I have recently completed to develop the ninth topic (themes) in my ASP.NET on going course. You can find the community version of this course available for free personal and academic usage at www.abelski.com. More information about the professional version can be found at www.xperato.com. The following video clips were prepared as part of the community version of my ASP.NET course.
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.