Introduction :

          I found two posts in ASP.NET MVC forums complaining that they are getting exception, The type XXX is defined in an assembly that is not referenced, after upgrading thier application into Visual Studio 2010 and .NET Framework 4.0 at here and here .

  Description :

           The reason why they are getting the above exception is the use of new clean web.config without referencing the assemblies which were presents in ASP.NET 3.5 web.config. The quick solution for this problem is to add the old assemblies in new web.config.

          <assemblies>
             <add assembly="System.Web.Abstractions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
             <add assembly="System.Web.Routing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
             <add assembly="System.Web.Mvc, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
              <add assembly="System.Data.Entity, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
              <add assembly="System.Data.Linq, Version=4.0.0.0, Culture=neutral, publicKeyToken=b77a5c561934e089" />
          </assemblies>

   How It works :

           Currently i have not tested the above scenario in ASP.NET 4.0 because i have not yet get it. But the above scenario can easily be tested and verified in VS 2008. Just Open the root web.config and remove

          <add assembly="System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>

            Even you add the reference of System.Core in your project, you will still get the above exception because aspx pages are compiled in separate assembly. You can check this yourself by checking Show Detailed Compiler Output: below in the yellow screen of death, you will find something,

/out:"C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\e907aee4\5fa0acc8\App_Web_y5rd6bdg.dll"

             This shows that aspx pages are compiled in separate assembly in Temporary ASP.NET Files.

Summary :

             After getting the above exception make sure to add the assemblies in web.config or add the Assembly directive at Page level. Hopefully this will helps to solve your problem.