Why is Task<T> not co-variant?

问题

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.

回答1:

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.

回答2:

I realize I'm late to the party, but here's an extension method I've been using to account for this missing feature:

/// <summary>
/// Casts the result type of the input task as if it were covariant
/// </summary>
/// <typeparam name="T">The original result type of the task</typeparam>
/// <typeparam name="TResult">The covariant type to return</typeparam>
/// <param name="task">The target task to cast</param>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static async Task<TResult> AsTask<T, TResult>(this Task<T> task) 
    where T : TResult 
    where TResult : class
{
    return await task;
}

This way you can just do:

class ResultBase {}
class Result : ResultBase {}

Task<Result> GetResultAsync() => ...; // Some async code that returns Result

Task<ResultBase> GetResultBaseAsync() 
{
    return GetResultAsync().AsTask<Result, ResultBase>();
}

来源：https://stackoverflow.com/questions/30996986/why-is-taskt-not-co-variant