I have just completed to develop the first topic of the Silverlight Basics course I am working on these days. You can find its community version available for free personal usage at www.abelski.org. The professional version is available at www.abelski.com. The following video clips were prepared as part of this topic.
The Visitor Design Pattern
The visitor design pattern allows us to add operations to objects that already exist without modifying their classes and without extending them.