EF и расширение классов

Extending Partial Data Classes (link)

Types that are defined in an EDM do not have associated methods like the classes used in object-oriented programming. Instead, they only contain properties that are defined in the EDM. You can add functionality to objects by extending the generated partial data classes. When data classes are generated by the Entity Data Model tools, they are implemented in partial classes. A partial class splits the definition of a class over two or more source files. Each source file contains a section of the class definition, and all sections are combined when the application is compiled. For more information, see Partial (Visual Basic) or partial (Type) (C# Reference).

Having partial classes enables you to extend these classes with custom methods and properties in a separate source file without having to worry about losing your customization when the generated files are refreshed.