C# 调用 C++ DLL 中的委托，引发“对XXX::Invoke类型的已垃圾回收委托进行了回调”错误的解决办法

      C++

public delegate bool EventCallBack(int index, int type, int param1, int param2);
[DllImport(WAPA_DLL, EntryPoint = "WAPA_IPC_SetEventCallBack", CallingConvention = CallingConvention.Cdecl)]
public static extern int WAPA_IPC_SetEventCallBack(EventCallBack fEvent);

　　C#调用

        public ctlCamera()
        {
            InitializeComponent();
            WAPAMethods.WAPA_IPC_Init(WAPAMethods.TVFORMAT.PAL);
            WAPAMethods.WAPA_IPC_SetEventCallBack(IpcOnEvent);
        }
        private bool IpcOnEvent(int index, int type, int param1, int param2)
        {
            var status = "未";
            switch (type)
            {
                case 0:
                    status = "断";
                    break;
                case 1:
                    status = "断";
                    break;
                case 2:
                    status = "移";
                    break;
                case 3:
                    status = "警";
                    break;
            }
            Console.Write(string.Format("索引：{0}，类型：{1}，参数1：{2}，参数2：{3}", index, type, param1, param2));
            return true;
        }

　　出现错误：   对“XXX::Invoke”类型的已垃圾回收委托进行了回调。这可能会导致应用程序崩溃、损坏和数据丢失。向非托管代码传递委托时，托管应用程序必须让这些委托保持活动状态，直到确信不会再次调用它们。

解决方法：

将委托赋值给类中的成员，再调用此成员

        private static WAPAMethods.EventCallBack eventCallBack;
        public ctlCamera()
        {
            InitializeComponent();
            WAPAMethods.WAPA_IPC_Init(WAPAMethods.TVFORMAT.PAL);
            eventCallBack = IpcOnEvent;
            WAPAMethods.WAPA_IPC_SetEventCallBack(eventCallBack);
        }