实体框架通用继承

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?

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

Answer 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
    }

Answer 2

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.