Entity Framework Generic Inheritance

Question

I'm trying to assign a DbSet of class that is inherited from a more generic class. The example below demonstrates what I'm trying to achieve:

DbSet<Animal> dbSet;

switch (selectedAnimal)
{
    case "Dog":
        dbSet = _Context.Dog; // DbSet<Dog>
        break;
    case "Cat":
        dbSet = _Context.Cat; // DbSet<Cat>
        break;
    case "Pig":
        dbSet = _Context.Pig; // DbSet<Pig>
        break;
}

The Dog, Cat, Pig class is an inherited class of Animal, as follows:

public class Dog : Animal { }

But I am getting a compile-time error of Cannot implicitly convert type DbSet<Dog> to DbSet<Animal>

How can I implement my design without getting the error?

P.S. The code above is just to illustrate what I'm trying to achieve, so forgive me if it doesn't make sense.

Possible duplicate of http://stackoverflow.com/questions/19082671/why-isnt-dbset-covariant — ChrisV, Jan 28 '15 at 06:45
Interesting.. Covariance is something new to me, will read up about it. — Dan, Jan 28 '15 at 06:49

Szer · Answer 1 · 2015-01-28T06:38:53.760

1

Because compiler cant cast derived classes to base class w/o information loss. You simply cant put DbSet of Cat into DbSet of Animal where Cat : Animal

But you CAN put DbSet of Animal into DbSet of Cat

Try this one:

var dbSet = GetDbSet(selectedObject) //selectedObject is Cat,Dog,Pig whatever

And the method:

    private DbSet<T> GetDbSet<T>(T selectedAnimal) where T : Animal
    {
        return this.Set<T>(); //this == EF Context
    }

edited Jan 28 '15 at 06:38

answered Jan 28 '15 at 06:33

Szer

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This doesn't answer my question. I'm trying to set a DbSet of derived class into a DbSet of base class, which according to @ChrisV, covariance. I don't see how your answer can do that, thanks anyway! – Dan Jan 28 '15 at 08:14

score 1 · Accepted Answer · answered Jan 28 '15 at 06:52

1

What you're referring to is called covariance. In C# only interfaces can be covariant. So IEnumerable<Cat> is a subtype of IEnumerable<Animal>. The problem with declaring a DbSet<Animal> and then putting a variable of type DbSet<Cat> in it is that a variable of type DbSet<Animal> has methods that allow, for instance, adding an Animal entity. That Animal might be a Dog. But the variable you put in there was a DbSet<Cat> and that can't contain Dog. It would be a violation of strong typing.

answered Jan 28 '15 at 06:52

ChrisV

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So according to what you are saying, will it work if I change `Animal` into an interface? It doesn't seem to work even after changing `Animal` from a class to interface. – Dan Jan 28 '15 at 06:59
No, I'm saying that it would work if `DbSet` were an interface (and if it were declared in a certain manner, read the wiki page on covariance and contravariance). What you're trying to do can't be done. – ChrisV Jan 28 '15 at 11:19

Entity Framework Generic Inheritance

2 Answers2