Program.cs

using System;
using System.IO.Ports;  //Requires Windows Compatibility from NuGet

//Make sure you have working drivers for your USB to serial port adapter
//  Be aware some chipsets for older adapters have been deliberately disabled due to counterfeiting and will throw an error
//  If you crash the code before a clean QUIT, the serial port will be held open. Disconnect and re-connect to free it up.
namespace SerialPortWriter
{
    internal class Program
    {
        static bool _continue;
        static SerialPort _serialPort;
        public static void Main(string[] args)
        {
            Console.WriteLine("Serial Port Writer");
            Console.WriteLine("==================");
            Console.WriteLine();

            string message;
            StringComparer stringComparer = StringComparer.OrdinalIgnoreCase;
            Thread readThread = new Thread(Read);
            // Create a new SerialPort object with default settings.
            _serialPort = new SerialPort();

            // Allow the user to set the appropriate properties.
            _serialPort.PortName = SetPortName(_serialPort.PortName);
            _serialPort.BaudRate = SetPortBaudRate(_serialPort.BaudRate);
            _serialPort.Parity = SetPortParity(_serialPort.Parity);
            _serialPort.DataBits = SetPortDataBits(_serialPort.DataBits);
            _serialPort.StopBits = SetPortStopBits(_serialPort.StopBits);
            _serialPort.Handshake = SetPortHandshake(_serialPort.Handshake);
            _serialPort.ReadTimeout = 500;
            _serialPort.WriteTimeout = 500;
            _serialPort.Open();
            _continue = true;
            readThread.Start();

            Console.WriteLine("Type QUIT to exit");
            while (_continue)
            {
                message = Console.ReadLine();
                if (stringComparer.Equals("quit", message))
                {
                    _continue = false;
                }
                else
                {
                    _serialPort.WriteLine(String.Format("{0}\r", message));
                }
            }
            readThread.Join();
            _serialPort.Close();
        }

        public static void Read()
        {
            while (_continue)
            {
                try
                {
                    string message = _serialPort.ReadLine();
                    Console.WriteLine(message);
                }
                catch (TimeoutException) { }
            }
        }

        public static string SetPortName(string defaultPortName)
        {
            string portName;
            Console.WriteLine("Available Ports:");
            foreach (string s in SerialPort.GetPortNames())
            {
                Console.WriteLine("   {0}", s);
            }
            Console.Write("COM port({0}): ", defaultPortName);
            portName = Console.ReadLine();
            if (portName == "")
            {
                portName = defaultPortName;
            }
            return portName;
        }

        public static int SetPortBaudRate(int defaultPortBaudRate)
        {
            string baudRate;
            Console.Write("Baud Rate({0}): ", defaultPortBaudRate);
            baudRate = Console.ReadLine();
            if (baudRate == "")
            {
                baudRate = defaultPortBaudRate.ToString();
            }
            return int.Parse(baudRate);
        }

        public static Parity SetPortParity(Parity defaultPortParity)
        {
            string parity;
            Console.WriteLine("Available Parity options:");
            foreach (string s in Enum.GetNames(typeof(Parity)))
            {
                Console.WriteLine("   {0}", s);
            }
            Console.Write("Parity({0}):", defaultPortParity.ToString());
            parity = Console.ReadLine();
            if (parity == "")
            {
                parity = defaultPortParity.ToString();
            }
            return (Parity)Enum.Parse(typeof(Parity), parity);
        }

        public static int SetPortDataBits(int defaultPortDataBits)
        {
            string dataBits;
            Console.Write("Data Bits({0}): ", defaultPortDataBits);
            dataBits = Console.ReadLine();
            if (dataBits == "")
            {
                dataBits = defaultPortDataBits.ToString();
            }
            return int.Parse(dataBits);
        }

        public static StopBits SetPortStopBits(StopBits defaultPortStopBits)
        {
            string stopBits;
            Console.WriteLine("Available Stop Bits options:");
            foreach (string s in Enum.GetNames(typeof(StopBits)))
            {
                Console.WriteLine("   {0}", s);
            }
            Console.Write("Stop Bits({0}):", defaultPortStopBits.ToString());
            stopBits = Console.ReadLine();
            if (stopBits == "")
            {
                stopBits = defaultPortStopBits.ToString();
            }
            return (StopBits)Enum.Parse(typeof(StopBits), stopBits);
        }

        public static Handshake SetPortHandshake(Handshake defaultPortHandshake)
        {
            string handshake;
            Console.WriteLine("Available Handshake options:");
            foreach (string s in Enum.GetNames(typeof(Handshake)))
            {
                Console.WriteLine("   {0}", s);
            }
            Console.Write("Handshake({0}):", defaultPortHandshake.ToString());
            handshake = Console.ReadLine();
            if (handshake == "")
            {
                handshake = defaultPortHandshake.ToString();
            }
            return (Handshake)Enum.Parse(typeof(Handshake), handshake);
        }
    }
}