Covariant Returns and Implicit Interface Implementations

[alrz]

This is currently not support due to a breaking change.

This is technically a breaking change, as the program below prints "C1.M" today, but would print "C2.M" under the proposed revision.

using System;
interface I1 { object M(); }
class C1 : I1 { public object M() { return "C1.M"; } }
class C2 : C1, I1 { public new string M() { return "C2.M"; } }
class Program
{
    static void Main()
    {
        I1 i = new C2();
        Console.WriteLine(i.M());
    }
}

Due to this breaking change, we might consider not supporting covariant return types on implicit implementations.

Can we allow it with some modifier on declaration site so that it's only an opt-in feature?

interface I1 { [PermitCovariantReturn] object M(); }

[YairHalberstadt]

If you're going to have to explicitly opt in, explicit implementations seem like they solve the problem reasonably well.

[alrz]

For context, this is what I'm trying to do.

public interface IChild { /* a subset of properties */ }
class Child : IChild { /* actual properties */ }
public interface IParent { IChild Child { get; } }
class Parent : IParent {
    public Child Child { get; set; }
}

Using interfaces to define a limited public contract to internal types.

[YairHalberstadt]

Source generators seem like they could do this for you. They could detect [PermitCovariantReturn] and generate the explicit implementations for you.

[alrz]

That is definitely an option. Though I see this as exposing a runtime feature currently unavailable in language. We don't really need a double invocation because runtime supports this directly.

[YairHalberstadt]

I doubt the double invocation will have much impact. Since it's to a non virtual instance member, if it's small it will be inlined, and if long, the cost of the invocation will be negligible.

[YairHalberstadt]

Though I see this as exposing a runtime feature currently unavailable in language.

I think it would be really nice if this just works out the box. I'm just not a fan of an opt in only magic attribute