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Editorial Team
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Asked: 2026-05-31T01:50:51+00:00

I have an old ASP.NET web application on a server, which was developed in

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I have an old ASP.NET web application on a server, which was developed in Visual Studio 2008 in the year 2008. The server now has .NET 4.0 installed also. When I try to access the web app I get:

Compilation Error
Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.

Compiler Error Message: CS1703: An assembly with the same identity 'System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' has already been imported. Try removing one of the duplicate references.

Source Error:

[No relevant source lines]

Is there anything I can do to get it to work without access to the original Visual Studio solution? I believe it has to do with the fact that .NET 4.0 is also installed on the server.

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1 Answer

  1. Editorial Team

    1) Have you checked the application pool your application is running under? Is it still .net 2.0 integrated/classic, or has it automagically changed to 4.0? If so, try changin back to 2.0.

    2) Do you by any chance have a System.Web.Extensions.dll in the bin folder of the application? If so, try renaming it (similar situation as here)

    3) If 1 is not an option and 2 is not the case, do you have any explicit references to the System.Web.Extensions in the app’s config file? Are they requesting a different version of the file?

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