagedType.LPStr)] string strParam)
    {
        if (Dispatcher.CheckAccess() == false)
        {
            //Updating the text block with the received message
            Dispatcher.BeginInvoke(() => MyTextBlock.Text = strParam);
        }
        else
        {
            MyTextBlock.Text = "Update directly";
        }
    }
}