I started getting a compilation error telling with a message telling me "CS0012: The type 'System.Data.Objects.DataClasses.EntityObject' 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'". This really confused me since the solution built fine but the error only occurred at run time.