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Editorial Team
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Asked: 2026-05-16T21:44:20+00:00

I am trying to build an ASP.NET project, but Visual Studio keeps giving me

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I am trying to build an ASP.NET project, but Visual Studio keeps giving me the error

(0): Build (web): Could not load file or assembly 'System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

The System.Web.Extensions.dll assembly is located in my GAC (and I added it to the bin folder just to be sure that it wasn’t having trouble finding it). The interesting thing to note here is that in my web.config I reference the assembly like so:

<add assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>

but, while this references Version=3.5.0.0, when the build is attempted, it is evidently looking for Version=1.0.61025.0.

So far, I have tried copying the assembly to the bin folder, referencing that assembly in the bin folder through Add Reference, and checking to make sure that the target framework is .NET 3.5 (which it appears to be). Can anyone tell me what is going on here?

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1 Answer

  1. Editorial Team

    System.Web.Extensions.Design, Version=1.0.61025.0 is an assmebly included
    in ASP.NET Ajax Extension component. In order to use it, you have to
    install it in your development environment.

    If you have not installed it before, please download and install it at
    http://www.microsoft.com/downloads/details.aspx?FamilyID=ca9d90fa-e8c9-42e3-aa19-08e2c027f5d6&displaylang=en

    After installation, please restart Visual Studio and build the solution
    again.

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