C# – Why is Task not co-variant

ccovariancetask

class ResultBase {}
class Result : ResultBase {}

Task<ResultBase> GetResult() {
    return Task.FromResult(new Result());
}

The compiler tells me that it cannot implicitly convert Task<Result> to Task<ResultBase>. Can someone explain why this is? I would have expected co-variance to enable me to write the code in this way.

Best Answer

According to someone who may be in the know...

The justification is that the advantage of covariance is outweighed by the disadvantage of clutter (i.e. everyone would have to make a decision about whether to use Task or ITask in every single place in their code).

It sounds to me like there is not a very compelling motivation either way. ITask<out T> would require a lot of new overloads, probably quite a bit under the hood (I cannot attest to how the actual base class is implemented or how special it is compared to a naive implementation) but way more in the form of these linq-like extension methods.

Somebody else made a good point - the time would be better spent making classes covariant and contravariant. I don't know how hard that would be, but that sounds like a better use of time to me.

On the other hand, somebody mentioned that it would be very cool to have a real yield return like feature available in an async method. I mean, without sleight of hand.

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