We have learned fthat an MS AJAX client control represents a DOM element as a client object and extends a markup representation or provides additional functionality for the element. In a word, a client control encapsulates JavaScript code that is intended to be reusable across applications. In fact, the MS AJAX client side framework has already supplied a good many built-in client controls, which includes not only the basic controls, such as Label, Button, InputControl, TextBox, Image, HyperLink, CheckBox, and Select, but also the advanced controls, ListView, ItemView, and XSLTView.

On the other hand, by deriving from the Control base class, we can create our own custom controls which automatically inherit many built-in cross-browser features from the parent. In the following section we are to build up a menu control that will be rendered using dynamic contents.