using System;

public static class PointExtensions{   

  public static double DistanceTo(this Point p1, Point p2){   
    return Math.Sqrt((p1.X - p2.X) * (p1.X - p2.X) +          
                     (p1.Y - p2.Y) * (p1.Y - p2.Y));          
  }                                                           

}

 
 
The class must be static.
 
The method must be static, and the 
first parameter becomes the receiver
of the message (the target of the instance
method). Notice the this modifier!