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 others here, if the referenced assembly is targeted to 'Any CPU', while the current assembly is targeted to 'Any CPU' will cause the problem (at least on a 64 bit machine).