How to pass variable length structure to pinvoke C function in C#

Question

My C structure format is this:

typedef struct pt_data {
  int Length;   ///< Length of the Data field in bytes
  uchar Data[1];    ///< The data itself, variable length
} PT_DATA;

My C function is this:

PT_STATUS PTSetFingerData (
IN PT_CONNECTION hConnection,
IN PT_LONG lSlotNr,
IN PT_DATA *pFingerData
)

Now I want to put a wrapper for this function in C#.

How can I do this? In particular, how can I do this for passing the C# PT_DATA struct to PT_DATA C struct?

Answer 1

You need to marshal the data manually. A variable length struct cannot be marshalled by the p/invoke marshaller.

In your case the obvious way to do this would be to declare the PTDATA* argument as byte[] in your p/invoke. Then you just need to populated the byte array before calling the function. The first 4 bytes are the length of the subsequent data.

static byte[] GetPTData(byte[] arr)
{
    byte[] len = BitConverter.GetBytes(arr.Length);
    byte[] data = new byte[sizeof(int) + arr.Length];
    Array.Copy(len, data, sizeof(int));
    Array.Copy(arr, 0, data, sizeof(int), arr.Length);
    return data;
}