 <p>I have a project called <a href="http://yaam.codeplex.com/" rel="nofollow">Yaam on CodePlex</a> that uses C# to send data through the serial port. Check that out for an example. On the C# side (see Yaam\Yaam.xaml.cs), simply use the <code>SerialPort</code> class in the <code>System.IO.Ports</code> namespace. Once you instantiate the object and set the properties (baud rate, com port, etc), simply call<code>.Open()</code> . There are also plenty of other examples on the web. Take a look at these:</p> <ul> <li><a href="http://jtoee.com/2009/02/talking-to-an-arduino-from-net-c/" rel="nofollow">http://jtoee.com/2009/02/talking-to-an-arduino-from-net-c/</a></li> <li><a href="http://www.instructables.com/id/Interfacing-your-arduino-with-a-C-program/" rel="nofollow">http://www.instructables.com/id/Interfacing-your-arduino-with-a-C-program/</a></li> </ul>