I have just completed to develop the Dependency Properties topic in my Silverlight course. The slides and the relevant video clips can be found for free personal usage in their community free version at www.abelski.org. The professional version of this course is available at www.abelski.com. The video clip below was 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.