Question

I have the following entities:

public interface IMyEntity
{
    [Key]
    int Id { get; set; }
    IMyDetail MyDetail { get; set; }
    ICollection<IMyDetail> CollectionOfReferences { get; set; }
}

public interface IMyDetail
{
    [Key]
    int Id { get; set; }
    int IntValue { get; set; }
}

public class MyEntity : IMyEntity
{
    [Key]
    public virtual int Id { get; set; }
    public virtual IMyDetail MyDetail { get; set; }
    public virtual ICollection<IMyDetail> CollectionOfReferences { get; set; }
}

public class MyDetail : IMyDetail
{
    [Key]
    public virtual int Id { get; set; }
    public virtual int IntValue { get; set; }
}

I want to use EF CodeFirst to access the database and to create database schema. But CodeFirst doesn’t allow to use interface types for relations between entities. Therefore it doesn’t create relation between MyEntity and MyDetail. I can’t change interfaces therefore I can’t change the type of property to MyDetail instead of IMyDetail. But I know that the client of this model will use only one implementation of each interface.

I’ve found a workaround for properties of type IMyDetail. I can create a property of type MyDetail and explicitly implement property of interface:

    private MyDetail _myDetail;

    public virtual MyDetail MyDetail
    {
        get
        {
            return this._myDetail;
        }
        set
        {
            this._myDetail = value;
        }
    }

    IMyDetail IMyEntity.MyDetail
    {
        get
        {
            return this._myDetail;
        }
        set
        {
            this._myDetail = (MyDetail)value;
        }
    }

It works fine. But this solution doesn’t work with ICollection<IMyDetail> because I can’t cast it to ICollection<MyDetail>.

Are there any solutions for this?

Answer 1

An imperfect solution is to just merge these interfaces you want to persist into base classes and break down the underlying objects with subclasses. EF does support this, and if you go with Table Per Hierarchy (the default), you can sort all of the underlying subclassed objects by a shared property using a regular LINQ query from EF instead of having to get crafty and do things like write raw SQL or get multiple lists into memory and sort the union without the help of the DB, like you would with Cel’s solution of interfaces and adapters.

You could also take the child/parent types of interfaces as generics, so that when the implementer uses concrete classes in the Db they can mostly use your interfaces, but tell EF to use concrete classes:

public interface IParent<out TChild>
    where TChild : IChild
{
    ICollection<TChild> Children { get; set; }

Someone could create their Db classes like:

public class Parent : IParent<Child>
. . .

But still use them like:

IParent<IChild> parents = db.Parents.Include(p => p.Children).ToArray();

Because the generic is marked out, the generic is covariant and therefore can take anything that meets the generic’s restrictions, including the above cast up the type tree to the IChild interface.

That said, if you really want to persist interfaces, the right answer is probably to use NHibernate:
How to map an interface in nhibernate?

And some coders recommend you keep interfaces on entities in any ORM limited to a few shared properties, or risk misuse:
Programming to interfaces while mapping with Fluent NHibernate