Returning the derived class instead of an interface

Question

Is it possible to return the derived class in an interface definition in C#? Some kind of return type covariance.

For example:

public interface IAnimal
{
    this GetAnimal();
}

public class Dog : IAnimal
{
    Dog GetAnimal() { return new Dog(); }
}

The simple answer is "you can't". – Eldar Dordzhiev Mar 15 '16 at 22:49 — Eldar Dordzhiev, Mar 15 '16 at 22:49

Christos · Answer 1 · 2016-03-15T22:50:43.587

2

Is it possible to return the derived class in an interface definition in C#?

Yes it is, if you define a generic interface.

public interface IAnimal<T>
{
    T GetAnimal();
}

public class Dog : IAnimal<Dog>
{
    public Dog GetAnimal() { return new Dog(); }
}

You could read further on this here.

edited Mar 15 '16 at 22:50

answered Mar 15 '16 at 22:48

Christos

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I'm pretty sure that is not what OP expects. – Eldar Dordzhiev Mar 15 '16 at 22:50
@EldarDordzhiev you are correct. I just corrected correspondingly my answer. Thanks – Christos Mar 15 '16 at 22:51
I had the same idea, but well, it is not as clean as I would like to have it. This is a really simplified example which gets quiet complicated in the real use case :/ – Nicolas Mar 15 '16 at 22:54

score 1 · Answer 2 · answered Mar 15 '16 at 22:51

1

Sure if you pass the derived class as a type constraint :)

public interface IAnimal<T>
{
    T GetAnimal();
}

public class Dog : IAnimal<Dog>
{
    public Dog GetAnimal() { return new Dog(); }
}

answered Mar 15 '16 at 22:51

Spark

11
2

score 1 · Accepted Answer · edited Jun 20 '20 at 09:12

C# doesn't support return type covariance, at least as of C# 6. It is a very commonly requested feature on the Roslyn GitHub, and was long before Roslyn even existed.

Eric Lippert wrote in his answer:

The feature is not implemented because no one here ever implemented it. A necessary but insufficient requirement is that the feature's benefits exceed its costs.

The costs are considerable. The feature is not supported natively by the runtime, it works directly against our goal to make C# versionable because it introduces yet another form of the brittle base class problem.

So sit back, hold tight, and cross your fingers for C# 7/8/9. But don't get your hopes up too much - as said, it's a high-cost low-benefit feature that would likely require modifications to the CLR.

For now, take a look at Christos' answer.

Returning the derived class instead of an interface

3 Answers3