I was playing with AJAX enabled applications for some time but not really done any thing major yet. I was working on a small project that required me to add some AJAX functionality using simple updatepanels. I dropped the necessary controls like scriptmanager and updatepanel in to the page and added the references to System.Web.Extensions.

I ran the application and ran in the error message mentioned above.

Error: “Sys” is Undefined Error

I read some blogs and added another reference to “System.Web.Extensions.Design” and added some

<handler> tags to the web.config file.

Nothing seemed to work and I still had the issue.

I got smart at this time and created a new AJAX enabled web site in Visual Studio and copied the web.config file over to my existing application.

The error message is gone!!!