调用C++DLL，回调不触发,该如何解决

调用C++DLL，回调不触发
这是C++的定义：

typedef unsigned char           HI_U8;

typedef unsigned char           HI_UCHAR;

typedef unsigned short          HI_U16;

typedef unsigned int            HI_U32;

typedef unsigned long           HI_ULONG;

typedef unsigned __int64        HI_U64;


typedef char                    HI_S8;

typedef short                   HI_S16;

typedef int                     HI_S32;

typedef long                    HI_LONG;

typedef long                    HI_HANDLE;

typedef __int64                 HI_S64;


typedef char                    HI_CHAR;

typedef char*                   HI_PCHAR;


typedef float                   HI_FLOAT;

typedef double                  HI_DOUBLE;

typedef void                    HI_VOID;


typedef enum hiMD_TYPE_E

{

	DATA_MOTION_DETECTION = 0,

	DATA_INPUT_ALARM,

	DATA_KEEP_ALIVE,

	DATA_DEC_STATE,

	DATA_ALADIO_ALARM,

	DATA_BUTT

}MD_TYPE_E;


typedef LONG (HISDK_APICALL* OnMessageCallBack)(

		HI_U32 u32Chn,

		MD_TYPE_E eDataType,

		HI_U8*  pu8Buffer,

		HI_U32 u32Length,

		HI_VOID* pUserData);



HISDK_API HI_S32 HISDK_APICALL HI_SDK_SetMessageCallBack(HI_HANDLE lHandle, HI_U32 u32Chn, OnMessageCallBack CallBack, HI_VOID *pUserData);

我在C#中这样调用：

 public enum MD_TYPE_E

        {

            DATA_MOTION_DETECTION = 0,

            DATA_INPUT_ALARM,

            DATA_KEEP_ALIVE,

            DATA_DEC_STATE,

            DATA_ALADIO_ALARM,

            DATA_BUTT

        }


 public delegate int MessageCallBack(int chn, MD_TYPE_E type, IntPtr areas, int length, IntPtr userData);


[DllImport(@"HISDK.dll")]

        public static extern int HI_SDK_SetMessageCallBack(IntPtr handle, int chn, MessageCallBack callback, IntPtr userData);


var result=HISDKInvoker.HI_SDK_SetMessageCallBack(_handle, _handle.ToInt32(), new MessageCallBack(MessageCallBack), this.Handle);//返回0，表示成功

public static int MessageCallBack(int chn, IPCAMERATest2.Core.HISDKInvoker.MD_TYPE_E type, IntPtr areas, int length, IntPtr userData)

        {

            return 0;

        }

但是无论怎样，MessageCallBack回调都不触发，看了下提供的C++DEMO， 是可以触发的，不知道是不是c#中的类型不对还是我的写法有问题？
------解决思路----------------------
MessageCallBack 怎么会有两个？
改一下名字吧