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

This also happens when a lower library is using a different versionof the .NET Framework. Had a similar issue and when I updated the Lower Libraries to 3.5 framework and the actual library to the 3.5 framework the problem went away.

Answer 2

This also happens when an assembly is missing which is referenced by some other assembly in your project