I just ran into an odd issue with a VS2010 project. In my case it was an MVC 2 application I was upgrading from VS2008. One of the built-in controllers (ProfileController) was failing to compile because it could not resolve the Linq extension method symbols Single() and Matches(). These are located in the System.Core assembly. I checked my project references in Solution Explorer, and System.Core was not listed. So I tried Add Reference, and System.Core was listed as included and gave me the option of to Remove it.
After some searching, I found this blog post, aptly named Do NOT remove the reference to System.Core from your VS2010 Project. It pointed out that this file is special and that if you remove it (or it gets lost somehow) you need to re-add it manually to your project using this syntax:
<ItemGroup>
<Reference Include="System.Core" />
</ItemGroup>
There is also a connect issue describing this bug (which is currently marked as Postponed), Cannot remove System.Core.dll reference from a VS2010 project.
In my case this appears to have been an upgrade issue, as the project worked fine in VS2008 before I ran the upgrade wizard to VS2010, and then in the course of trying to get it to compile for the first time, I ran into this issue. Adding the assembly to the project file by hand (via Notepad) solved the problem in my case.
