AJAX fun with the dreaded 'sys is undefined' error

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AJAX fun with the dreaded 'sys is undefined' error

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If you have started to play with AJAX then you have most likely come across the dreaded 'sys is undefined' error.  I found numerous articles on how to get this working but the best ones all point to making sure you have the correct entries in your web.config.   So to summarize what I have read:

1)  Go to C:\Program Files\Microsoft ASP.NET\ASP.NET 2.0 AJAX Extensions\{your version of AJAX}

2)  Open the sample web.config file and make sure you have the same entries in your web.config

3)  If you don't then copy and paste is your friend :)

4)  See if the app works now

 Here are a couple of articles to assist if you are still having issues:

Chris Riccio's blog   http://weblogs.asp.net/chrisri/archive/2007/02/02/demystifying-sys-is-undefined.aspx

Lorin Thwaits's blog  http://geekswithblogs.net/lorint/archive/2007/03/28/110161.aspx


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  • I changed as per your instructions but it doesn't work. But i found a soltuion read below:

    This problem comes if your web.config file is not in root folder or the folder is not an application in IIS. To solve this problem open IIS manager and browse to folder where your web.config is and right click the folder and choose properties from popup menu. Now if application name is not there then click on create button and click ok. That's all it will work.

  • Thanks Raj.  I couldn't figure this out for a few hours, and finally saw your post.  I had my web.config in a subfolder instead of the root folder the website.



  • Thanks, this worked.

  • It turns out for me, I had Macromedia coldfusion installed on the same server.  This required me to add aspnet_isapi.dll as a wildcard handler or ScriptResource.axd returned a 404

  • Thanks alot.  This is one post that actually had the solution.

  • Happy to help :)

  • I've been trying to figure out how to use AJAX based on another application from my company, and this was the last step I couldn't figure out.  Thanks so much for saving my sanity.

  • Thank you. It's work and easy. :)

  • I was having a tough time with this error. I was running this on IIS 7 on Vista and had already changed my app pool to "Classic" for previous AJAX functionality in the application. For some reason, when I changed the app pool back to default, restarted IIS then changed it BACK again to "Classic" everything worked fine. Apparently you have to make the app pool change after all AJAX functionality is added. Hope this helps someone.

  • Just another little one I came across when trying to add some AJAX/UpdatePanels to an existing web application.  The web.config file had the following line which caused the Sys.WebForms undefined problem in this case.

    <xhtmlConformance mode="Legacy"/>

    Since I didn't see that mentioned anywhere I thought I would throw it out for the rare case when someone else runs across it.

  • You need to create 2 empty files in your .net project:



    That fixed the 'sys' problem for me.

  • Hi

    I have changed sharepoint site web.config according to the web.config from %Program Files%\Microsoft ASP.NET\ASP.NET 2.0 AJAX Extensions\%version%\

    Now its working fine



  • yay, this really works!


  • Thank You!  This one had me scratching my head.  Updading the entries in the web.config solved the problem.

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