Upgrade to 4.0 Framework, then downgraded, now can't find Linq namespace?

Question

I just upgraded from VS2008 to VS2010. In the process, I allowed it to upgrade my DLL's to 4.0. I've decided that wasn't a good idea and now I've rolled back.

It all works fine in VS2010, but when it hits my CI server (CruiseControl.Net), I get an error of:

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

3.5 is on the server, and other projects that use 3.5 build fine, so I don't think it's a framework issue.

Anyone have any hints?

Answer 1

Make sure you have a reference to System.Core.dll.

Answer 2

I do have a reference to System.Core, but it turned out for one of my projects, when I downgraded, it did not add this bit back into my project file:

<Reference Include="System.Core">
  <RequiredTargetFramework>3.5</RequiredTargetFramework>
  <Private>False</Private>
</Reference>

The key part there being the RequiredTargetFramework. I was clued into this because it wasn't only Linq that wasn't being found, but also Func<> and Extension method attributes.

It was all 3.5 stuff that was missing.