当从c++代码发送回调到c#代码时，访问冲突读取位置0x00000004

我正在开发一个windows 8手机应用程序，其中我需要从c++代码发送回调事件消息到c#中的UI代码。下面是c#代码，我在其中创建回调函数并将其指针发送给c++代码。c++代码可以使用这个函数指针向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";
            }
        }
}

下面是将事件消息发送给c#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#中获得事件消息，但是在发送650-700回调后，我的应用程序给出了访问违反异常，之后没有任何工作。我怀疑我从c#传递函数指针到c++的方式，但无法解决它。

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";
        }
    }
}