ASP.Net Error: Could not load file or assembly ‘System.Web.Extensions…’


Recently I deployed an ASP.Net application to a production server and received an error message that began, “Could not load file or assembly ‘System.Web.Extensions…'”.

The Reason  for the Error

The application uses Microsoft ASP.NET 2.0 AJAX Extensions 1.0, but they had not been installed on the production machine.

The Obvious Solution

The obvious solution was to download the ‘ASPAJAXExtSetup.msi‘ installer from Microsoft and run it on the live server. The required .dll files would then be inserted into the GAC.

The Hitch 

The production machine is a Windows 2000 server. In order to access these files, the IIS instance on the server would need to be restarted. Unfortunately, this was not an option. The target server hosts various applications that cannot be brought down without disrupting the users.

The Final Solution

The development machine did not exhibit the problem, so this machine obviously had the dll files present. The simple answer is to place a copy of these files in the application’s ‘bin’ folder on the production server. When I did this, the error message went away immediately.

So, what files did I need, and where did I find them?
Well, since the installer copies the dlls into the GAC, they can obviously be found there.

For reference, the GAC on my machine is at:


Unfortunately, you can’t see the individual files using Windows Explorer. However, the files can be viewed using the command prompt. It is a bit of a messy process, but it is possible to use the Command Prompt to extract the files.

The good news is that there is an easier way. A colleague pointed out that the installer also places another copy of these files outside the GAC. From this location, they can simply be copied and pasted into the bin folder of the target application.

On my machine the required files are here:

C:\Program Files\Microsoft ASP.NET\ASP.NET 2.0 AJAX Extensions\v1.0.61025\

The files I needed were:


Many thanks to GK for pointing this out to me.


59 Responses to ASP.Net Error: Could not load file or assembly ‘System.Web.Extensions…’

  1. abdulrehman says:

    please send me this dll i really need it

  2. kramii says:


    I am pleased that you found this post informative.

    I am sorry, I cannot send you the files at the moment. If you really need them, you can install AJAX and extract them yourself. It really isn’t hard. See the post above for details on how to achieve this.

  3. I was having the same problem and this solved it. Thank you for posting your solution, it was very helpful.

  4. kramii says:

    Dennison Uy – Graphic Designer:

    I am delighted that you found this post useful.

  5. Chris Peters says:

    I am VERY glad to come to this page to find definitive, no-nonsence answer. I resolved my problem using information provided here. Greate job. Guess what – You are bookmarked!

  6. Will says:

    Thanks, this worked great. I tried for 20 minutes to copy the dlls from c:\WINDOWS\assemblies\ with no luck. The copies were right where you said they would be, and once i copied them to my remote application’s /bin/ folder, the application sprang to life!



  7. Nelson says:

    This fixed part of the problem – I am trying to use the new ASP.NET 3.5 Extensions.
    Now I am getting a permission error:

    Configuration Error
    Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.

    Parser Error Message: Could not load file or assembly ‘System.Web.Extensions, Version=, Culture=neutral, PublicKeyToken=31bf3856ad364e35’ or one of its dependencies. Failed to grant minimum permission requests. (Exception from HRESULT: 0x80131417) (D:\inetpub\oldebeaufortseaport\bbc\web.config line 79)

    Source Error:

    Line 77:
    Line 78:
    Line 79:
    Line 80:
    Line 81:

    Source File: D:\inetpub\oldebeaufortseaport\bbc\web.config Line: 79

  8. kramii says:

    Will & Chris Peters:

    I am pleased that the article helped you. Thank you both for taking the time to give me positive feedback.


    I am sorry that things didn’t work out for you. Unfortunately, I don’t have ASP.Net version 3.5 (it won’t even load on my PC) so I am unable to reproduce the problem. My best guess is that a different version of the dll’s is required for later versions of ASP.Net? I hope you find an answer.

  9. Pranay says:

    Thanks a lot! that resolved my problem!!

  10. sujith says:

    hi thr.. it worked for me.. thanks

  11. Howzilla says:

    This post solved most of my problems but upon going to the parking lot my car was still dirty and the gas tank was low. When I got home the dog had crapped everywhere and got in the refridgerator and drank all my beer. Got a dll for that? Thanks dude.

  12. Steve says:

    You just spared me about an hour of fruitless Googling. Thank you, sir!

  13. gangs says:

    That worked for me. I owe you a beer man.

  14. kramii says:

    Pranay / sujith / Howzilla / Steve / gangs:

    Thanks for your feedback. I appreciate you taking the trouble. I’m glad the article helped.


    LOL! I can’t help with the car, but for the messy beer-drinking dog that can open the refrigerator on its own, I suggest you re-install ObedienceTraining.dll and then register her with the circus – she would go down a storm with the clowns. Alternatively, you could always just try switching her off and then back on again…

  15. CrazyRabbit says:

    Thanks For the tips used 2 out of the 3 given. I installed the msi file on my dev machine and copied the 2 dll’s into my bin folder for the production server. Both solutions worked, no mess no fuss.

    Thanks 😉

  16. Thank a lot. I have always been troubled by the error. Finally I have found the good explanation. Thank you again. In fact, I am always wandering between the the errors given by the use of ajax in vs 2005. But in VS 2008, most errors do not occur like in vs 2005. Thanks.

  17. Thanks!


    PS Howzilla and kramii – thanks for the funnies, brought tears to my eyes 🙂

    PPS you really need to install either ToiletTrainDog.dll or GetANappy4Dog.dll to fix the crapping issues!

  18. ar! says:

    Thank you very much! this post help me a lot!

  19. jerry says:

    Thanks a lot
    we used the concept to fix problem in our gogrid server.

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  28. Tom says:

    Unfortunately, in .NET 3.5, these files have been superseded and require replacement…..

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