AJAX Frameworks for ASP.NET

If you are planning to AJAX-enable a ASP.NET website, a good option is of course the ASP.NET AJAX Framework from Microsoft due to it's better integration with the Visual Studio IDE and documentation. Occasionally I see questions on the Forums from folks having issues making ASP.NET AJAX work. Very often this is because they have not picked the correct version of ASP.NET AJAX to go with the ASP.NET Framework version.

So here is a quick summary. If your web application is based on ...

• ASP.NET 3.5 - you need ASP.NET AJAX 3.5 and it comes built-in with VS 2008. If you need specialized controls from the AJAX Control Toolkit use Toolkit version 3.5. If you are specifically building on top of .NET Framework 3.5 SP1 using Visual Studio 2008 SP1, use Toolkit version 3.5.20820. (System.Web.Extensions, Version= 3.6.0.0 corresponds to ASP.NET 3.5 Extensions December CTP)
  
• ASP.NET 2.0 - you need ASP.NET AJAX 1.0 and you have to manually install it in VS 2008 or 2005 and make sure the web.config of that application correctly points to the 1.0 assembly. Toolkit version 1.0 will only work with ASP.NET AJAX 1.0.
• ASP.NET 1.1 - ASP.NET AJAX is not supported. You would have to choose a Framework like the free, cross-browser Anthem.NET which works works with both ASP.NET 1.1 and 2.0.

For implementing simple AJAX functionality in any of the ASP.NET Framework versions, you can also consider using jQuery. Some of the jQuery plugins fare better than the Extenders in the ASP.NET AJAX Control Toolkit.

If you want to compare AJAX Frameworks for ASP.NET, this outdated article on the comparative features of various Frameworks can give you some pointers to base your research on.

Related links:
Free ASP.NET AJAX E-learning course
HOW TO implement AutoComplete with jQuery & ASP.NET
Is ASP.NET AJAX truly cross browser?