Is there a C# class like Queue that implements IAs

问题:

Both Queue and ConcurrentQueue implement IEnumerable but not IAsyncEnumerable. Is there a standard class or class available on NuGet which implements IAsyncEnumerable such that, if the queue is empty, the result of MoveNextAsync does not complete until something next is added to the queue?

回答1:

If you are using the .NET Core platform there are at least two built-in options:

1. The System.Threading.Tasks.Dataflow.BufferBlock<T> class, part of the TPL Dataflow library. It doesn't implement the IAsyncEnumerable<T> natively, but it exposes the awaitable OutputAvailableAsync() method, doing it trivial to implement a ToAsyncEnumerable extension method.

2. The System.Threading.Channels.Channel<T> class, the core component of the Channels library. It exposes an IAsyncEnumerable<T> implementation via its Reader.ReadAllAsync()¹ method.

Both classes are also available for .NET Framework, by installing a nuget package (different for each one).

An implementation of IAsyncEnumerable<T> for BufferBlock<T>:

public static async IAsyncEnumerable<T> ToAsyncEnumerable<T>(
    this IReceivableSourceBlock<T> source,
    [EnumeratorCancellation]CancellationToken cancellationToken = default)
{
    while (await source.OutputAvailableAsync(cancellationToken).ConfigureAwait(false))
    {
        while (source.TryReceive(out T item))
        {
            yield return item;
        }
    }
    await source.Completion.ConfigureAwait(false); // Propagate possible exception
}

¹ (not available for NET Framework, but easy to implement in a similar way)