DllImport error: unable to find an entry point named

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C++

C#

DLL

I have an DLL file named WD_SDK.dll ( it go with an SDK.h file ). I open the SDK.h and I see

typedef void (CALLBACK * VideoCaptureCB_Ptr)(PVOID pContext, BYTE * apData[3],
VideoSampleInfo_T * pVSI);

typedef struct _VideoSampleInfo_T
{
    ULONG   idFormat; // 
    ULONG   lSignalState;
    int     nLen; // not used for raw video data(e.g. YUV420)
    int     nWidth;
    int     nHeight;
    int     anPitchs[3]; // only used for raw video data(e.g. YUV420)
    ULONG   dwMicrosecsPerFrame; // 1000*1000/FPS
    ULONG   field;
    int     iSerial;

} VideoSampleInfo_T;

WD_RegisterVideoPreviewCB(HANDLE hChannel, PVOID pContext, VideoCaptureCB_Ptr pCB);

I want to call the WD_RegisterVideoPreviewCB in C#. So I use the DllImport to declare it in C#. It look like this:

[DllImport("WD_SDK.dll", EntryPoint = "_WD_RegisterVideoPreviewCB@12", ExactSpelling = true)]
        static extern int WD_RegisterVideoPreviewCB(IntPtr hChannel,  object pContext, VideoCaptureCB_Ptr pCB);

Then i re-declare the C++ struct and CALLBACK in C# like this:

public delegate void VideoCaptureCB_Ptr(IntPtr pContext, byte[] apData, VideoSampleInfo_T pVSI);

[StructLayout(LayoutKind.Sequential)]
    public struct VideoSampleInfo_T
    {
        public uint idFormat;
        public uint lSignalState;
        public int nLen; // not used for raw video data(e.g. YUV420)
        public int nWidth;
        public int nHeight;
        public int[] anPitchs; // only used for raw video data(e.g. YUV420)
        public uint dwMicrosecsPerFrame; // 1000*1000/FPS
        public uint field;
        public int iSerial;

    }

And one delegate implement

When i run my code

WD_RegisterVideoPreviewCB(m_ahChannels[i],  m_aMediaHandler[i], HandleVideoStatic);

THe program show me an error "does not match the unmanaged target signature. "

I thinks encountered problem with the PVOID pContext, I changed it to object pContext. But i have to pass the m_aMediaHandler[i] ( It's an instance of a class not a pointer ) to it, the error raised " .."invalid agruments"

Please help me to correct this code. Thanks in advance.