Covariance issue when returning Task from a method

Question

In .NET 6, I have an interface and a type:

internal interface IProvider
{
}

public class TProvider : IProvider
{
}

I want to use a covariant T in another interface like this:

internal interface IFactory<out T>
{
    Task<T> Create();
}

But on the line Task<T> Create(), I am getting an error:

The type parameter 'T' must be invariantly valid on 'IFactory.Create()'. 'T' is covariant

If I change the method call to a synchronous version -- T Create() -- it compiles.

Is there a way to keep T as covariant and return Task<T> as part of an asynchronous method signature?

Natively this isn't possible, it would be good to know more about your use case on what you need this for. There may be alternatives. There's also a discussion here regarding this state https://github.com/dotnet/roslyn/issues/2981 — itsdaniel0, Mar 02 '23 at 07:34
Related: [Why isn't Task covariant?](https://stackoverflow.com/questions/30996986/why-is-taskt-not-co-variant#:~:text=The%20justification%20is%20that%20the,single%20place%20in%20their%20code). — John Wu, Mar 02 '23 at 08:06

score -1 · Answer 1 · answered Mar 02 '23 at 07:32

-1

Change this:

internal interface IFactory<out T>
{
    Task<T> Create();
}

To :

internal interface IFactory<T>
{
    Task<T> Create();
}

answered Mar 02 '23 at 07:32

Siba Daki

3

The question explicitly says "to keep T as covariant", which is precisely what this *doesn't do* – Marc Gravell Mar 02 '23 at 07:36
@MarcGravell, it won't work as covariant – Siba Daki Mar 02 '23 at 08:16

1 Answers1