C# Error “Is not supported by the language” after migration to .Net4

Question

I\'m trying to migration our website from .Net 3.5 to 4 and I\'m encountering a very weird issue.

Code that works just fine in 3.5 does not anymore once I target .Net4,

Answer 1

Similar to some of the other posts, in my case I was completely missing a reference to an assembly. It wasn't directly accessed from the project I was working with, but was accessed in another, linked project that I was referencing.