In one of my projects I saw that the Microsoft.Csharp reference is not found.
It was strange because for other projects the reference is resolved fine.
Resolution: Somehow my current project framework got pushed to v3.5.
I changed it to v4.0 and that fixed the issue.
It is important that you save the changes before the project framework is changed because the project will re load and you may lose any unsaved work.
Random Thoughtz 