I've got an ASP.NET Model-View-Presenter website that was generated by the WCSF (Windows Client Software Factory). When it created the solution a few weeks ago, WCSF laid down my test project, yet didn't automatically include a reference to System.Core.dll. Aside from a "The type or namespace name 'Linq' does not exist in the namespace 'System'" error, this wouldn't typically cause me problems in a test project. In fact, when it first occurred, I removed the "Using System.Linq;" directive and rebuilt and didn't think about it again.
Until I decided to use Moq.
I was starting to think that I'd been sipping on grampy's cough medicine, when my test class recognized the Moq.Mock class, yet when I'd call Setup() and try to enter my predicate, it wouldn't allow a predicate to be typed - nor would it give me Intellisense for any of my interface methods.
After hunting around for a while, I figured out that the System.Linq error message that I saw a few weeks ago was the root of the problem. Without a reference to System.Core.dll, the Linq libraries that support predicate recognition weren't loaded, and I wasn't able to run my mock setups.
Ahh, good times with WCSF.