The type EntityState is defined in an assembly that is not referenced. You must add a reference to assembly System.Data.Entity

The type 'EntityState' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Data.Entity, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'. 

I got the following error on my web controller page.Finally I do the following changes to my project. It works fine.

1.      Right click on your project.

2.      Choose Add Reference

3.      On .NET tab, select reference System.Data.Entity and click OK