.Net 4.0 introduces the concept of Client Profile. I came to know about it when I was adding references to my project file. Even though, I had added a few assemblies in the project reference, the compiler was complaining of not finding the namespaces in the references that I have added during compilation.
The project was a Windows Form Application. I opened up the Project Settings Page to see that the
Target Framework was .Net 4.0 Client Profile.
I changed it to .Net 4.0 Framework. Everything worked fine.