With traditional object-oriented development, extending classes is typically achieved by deriving from a base class and enhancing the functionality in the derived class. Visual Basic® still supports these object-oriented concepts, but classes are sometimes marked NotInheritable to prevent modification of their behavior through inheritance. As a result, there is no way to customize these classes. An example of this is the System.String class. A new feature made available in Visual Basic 2008, however, lets you extend any existing type's functionality, even when a type is not inheritable.
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