sachin0005,

I use the FTD2XX_NET library with .NET 4.5.2 and use the Write() and GetRxBytesAvailable() functions without problem.

Apologies, but a basic question up front.  Have you opened the FTDI device?   Your example code does not indicate that you have.

            UInt32 numDevs = 0;
            FTDI.FT_STATUS ftStatus = ftdi.GetNumberOfDevices(ref numDevs);
            FTDI.FT_DEVICE_INFO_NODE[] deviceList = new FTDI.FT_DEVICE_INFO_NODE[numDevs];
            ftStatus = ftdi.GetDeviceList(deviceList);
            foreach (FTDI.FT_DEVICE_INFO_NODE node in deviceList)
            {
                    ftStatus = ftdi.OpenByDescription(node.Description);
                    string comPort;
                    ftStatus = ftdi.GetCOMPort(out comPort);

                    //.....
             }

tom