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

Similiar to Jonathan Perry's answer, in my case I had a reference to an old assembly, not the compiled one. I removed the reference and added it again pointing to the correct dll.