Before getting straight into developing your favorite Hello, World! application in ASP.NET, we feel it’s important that you become familiar with the underlying structure of ASP.NET. This section describes some of the most important classes packaged in the System.Web.UI namespace in the ASP.NET framework.
The System.Web.UI namespace defines classes and interfaces used in constructing and rendering elements on a Web Form. The most important class in the System.Web.UI is the Control class, which defines properties, methods, and events that are common in all server controls in the Web Forms framework. Another important class in this namespace is Page, which is a derivative of the Control class. All ASP.NET web pages are instances of derivatives of the Page class. To have an extensible framework, the System.Web.UI namespace also includes the UserControl class, which is similar to the Page class except that it is mainly used as the base class for user controls. We will make use of the UserControl and Page classes in “Custom Server Controls” and “ASP.NET Application Development” later in this chapter.
The Control class is the root of all controls. For example, a text box is a control; a button or a combo box is also a control. The Control class basically encapsulates common functionalities and properties of all user-interface widgets. As you get deeper into ASP.NET development, everything you see is a Control derivative of some sort.