Champion "Covariant Return Types" (VS 16.8, .NET 5)

[gafter]

• Proposal added: proposals/csharp-9.0/covariant-returns.md
• Discussed in LDM
• Decision in LDM
• Finalized (done, rejected, inactive)
• Spec'ed (Draft at Covariant Return Types - Draft Specification #2844)

[Joe4evr]

This would apply to methods and properties, and be supported in classes and interfaces.

The part about support in interfaces, does this mean something like the following?

public interface IFoo { }
public interface IBar
{
    IFoo Get();
}
public interface IBaz : IFoo { }
public interface IQux : IBar
{
    override IBaz Get(); //or would it be 'override IBaz IBar.Get();'?
}

I guess it's lucky that discussions around Default Interface Implementations have pushed back a little bit on using override as a keyword for just that. Even if it wasn't, there probably could've been a way for the two to co-exist, but if you're talking about adding one new thing to interfaces, it would be good to consider the other things that are proposed to be added to interfaces, and make sure that one design choice doesn't preclude any other features entirely.

[JonHanna]

I find myself wishing C# supported covariant returns very often, and often with code that isn't particularly "clever".

[TylerBrinkley]

Covariant returns would be very useful for typed cloning methods, preventing the need to manually create unique protected cloning methods.

@gafter @Joe4evr @JonHanna @TylerBrinkley @srivatsn
Or better:

public interface IQux : IBar
{
    this Get();
}

or better #1566 :

public interface IQux : IBar
{
    [IBaz] Get(); 
}

Why didn't this happen yet?
It seems popular enough.

[HaloFour]

@MohammadHamdyGhanem

Why didn't this happen yet?
It seems popular enough.

Because you haven't written it yet.

Seriously, there are a million "good" ideas, modifying the language is a very expensive process and the team has finite resources. Things take time.

@HaloFour

Because you haven't written it yet.

If I can write "it", I would rather write a new S# (Smart Sharp) language, where I do whatever I want :).

Anyway, may God help the team :)

[yaakov-h]

Would this need CLR changes, or compiler changes only?