await IAsyncEnumerable where is it used?

Question

It says here:

Starting with C# 8.0, IAsyncEnumerable<T>, for an async method that returns an async stream.

Question. In addition to the specified example with foreach, is it possible to use more the await with IAsyncEnumerable<T> somehow, or is it designed specially for foreach? I think yes, but not sure. Perhaps there are other purposes.

await foreach (var number in GetNumbersAsync())
{
    Console.WriteLine(number);
}

async IAsyncEnumerable<int> GetNumbersAsync()
{
    for (int i = 0; i < 10; i++)
    {
        await Task.Delay(100);
        yield return i;
    }
}

As with `IEnumerable` you can call the methods of the interface manually. — Jeremy Lakeman, Aug 22 '22 at 05:18
@JeremyLakeman, Can you give an example with await? Nothing else is of interest. — , Aug 22 '22 at 05:19
[`GetAsyncEnumerator`](https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.iasyncenumerable-1.getasyncenumerator?view=net-6.0) returns [`IAsyncEnumerator`](https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.iasyncenumerator-1?view=net-6.0). You can use the enumerator to manually enumerate an enumerable. — ProgrammingLlama, Aug 22 '22 at 05:23

score 2 · Accepted Answer · answered Aug 22 '22 at 05:25

2

As Jeremy Lakeman said in the comments, you can use it however you want. There is nothing magic about it. Simply call .GetAsyncEnumerator() on your IAsyncEnumerable and then you can use that as you would a regular enumerator, but with async support.

Example:

IAsyncEnumerator<int> e = GetNumbersAsync().GetAsyncEnumerator();
try
{
  while (await e.MoveNextAsync())
    Console.Write(e.Current + " ");
}
finally {
  if (e != null)
    await e.DisposeAsync();
}

The sample was taken from here: https://learn.microsoft.com/en-us/archive/msdn-magazine/2019/november/csharp-iterating-with-async-enumerables-in-csharp-8

answered Aug 22 '22 at 05:25

Karl-Johan Sjögren

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Or `using (IAsyncEnumerator e = GetNumbersAsync().GetAsyncEnumerator()) { }` instead of the try/catch stuff, though I appreciate you didn't write the sample :) – ProgrammingLlama Aug 22 '22 at 05:26
Thanks, but everything revolves around the foreach purpose? – Aug 22 '22 at 05:27
1

@NikVladi What other use case do you have in mind? There's no `foreach` in the example here. I'm not sure why you'd have an enumerable if you don't want to enumerate it. Can you elaborate on what you're thinking? – ProgrammingLlama Aug 22 '22 at 05:28
1

@DiplomacyNotWar You'd need `await using(..)` in your sample since it is an async disposable :) – Karl-Johan Sjögren Aug 22 '22 at 05:30
Good point! :-) – ProgrammingLlama Aug 22 '22 at 05:35
@DiplomacyNotWar, I'm trying to figure out what await-related end goal is IAsyncEnumerable. – Aug 22 '22 at 05:35
@NikVladi Imagine that each call to `MoveNextAsync` is actually a web request or database query. While you're waiting for the web service or database to respond, you don't need the thread, right? So making it async allows you to free up a thread pool thread to do something else until the service/database responds. – ProgrammingLlama Aug 22 '22 at 05:37
@DiplomacyNotWar, That is, I correctly understand that the goal is a convenient asynchronous use of the foreach construct? – Aug 22 '22 at 05:40
1

I'd think one of the goals is that, yes. – ProgrammingLlama Aug 22 '22 at 05:41

score 2 · Answer 2 · answered Aug 22 '22 at 05:27

When compiled, foreach becomes like:

IAsyncEnumerator<int> e = RangeAsync(10, 3).GetAsyncEnumerator();
try
{
  while (await e.MoveNextAsync()) Console.Write(e.Current + " ");
}
finally { if (e != null) await e.DisposeAsync(); }

Further

the github.com/dotnet/reactive project includes the System.Linq.Async library, which provides a full set of such extension methods for operating on IAsyncEnumerable. You can include this library from NuGet in your project, and have access to a wide array of helpful extension methods for operating over IAsyncEnumerable objects.

await IAsyncEnumerable where is it used?

2 Answers2