The ASP.NET Web Forms page framework is a scalable common language runtime programming model that can be used on the server to dynamically generate Web pages.
Intended as a logical evolution of ASP (ASP.NET provides syntax compatibility with existing pages), the ASP.NET Web Forms framework has been specifically designed to address a number of key deficiencies in the previous model. In particular, it provides:
The ability to create and use reusable UI controls that can encapsulate common functionality and thus reduce the amount of code that a page developer has to write.
The ability for developers to cleanly structure their page logic in an orderly fashion (not "spaghetti code").
The ability for development tools to provide strong WYSIWYG design support for pages (existing ASP code is opaque to tools).
This section of the QuickStart provides a high-level code walkthrough of some key ASP.NET Web Forms features. Subsequent sections of the QuickStart drill down into more specific details. Writing Your First Web Forms Page
ASP.NET Web Forms pages are text files with an .aspx file name extension. They can be deployed throughout an IIS virtual root directory tree. When a browser client requests .aspx resources, the ASP.NET runtime parses and compiles the target file into a .NET Framework class. This class can then be used to dynamically process incoming requests. (Note that the .aspx file is compiled only the first time it is accessed; the compiled type instance is then reused across multiple requests).
An ASP.NET page can be created simply by taking an existing HTML file and changing its file name extension to .aspx (no modification of code is required).
Important: Note that nothing happens yet when you click the Lookup button. This is because the .aspx file contains only static HTML (no dynamic content). Thus, the same HTML is sent back to the client on each trip to the page, which results in a loss of the contents of the form fields (the text box and drop-down list) between requests. Using ASP <% %> Render Blocks
ASP.NET provides syntax compatibility with existing ASP pages. This includes support for <% %> code render blocks that can be intermixed with HTML content within an .aspx file. These code blocks execute in a top-down manner at page render time.
Important: Unlike with ASP, the code used within the above <% %> blocks is actually compiled--not interpreted using a script engine. This results in improved runtime execution performance.
ASP.NET page developers can utilize <% %> code blocks to dynamically modify HTML output much as they can today with ASP.
Important: While <% %> code blocks provide a powerful way to custom manipulate the text output returned from an ASP.NET page, they do not provide a clean HTML programming model. As the sample above illustrates, developers using only <% %> code blocks must custom manage page state between round trips and custom interpret posted values. Introduction to ASP.NET Server Controls
In addition to (or instead of) using <% %> code blocks to program dynamic content, ASP.NET page developers can use ASP.NET server controls to program Web pages. Server controls are declared within an .aspx file using custom tags or intrinsic HTML tags that contain a runat="server" attribute value. Intrinsic HTML tags are handled by one of the controls in the System.Web.UI.HtmlControls namespace. Any tag that doesn't explicitly map to one of the controls is assigned the type of System.Web.UI.HtmlControls.HtmlGenericControl.
Important: Note that these server controls automatically maintain any client-entered values between round trips to the server. This control state is not stored on the server (it is instead stored within an form field that is round-tripped between requests). Note also that no client-side script is required.
In addition to supporting standard HTML input controls, ASP.NET enables developers to utilize richer custom controls on their pages.
Handling Server Control Events
Each ASP.NET server control is capable of exposing an object model containing properties, methods, and events. ASP.NET developers can use this object model to cleanly modify and interact with the page.
