I have recently completed to develop the eighth topic in my ongoing ASP.NET course. It focuses on the ASP.NET web controls and explains how to use the more complicated ones. You can find the community version of this ongoing course available for free at www.abelski.com. The professional version is available at www.xperato.com. You can download the slides and watch the relevant video clips below.
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.