Marshalling structures containing byte array

Question

I have been struggling 2nd day with the task how to pass a structure, containing array of byte to c++ dll. Namely, I don't know how to marshal C# structure. Here is my code:

in C++ dll:

    struct Image
    {
        int Width;
        int Height;
        int Depth;
        uchar *Data;
    };
    .....
__declspec(dllexport) void DirectTransform(Image *InImage, Image*DestImage)  
{
...Implementation...
}

in C# program:

    [StructLayout(LayoutKind.Sequential)]
     struct ImageData
    {
        public int Width;
        public int Height;
        public int Depth;
        [MarshalAs(UnmanagedType.LPArray)]
        public byte[] Data;
    }
 [DllImport("MyDll.dll",CallingConvention=CallingConvention.Cdecl)]
        public static extern void DirectTransform(ImageData Src, ImageData Dest);

//Fill out both structures..

        DirectTransform(Image, DestImage);

Exception throws at calling of DirectTransform and says that:

Cannot marshal field 'Data' of type'ImageData': Invalid managed/unmanaged type combination (Array fields must be paired with ByValArray or SafeArray).

When I change LPArray to ByValArray and point the size of array(in this case 202500) it also doesn't work, because the size is too large. When SafeArray is used, the program fails inside DLL with the message :

Attempted to read or write protected memory. This is often an indication that other memory is corrupt. And data in structures are erroneous.

Could anyone help me?

It has to be IntPtr, initialize it with Marshal.AllocHGlobal(). The Dest argument is however a problem, you either don't know how large an array to allocate or the C code allocates it and then you have a memory leak. — Hans Passant
– Hans Passant, Commented Oct 3, 2014 at 8:37

Community · Accepted Answer · 2017-05-23 10:33:34Z

0

Look at this question

You can try passing a pointer to your array and, for example an integer length field. Doing that, you'll need to manually allocate unmanaged memory using Marshal.AllocHGlobal, call your function and after that in finally block - clear after yourself with Marshal.FreeHGlobal

With Marshal.Copy method you are copying your data into allocated memory.

edited May 23, 2017 at 10:33

CommunityBot

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answered Oct 3, 2014 at 8:45

steavy

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