[DllImport("kernel32.dll")]
        private static extern int CreateFile(
            string lpFileName, 
            uint dwDesiredAccess,
            int dwShareMode,
            int lpSecurityAttributes, 
            int dwCreationDisposition,
            int dwFlagsAndAttributes, 
            int hTemplateFile
            );
hComm = CreateFile(PortNum, GENERIC_READ | GENERIC_WRITE, 0, 0, OPEN_EXISTING, 0, 0);
...

PurgeComm(hComm, PURGE_RXCLEAR | PURGE_TXCLEAR | PURGE_RXABORT | PURGE_TXABORT);