1

I have a function that returns an IAsyncEnumerable. I want to await it for completion, but I do not care about any of the results. I only care about the side effects of calling the function.

How do can I easily await the IAsyncEnumerable for completion without using a for loop or turning it into a container (e.g. ToListAsync)?

Theodor Zoulias
  • 34,835
  • 7
  • 69
  • 104
Rick de Water
  • 2,388
  • 3
  • 19
  • 37
  • 3
    You'll need to loop over it. You won't know it's "completed" until the very last call to `MoveNextAsync` returns a `ValueTask` which completes with `false` – canton7 Apr 13 '21 at 09:59

2 Answers2

3

You could use the LastOrDefaultAsync operator from the System.Linq.Async package, and then convert the resulting ValueTask<T> to a Task by using the AsTask extension method. For extra convenience you could combine these two operations to one extension method ToTask:

/// <summary>
/// Retrieves a Task object that represents the completion of this IAsyncEnumerable.
/// </summary>
public static Task ToTask<T>(this IAsyncEnumerable<T> source)
{
    return source.LastOrDefaultAsync().AsTask();
}

But honestly doing a loop manually is simpler and probably more efficient:

public async static Task ToTask<T>(this IAsyncEnumerable<T> source)
{
    await foreach (var _ in source.ConfigureAwait(false)) { }
}

The LastOrDefaultAsync operator will preserve in memory the last emitted element during the enumerable's lifetime, which may prevent the element from being garbage collected in a timely manner. You could prevent this from happening by including in the query the IgnoreElements operator from the System.Interactive.Async package, but I think it becomes too much of a hassle.

Theodor Zoulias
  • 34,835
  • 7
  • 69
  • 104
0

Short answer: You can't.

Loger answer: Asking

How do can I easily await the IAsyncEnumerable for completion without using a for loop or turning it into a container (e.g. ToListAsync)?

Is like asking

How can I wait for the completion of a Task without using await or something like .Result or .Wait()?

The way to know when an IAsyncEnumerable is done is calling await MoveNextAsync() on it's iterator and it returning false. The concisest way to do that is

await foreach (var _ in AsyncGenerator()) ; // The semicolon is necessary
MindSwipe
  • 7,193
  • 24
  • 47