Question

I’m really enjoying working with C# 5.0 asynchronous programming. However, there are a few places where updating old code to be consistent with the TAP model is causing problems for me.

Here’s one of them – I’m not sure exactly why Task<TResult> is not covariant in TResult, but it’s causing problems for me when trying to update a covariant interface to move from a synchronous to an asychronous pattern:

Old code:

public interface IInitializable<out T> // ** out generic modifier **
{
    /// <summary>
    /// Boolean to indicate if class is ready
    /// </summary>
    bool IsInitialized { get; }

    /// <summary>
    /// Calls for instance to be initialized using current parameters
    /// Driver initialization can be done in the default constructor if desired
    /// </summary>
    T Initialize();
}

New code (won’t compile):

public interface IAsyncInitializable<out T> // ** out generic modifier...broken **
{
    /// <summary>
    /// Boolean to indicate if class is ready
    /// </summary>
    bool IsInitialized { get; }

    /// <summary>
    /// Calls for instance to be initialized using current parameters
    /// Driver initialization can be done in the default constructor if desired
    /// </summary>
    Task<T> InitializeAsync(); // ** breaks because Task<TResult> is invariant in TResult **
}

Is there is a reasonable way around this without modifying my APIs too drastically? (Bonus: why is Task not covariant?). There’s no IAwaitable interface, but I suppose I could make one and create an extension method that converts to a wrapped, covariant, awiatable task object. Or am I doing it wrong?

Answer 1

Task<T> can’t be covariant in T, because it’s a class. Only interfaces and delegates can have generic variance.

As for whether it’s worth doing the wrapping… I guess that depends on how much you use the covariance within your project. I suspect you’ll find all the wrapping and unwrapping confusing over time, to be honest – if it’s not too bad to just take the hit of removing the covariance, I’d do that.