title: this parameter modifier in C#?
body: <p>I'm curious about this code snippet: </p>
  
  <pre><code>public static class XNAExtensions
  {
      /// &lt;summary&gt;
      /// Write a Point
      /// &lt;/summary&gt;
      public static void Write(this NetOutgoingMessage message, Point value)
      {
          message.Write(value.X);
          message.Write(value.Y);
      }
      // ...
  };
  </code></pre>
  
  <p>What does the <code>this</code> keyword mean next to the parameter type? I can't seem to find any information about it anywhere, even in the C# specification.</p>