C# with Arduino on Facebook

.Net2.0 gives you the easy to use System.IO.Ports so a programm that writes everything you type to the arduino is quickly written :

 using System;
 using System.IO.Ports;

 namespace test
 {
    class Program
    {

        static void Main(string[] args)
        {
            SerialPort port = new SerialPort("COM4", 9600);
            port.Open();
            while (true)
            {
                String s=Console.ReadLine();
                if (s.Equals("exit"))
                {
                    break;
                }
                port.Write(s+'\n');
            }
            port.Close();

        }

    }
 }

Detect Arduino com port and use Serial Comms

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Tutorial: sending instructions to Arduino using C# (portuguese with translation options) (source code available)

SerialTalk: send and receive data from Arduino (portuguese with translation options) (source code available)