Hi,

I've tried several approaches for a few days now & feel its time to
ask the gurus.

I am making a call to a C++ dll from C# via p/invoke, the api expects
an array of structs with pre-allocated buffers. The api populates the
buffers with raw byte data from a device.

The C++ struct definition is :

typedef struct {
  unsigned char * Buffer1;
  unsigned int      Buffer1size;
  unsigned char * Buffer2;
  unsigned int      Buffer2size;
  unsigned int      Type;
  unsigned char * Buffer3;
  unsigned int      Buffer3size;
  unsigned int      PacketNum;
} CapBuffer;

The C++ dll exported method has the following signature and expects
the above struct as an array of structs.

int CaptureMultiple( CapBuffer ** capBufferArr, unsigned int
numBuffer ..)

Buffer's 1, 2 & 3 are to be preallocated to 20000 bytes, and the dll
will populate these buffers.

I pass the arrray to structs parameter as:

internal static extern int CaptureMultiple( [In, Out] CapBuffer[]
capBufferArr, UInt32 numBuffers...)

I'm not sure how the individual buffers should be marshalled. I've
tried several techniques, some of which are as follows, in each case I
allocate memory (with a padded string buffer for strings or do a
Marshal.AllocHGlobal() for IntPtr)

[MarshalAs(UnmanagedType.LPStr, SizeConst = BUFFER_SIZE)]
public string Buffer1;

[MarshalAs(UnmanagedType.SysUInt)]
public IntPtr Buffer1;

[MarshalAs(UnmanagedType.ByValArray, SizeConst = BUFFER_SIZE)]
public byte[] Buffer1;

What am I doing wrong? What is the standard approach to pass a struct
with an embedded const char*?

Any advice would be much appreciated.

Thanks,
Lionel