Upgrade to 4.0: The type or namespace name 'Linq' does not exist in the namespace 'System' (are you missing an assembly reference?)

Question

I recently upgraded my solution from .net framework 3.5 to 4.0. All good except it gives me some errors in my website project.

The type or namespace name 'Linq' does not exist in the namespace 'System' 
(are you missing an assembly reference?)

The type or namespace name 'Linq' does not exist in the namespace 'System.Xml'
(are you missing an assembly reference?)

The type or namespace name 'Script' does not exist in the namespace 'System.Web' 
(are you missing an assembly reference?)

Here is what my web.config looks like:

<add assembly="System.Core, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
<add assembly="System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/>
<add assembly="System.Data.DataSetExtensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
<add assembly="System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
<add assembly="System.Web.Extensions.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
<add assembly="System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/>
<!--vjslib is needed for Google Transit-related code-->
<add assembly="vjslib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/>
<add assembly="Infragistics35.WebUI.Shared.v10.2, Version=10.2.20102.1011, Culture=neutral, PublicKeyToken=7dd5c3163f2cd0cb"/>
<add assembly="Infragistics35.WebUI.UltraWebGrid.v10.2, Version=10.2.20102.1011, Culture=neutral, PublicKeyToken=7dd5c3163f2cd0cb"/>
<add assembly="Infragistics35.WebUI.UltraWebTab.v10.2, Version=10.2.20102.1011, Culture=neutral, PublicKeyToken=7DD5C3163F2CD0CB"/>
<add assembly="Infragistics35.Web.v10.2, Version=10.2.20102.1011, Culture=neutral, PublicKeyToken=7DD5C3163F2CD0CB"/>
<add assembly="System.Web.Mobile, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"/>
<add assembly="mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
<add assembly="System.Xml, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
<add assembly="System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
<add assembly="System.Xml.Linq, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>

Why is it not able to find LINQ reference when i say using System.Linq or System.Xml.Linq? What am i missing here?

--Edit--

This happens only in website project. Other class libraries work fine.

Answer 1

I have a ASP.NET 3.5 WebSite project which I've tried to upgrade to .NET v4. Got the error. My solution:

1. Go to WebSite project property pages and set it back to . NET v3.5
2. Compile and see all is OK
3. Switch it back to .NET v4
4. Compile and see all is OK