ASP.NET AJAX, formerly code-named Atlas, is a set of extensions to ASP.NET developed by Microsoft for implementing Ajax functionality.

Including both client-side and server-side components, ASP.NET AJAX allows the developer to create web applications in ASP.NET 2.0 (and to a limited extent in other environments) that can update data on the web page without a complete reload of the page. The key technology which enables this functionality is the XMLHttpRequest object, along with Javascript and DHTML.

ASP.NET AJAX was released as a standalone extension to ASP.NET in January 2007 after a lengthy period of beta-testing. It was subsequently included with version 3.5 of the .NET Framework, which was released alongside Visual Studio 2008 in November 2007.

The core technology builds upon the same concept as Anthem.NET, which is one of the earliest serious implementations of WYSIWYG ASP.NET Ajax Technology. After an Ajax request, the HTML within some portions of the page (for ASP.NET Ajax this is the "update panel") are being completely re-rendered with new HTML and therefore it's very hard to keep client side state across Ajax Server Side event handlers. This can be seen by the fact that the Auto Completer in the ASP.NET Ajax Control library (which is an extension to ASP.NET Ajax) has to use WebServices to retrieve data instead of the far more intuitive way which would have been to use a server side event handler embedded as a method into the page the control resides within. A totally different approach can be seen in Gaia Ajax Widgets which has in fact mirrored every server side control's properties and methods in Client Side JavaScript and can therefore keep state across server side Ajax methods and still be able to manipulate control properties/values. This is far more difficult to do but reduces the bandwidth by far and also makes it possible to keep client side state across Ajax Callbacks.

Name change and road map

On September 11, 2006, Scott Guthrie, the General Manager in charge of the .NET platform, announced that ATLAS would be renamed, and launched as three products towards the end of the year. The new products will be called the Microsoft AJAX Library, containing the front end javascript library, the ASP.NET 2.0 AJAX Extensions, containing the server-side .NET code, and ASP.NET AJAX Control Toolkit, containing shared source controls that can be used to "get the most value from the ASP.NET AJAX Extensions".

A major change since its inception includes the change of namespace from Microsoft.Web.* to System.Web.*

They released the production version on 23 Jan 2007.

Browser support

ASP.NET AJAX supports multiple modern web browsers such as Internet Explorer, Firefox, and Safari. Mobile devices such as Windows Mobile are currently not supported.