Question

I have a Foo entity in Entity Framework. But I’m making it inherit from IFoo so that my business logic only knows IFoo – thus abstracting Entity Framework away.

The problem is that Foo has a collection of Bar entities. And this collection is of type EntityCollection<Bar> .

If I put this collection in IFoo as it is, I make IFoo dependent on Entity Framework. So I thought of putting it as ICollection<IBar>, but this doesn’t compile (naturally).

The only solution I can think of is to go to the concrete Foo implementation generated by the Entity Framework designer and change the collection from EntityCollection<Bar> to ICollection<IBar> there. But I dread the thought of the implications this will have on Entity Framework ‘behind the scenes’.

Is there any way for me to define IFoo and IBar independently of Entity Framework while still maintaining Foo and Bar as EF Entities that implement them? Do IFoo and IBar even make sense, if I cannot achieve this independence that I aim for?

Answer 1

The general concept you are referring to is ‘persistence ignorance’ (PI), although that generally applies directly to entities themselves rather than the code that consumes the entities.

In any case, Hibernate and NHibernate natively support PI, but the initial version of Microsoft’s Entity Framework does not. MS caught a lot of flak for this and PI is probably the #1 most discussed feature for the next version (whenever that is).

As far as what you are trying to do with interfaces, does the collection of Bars need to be modified after it is retrieved? If the answer is yes, there is no easy answer. Even covariance couldn’t help you here because ICollection<T> has an Add method.

If the collection is read-only, then you might consider exposing it as IEnumerable<IBar>. The Enumerable.Cast method makes this fairly convenient.

interface IFoo {     IEnumerable<IBar> Bars { get; } }  partial class Foo : IFoo {     IEnumerable<IBar> IFoo.Bars     {         get { return Bars.Cast<IBar>(); }     } } 

Also, I know of at least one effort to make the current version of EF support persistence ignorance.