Marshalling structures containing byte array - marshalling

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?

Answer 1

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.