Could not load type ‘System.Web.UI.ScriptReferenceBase’ from assembly

By | May 13, 2013
While deploying my application on a production server, I was facing the following issue:Exception: System.TypeLoadException
Message: Could not load type ‘System.Web.UI.ScriptReferenceBase’ from assembly 

‘System.Web.Extensions, Version=, Culture=neutral, PublicKeyToken=31bf3856ad364e35’.
Stack Trace:    at AjaxControlToolkit.ToolkitScriptManager.OnResolveScriptReference
(ScriptReferenceEventArgs e)
By looking into above details, it clearly shows that there is some problem in loading AjaxControlToolkit assembly.I did some online search for issue and verified all possible things on production server:

  • Verify the correct version of AjaxControlToolkit dll on server.
  • Verify the assembly entry in <assemblies> section in configuration file on server with correct version.
But finally find out the missing thing on production server. Production server was missing
ServicePack 1 for .NET 3.5. Because my local machine has .NET 3.5 SP1 where the release is build.
Although It took sometime to convince my System Admin for installing Service Pack 1 for .NET 3.5 on production machine :-).
So, In order the fix the above issue, verify the above mentioned items and install .NET 3.5 Service Pack 1.

Category: Ajax

About Web Development

Imran Abdul Ghani is working as Software Developer(Senior) with extensive knowledge in Web development technologies especially C#, ASP.NET, MVC, WCF, Web API, ADO.NET Entity Framework, jQuery etc. He has several years of experience in designing/developing enterprise level applications. He is Microsoft Certified Solution Developer for .NET (MCSD.NET) since 2005. You can reach his blogging at,, and

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