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We recently started getting an error when viewing a Crystal Report in a web based system Applogix developed and have been enhancing for some time now. We originally developed the system in Visual Studio 2005, (.net framework 2.0, Crystal Reports 2005) and recently upgraded our solution to Visual Studio and Crystal Reports 2008, but kept the target framework as .net 2.0.

The error does not occur locally on my development machine (Vista, VS2008) but happens once the solution is installed on out test servers: Server 2003, Windows XP etc:

Could not load file or assembly ‘System.Web.Extensions, Version=2.0.0.0

‘System.web.extensions’ refers to Microsoft Ajax.net components, so at face value the error seems relatively simple: you have not installed the ASP.NET 2.0 AJAX Extensions 1.0 on your destination server – so if you are currently troubleshooting a .net 2.0 app with this error make sure you have done that:

Download: http://go.microsoft.com/fwlink/?LinkID=77296 and install it on your server.

You can confirm that you have the Extensions installed by going to c:\Windows\Assembly and looking for System.web.extensions  with a version of 1.0.61025.0

NB – I know the error message said it could not find version 2.0.0.0 – but v 1.0.61025.0 is the correct one – the fact that your site is looking for a version that does not exist is part of the issue. » Read more: Solution: Could not load file or assembly ‘System.Web.Extensions, Version=2.0.0.0