FIrst, be sure to read my post “When is EF5 not EF5” so you understand the basis for this post.

When you use EF5, regardless of targeting .NET 4 or .NET 4.5, you’ll find the special EF related data annotations (such as MaxLength and NotMapped) in the System.ComponentModel.DataAnnotations.Schema namespace.

In EF4, they were in System.ComponentModel.DataAnnotations namespace