One of my favorite features in .Net 3.5 is the ability to extend a class using extension methods. Earlier you could only extend a class either by creating a new class that inherits from the first class or you could create a partial class. Of course if you have the source code of the original class you could just write the new code directly in it and recompile it. In many cases you don’t have access to the source code of a class and if the class is also sealed (NotInheritable in VB) you would not be able to extend its behavior using the other methods either. The string class is a typical example of a class that is sealed and you normally wouldn’t be able to extend at all. That is until extension methods entered the scene.