Question

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Does C# support return type covariance?

I’m not sure if I’m just being stupid…

If I have an interface:

public interface IMoop
{
    object Moop();
}

Why can’t I implement it like so (I guess this would use implicit Covariance?)

public class MoopImplementor : IMoop
{
    string Moop();
}

Any instance of MoopImplementor would meet the contract specified by IMoop, so it seems like this should be ok.

Please enlighten me 🙂

EDIT: To be clear- since the implementing class returns something that inherits from the return type of the Interfaced method – I feel this should work. Specifically, a string IS an object. (and the same goes for any other inhertiance chain).

Answer 1

C# does not support return type covariance for the purposes of interface implementation or virtual method overrding. See this question for details:

Does C# support return type covariance?

C# does support generic covariance and contravariance of interfaces and delegate types that are constructed wtih reference types for the type arguments as of C# 4.

And C# does support return type covariance when converting a method that returns a reference type to a delegate type whose return type is a compatible reference type. (And similarly it supports parameter type contravariance.)

If this subject interests you, I have written a great many articles discussing various versions of variance that C# does and does not support. See

https://blogs.msdn.microsoft.com/ericlippert/tag/covariance-and-contravariance/

for details.