Program.cs

using System;
using System.IO.Ports;
using System.Text;

if (args.Length == 0)
{
    Console.WriteLine("arduino-demo <portName> [<baudRate>=9600]");
    return;
}

// to get port name you can use SerialPort.GetPortNames()
string portName = args[0];
int baudRate = args.Length >= 2 ? int.Parse(args[1]) : 9600;

using SerialPort sp = new SerialPort(portName);
sp.Encoding = Encoding.UTF8;
sp.BaudRate = baudRate;
sp.ReadTimeout = 1000;
sp.WriteTimeout = 1000;
sp.Open();

bool finished = false;
Console.CancelKeyPress += (a, b) =>
{
    finished = true;
    // close port to kill pending operations
    sp.Close();
};

Console.WriteLine("Type '!q' or Ctrl-C to exit...");
Console.WriteLine("Example commands:");
Console.WriteLine("  DIR");
Console.WriteLine("    change direction of spinning");
Console.WriteLine("  RATE 300");
Console.WriteLine("    change rate to 300ms");
Console.WriteLine("  COL FF0000");
Console.WriteLine("    change LED color to red");
Console.WriteLine("  SPIN");
Console.WriteLine("    start/stop spinning");

while (!finished)
{
    string? line = Console.ReadLine();
    if (line is object && line == "!q")
        break;

    try
    {
        sp.WriteLine(line);
    }
    catch (TimeoutException)
    {
        Console.WriteLine("ERROR: Sending command timed out");
    }

    if (finished)
        break;

    // if RATE is set to really high Arduino may fail to respond in time
    // then on the next command you might get an old message
    // ReadExisting will read everything from the internal buffer
    string existingData = sp.ReadExisting();
    Console.Write(existingData);
    if (!existingData.Contains('\n') && !existingData.Contains('\r'))
    {
        // we didn't get the response yet, let's wait for it then
        try
        {
            Console.WriteLine(sp.ReadLine());
        }
        catch (TimeoutException)
        {
            Console.WriteLine($"ERROR: No response in {sp.ReadTimeout}ms.");
        }
    }
}