During the past 2 years I have been working on a short practical guide for learning C#. The guide includes texts and video clips. The texts were written in Hebrew. The video clips were taken in Hebrew as well. You can find it at www.CSharpBook.co.il. I have recently upgraded this website.
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.