When you first open an ASP.NET 2.0 website in Visual Web Developer / Visual Studio 2008, VS asks you if you want to upgrade the project to 3.5.
As some of you who are using Microsoft AJAX and who say YES to upgrade to 3.5 have noticed, when you go to run your newly upgraded application (which worked fine before the upgrade), you get build errors !!!
The build error complains that it can't load System.Web.Extensions Version 1.0.61025.0
That's the OLD version of Web.Extensions (AJAX) that works with ASP.NET 2.0.
You'll note a number of references in your web.config file to this old version. Since before the 2008 release wave, MS AJAX was loaded via separate assemblies and since it is now "baked in" to ASP.NET, it's hard for the upgrade tool to know that you don't need the referenced version.
So..... In your web.config change all the 1.0.61025.0 version references to 188.8.131.52 (unless you are using some CTP or Futures Version)
If you are using the AJAX Control Toolkit there is still a bit of work to do.
You will now get this error on your Toolkit Control instances.
Download the version of the AJAX Control Toolkit that is built for ASP.NET 3.5
Note: There are separate versions for ASP.NET 2.0 projects and ASP.NET 3.5 projects.
If your are running Windows Vista, Right-Click the .zip file, select properties, and "Unblock" the file.
Then unzip the files to the location of your choice.
Make sure the Visual Studio Toolbox Tab that you create for the controls is with the 3.5 version.
Now you need to upgrade your project to use the new tool kit. The easiest way to do this is to let Visual Studio do it for you.
SWITCH TO SOURCE VIEW and drag and drop any Ajax Control Toolkit control into your page.
When you get this dialog SELECT APPLY TO ALL ITEMS and click yes.
Then delete the control you just added.
Visual Studio will have updated your controls reference and you should now be good to go !