Calling unmarshalled DLL from C#

[bretta]

Hi

I am trying to make a series of calls to an unmarshalled DLL. The header file for the DLL reads as follows:

BOOL WINAPI	InitUSB	(void);
BOOL WINAPI	CloseUSB (void);
BOOL WINAPI USBReadData(BYTE Endpoint,DWORD BytesToRead,PBYTE DataBuffer);
BOOL WINAPI USBSendCommand(BYTE bCmd);
BOOL WINAPI USBGetCommand (BYTE bCmd, WORD wLength, PBYTE pData);

In my code I have defined the functions as follows:

		// declare function 
		[DllImport("mirwsp", CallingConvention= CallingConvention.StdCall)]
		private static extern Boolean InitUSB();
		[DllImport("mirwsp", CallingConvention= CallingConvention.StdCall)]
		private static extern Boolean CloseUSB();
		[DllImport("mirwsp", CallingConvention= CallingConvention.StdCall)]
		private static extern Boolean USBSendCommand(byte Cmd);
		[DllImport("mirwsp", CallingConvention= CallingConvention.StdCall)]
		private static extern Boolean USBGetCommand(byte Cmd, int wLen, ref IntPtr pData);
		[DllImport("mirwsp", CallingConvention= CallingConvention.StdCall)]
		private static extern Boolean USBReadData(byte EPNumber, long BytesToRead, ref IntPtr DataBuffer);

InitUSB, USBSendCommand and USBReadData appear to work correctly but when I call USBGetCommand I get a "Value is no longer valid" exception when I try to call Marshal.FreeHGl obal(data);

		private byte[] getCommand(int len, byte command)
		{
			IntPtr data = Marshal.AllocHGlobal(len);
                        try {
			        bool bSuccess = USBGetCommand(command,len,ref data);
			
			        byte[] returnBytes = new byte[len];
			        Marshal.Copy(data, returnBytes, 0, len);
			
				Marshal.FreeHGlobal(data);
			} catch (Exception) {
				
			}
			return returnBytes;
			
		}

If I ignore that error and continue running the code then the next time I make the call I get the same exception but in the previous line (Marshal.Copy(d ata, returnBytes, 0, len);) Which means I am unable to see the data returned by the function.

Any help would be appreciated as I am pretty new to marshalling in this way.

Thanks

Brett

[Plater]

That actually looks like how I would think to do it.
Couple of possibilities, can you marshal something other than an IntPtr?
Like marshal a byte[] and somehow get a ptr to that (I know it should be same thing, but it might make a difference)
Also, have you been checking the return value from your DLL function? Make sure you only attempt to copy/marshall the bytes if successfull?
The DLL function might be choking on the IntPtr for some reason and thus returning you a null pointer which you try to marshal?