This works:

  [StructLayout(LayoutKind.Sequential)] struct COPYDATASTRUCT
  {
    public IntPtr dwData;
    public int cbData;
    [MarshalAs(UnmanagedType.LPStr)] public string lpData;
  }

Then in a method:
  COPYDATASTRUCT cds;
  cds.dwData = (IntPtr) 100;
  cds.lpData = strMsg;
  cds.cbData = cds.lpData.Length + 1;
  SendMessage(hwnd, WM_COPYDATA, 0, ref cds);

If I change lpData to:
  [MarshalAs(UnmanagedType.LP.LPArray)] public byte[] lpData;

To send a byte array instead of a string I get a marshalling error.

What am I missing here?

Thanks,
John Dyer

   [StructLayout(LayoutKind.Sequential)] struct COPYDATASTRUCT
   {
     public IntPtr dwData;
     public int cbData;
     public IntPtr lpData;
   }

IntPtr p = Marshal.AllocHGlobal(Msg.Length);
Marshal.Copy(Msg, 0, p, Msg.Length);
COPYDATASTRUCT cds;
cds.dwData = (IntPtr) 100;
cds.lpData = p;
cds.cbData = Msg.Length;
SendMessage(hwnd, WM_COPYDATA, 0, ref cds);

Is this the correct way of handling these types of problems?
It would see that moving byte[] arrays would be a basic operation and
everything else would be built on top.  But it looks like most
operations revolve around strings instead.

Bewildered,
John Dyer