Champion "Async Streams" (including async disposable) (16.3, Core 3)

[gafter]

• Proposal added: https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/async-streams.md
• Discussed in LDM
• Decision in LDM
• Finalized (done, rejected, inactive)
• Spec'ed

See dev spec: https://github.com/dotnet/roslyn/blob/main/docs/features/async-streams.md
See also dotnet/roslyn#261

LDM notes:

• LDM 2017-08-30
• LDM 2017-01-18
• LDM 2015-09-08
• https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-05-21.md
• https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-10-03.md (interface shape)
• https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-10-15.md (syntax)
• https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-11-28.md#async-iterators-and-await-foreach
• https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-12-12.md#cancellation-in-getasyncenumerator (CancellationToken, pattern-based binding)
• https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-01-09.md#async-streams-getasyncenumerator-revisited
• https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-01-16.md (pattern-based disposal)
• https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-03-25.md#cancellation-tokens-in-async-streams (cancellationToken variable)
• https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-04-15.md (cancellation token using attribute)
• https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-04-22.md#defaultcancellationattribute-and-overridinghidinginterface-implementation (cancellation token using attribute)
• https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-04-29.md (cancellation, local functions)

[gafter]

This would be something like

interface IAsyncEnumerable<T>
{
        IAsyncEnumerator<T> GetEnumerator();
}
interface IAsyncEnumerator<T>: IAsyncDisposable
{
        Task<bool> MoveNextAsync();
        T Current { get; }
}

With the following elements yet to be designed

• IAsyncDisposable
• async foreach loops
• Declaring async iterator methods
• Some kind of query expression support

[clairernovotny]

@gafter I thought we settled on

interface IAsyncEnumerable<T>
{
        IAsyncEnumerator<T> GetEnumerator(CancellationToken cancellation);
}

And potentially

static class AsyncEnumerableExtensions
{
    static IAsyncEnumerator<T> GetEnumerator(this IAsyncEnumerable<T> source) => source.GetEnumerator(CancellationToken.None);
}

[juepiezhongren]

After the appearance of "IAsyncEnumerable" in Azure Service Fabric, we want more.........
We want the entire async implementation of .net collection BCL part!!!!!!
AsyncLinq: SelectAsync... AsyncParalellLinq: AsAsyncParalell... await yield!!!!!

As ecmascript is proposing with generator async, there is to be the fulfilment in .net!
And with what i have mentioned being implemented, .net(C# and F#) is going to be the most developer-friendly language for big-data, and service-fabric is gonna boosting a completely new ecosystem with a firm asynchronous programming foundation.

wish these come true earlier!!!!

[oliverjanik]

Any news on this? Seems this interface has been defined in several places already (IX, EF7)

[benaadams]

As brought up in roslyn thread

interface IAsyncEnumerator<T>: IAsyncDisposable
{
    Task<bool> MoveNextAsync();
    T Current { get; }
}

Has issues with parallelazation/concurrency/atomicity which is a desirable property of an async system (e.g. double gets on Current or missed items on Current)

[IvanKonov]

Maybe then:

interface IAsyncEnumerator<T> : IAsyncDisposable
{
    Task<bool> GetNextAsync(out T next);;
    Task<T> Current { get; }
}

[HaloFour]

An async method can't have out parameters.