The Web Forms Programming Model

Calc.aspx demonstrates three important principles of the Web Forms programming model:

• A Web form's user interface is "declared" using a combination of HTML and server controls. Controls can be customized by using control properties as attributes in the tags that declare the controls. Controls are also bona fide objects that are instantiated and executed each time the page that hosts them is requested.


• Server controls fire events that can be handled by server-side scripts. In effect, ASP.NET abstracts the divide between client and server by creating the illusion that events are fired and handled on the same machine. In reality, events fire on the server when an external stimulus (such as the click of a button) causes the form to post back to the server.


• Server-side scripts aren't scripts in the conventional sense of the word. Unlike ASP scripts, which are interpreted rather than compiled and therefore run rather slowly, server-side scripts in ASP.NET are compiled to common intermediate language (CIL) and executed by the common language runtime. Although ASP.NET pages incur more processing overhead than static HTML pages, they tend to execute much faster than ASP pages.

You probably noticed the RunAt="server" attributes sprinkled throughout Calc.aspx. RunAt="server" is the key that unlocks the door to the magic of Web forms; it signals ASP.NET to "execute" the tag rather than treat it as static HTML. RunAt="server" is not optional. It must be used in every tag that ASP.NET is to process, including the <form> tag that marks the beginning of a form containing server controls.

Web Controls

TextBox, Button, and Label are server controls. They're also examples of Web controls-server controls defined in the FCL's System.Web.UI.WebControls namespace. The Web controls family includes almost 30 different control types that you can use in ASP.NET Web forms. The following table lists the Web controls provided in version 1.0 of the .NET Framework class library:

Web Controls

Class Name	Description
AdRotator	Displays rotating banners in Web forms
Button	Generates submit buttons
Calendar	Displays calendars with selectable dates
CheckBox	Displays a check box in a Web form
CheckBoxList	Displays a group of check boxes
CompareValidator	Validates user input by comparing it to another value
CustomValidator	Validates user input using the algorithm of your choice
DataGrid	Displays data in tabular format
DataList	Displays items using single column or multicolumn lists
DropDownList	Generates HTML drop-down lists
HyperLink	Generates hyperlinks
Image	Displays images in Web forms
ImageButton	Displays graphical push buttons
Label	Generates programmable text fields
LinkButton	Generates hyperlinks that post back to the server
ListBox	Generates HTML list boxes
Literal	Generates literal text in a Web form
Panel	Groups other controls
RadioButton	Displays a radio button in a Web form
RadioButtonList	Displays a group of check boxes
RangeValidator	Verifies that user input falls within a specified range
RegularExpressionValidator	Validates user input using regular expressions
Repeater	Displays items in simple lists
RequiredFieldValidator	Verifies that an input field isnt empty
Table	Generates HTML tables
TextBox	Generates text input fields
ValidationSummary	Displays a summary of validation errors
Xml	Displays XML documents and documents generated from XML using XSLT

Some Web controls are simple devices that produce equally simple HTML. Others produce more complex HTML, and some even return client-side script. Calendar controls, for example, emit a rich mixture of HTML and JavaScript. It's not easy to add a calendar to a Web page by hand (especially if you want dates in the calendar to be clickable), but calendars are no big deal in Web forms: you simply include an <asp:Calendar> tag in an ASPX file. DataGrid is another example of a sophisticated control type. One DataGrid control can replace reams of old ASP code that queries a database and returns the results in a richly formatted HTML table. You'll learn all about the DataGrid and other Web controls in the next chapter.