需要帮助解决 AccessViolationException with C#/C 互操作

我需要一些帮助来修复持久的访问违规异常。

给定这样的 c 签名

struct message {
    char *topic;
    void *payload;
    int payloadlen;
};
__declspec(dllexport) void callback_set(struct data *dat, void (*on_publish)(struct data *, void *, const struct message *));

我有这个 C#

    public struct message 
    {
        public string topic;
        public IntPtr payload;
        public int payloadlen;
    };

    /* INTEROP ACCESS */
    public delegate void on_publish(IntPtr dat, IntPtr usrData, IntPtr messageData);
    [DllImport(dllPath, CallingConvention = CallingConvention.Cdecl)]
    public extern static void callback_set(IntPtr dat, IntPtr callback);
    /* IMPLEMENTATION OF OUR on_publish*/
    public static void MessageHandler(IntPtr dat, IntPtr usrData, IntPtr messageData)
    {
        var instance = (message)Marshal.PtrToStructure(messageData, typeof(message));
        string test = instance.topic; // <-- this is successfully received
        Console.WriteLine("message rec " + test);
    } //<-- as soon as I exit, the dll blows up with access violation

    /* REGISTERING MESSAGEHANDLER AS ON_PUBLISH */
    public static void RegisterMessageHandler(IntPtr dat) //<-- I promise, the pointer I send here is valid and not modified
    {
        messageHandler = new on_publish(MessageHandler);
        messageHandlerPtr = Marshal.GetFunctionPointerForDelegate(messageHandler);
        callback_set(dat, messageHandlerPtr); //<-- if I do not call this, everything works, no ADE
        Console.WriteLine("message handler registered");
    }
    //just tried to move to scope in order to retain their managed memory loc
    private static IntPtr messageHandlerPtr;  
    private static on_publish messageHandler;

运行时，并确保应收到消息 - 我得到了topic的正确字符串，但一旦MessageHandler返回，我就会得到可怕的异常。

我尝试过的事情：

• 更改调用约定
• 在托管callback_set定义中使用 on_publish 而不是 IntPtr
。
• 可能更多绝望的事情不应该产生影响

任何帮助非常感谢！

如果有人可以提供帮助，我可以分享该项目的压缩文件 - 它将像我试图与之互操作的 Mosquitto 一样获得 BSD 许可。