C# Async await in a function called without await

Question

I have a code to call a function in a loop, which would access endpoints asynchronously, coded like this:

public async Task HitEndPointsAsync()
{
    for (int i = 0; i < _urls.Length; i++)
    {
        await HitOneEndPointAsync(i);
    }
}

The simplified function HitOneEndPointAsync looks like this:

private async Task HitOneEndPointAsync(int i)
{
    var newRec = InsertRec(i);
    ExtURL extUrl = new ExtURL(_urls[i]);
    result = await extUrl.GetAsync(_parms[i]);
    UpdateRec(i, result);
}

If I remove the await in HitEndPointsAsync, then the await that is in HitOneEndPointAsync is no longer effective. If I need all endpoints to be called at once, but on each call await for the response to process that response further, would this be an option? Because as soon as I remove the await at the function call level, the await down the line is ignored. Thoughts?

Could you clarify what you mean by "no longer effective"? It's not clear to me what you're observing. — Jon Skeet, Feb 16 '20 at 18:44
Those await would schedule continuations (likely on another thread, that depends on the context). Is that not happening? — Theraot, Feb 16 '20 at 18:46
"If I remove the await in HitEndPointsAsync, then the await that is in HitOneEndPointAsync is no longer effective" - indeed, so... don't remove that? it is needed! can you clarify *why* you are trying to remove it? — Marc Gravell, Feb 16 '20 at 18:49
@Marc: Yes, removing it in an attempt to call all endpoints at once without awaitng the completion of each call. However, within each call I need to await for the response — Yogi, Feb 16 '20 at 18:54
@Jon: by no longer effective I meant that it is not awaiting for the response — Yogi, Feb 16 '20 at 18:55
@Theraot: The await works, but either both await are on, or no await at all. Can't do await on the lower level, and no await on the upper level — Yogi, Feb 16 '20 at 18:56
I'm sure it *is* awaiting at the lower level - but if you're not awaiting the result at the higher level, you may be getting confused by what's happening. — Jon Skeet, Feb 16 '20 at 19:01
@Jon, I can assure you it is not awaiting at the lower level without the await at the upper level. I do have a logging mechanism that logs the response of the httpClient. And the responses are null when the upper await is removed. — Yogi, Feb 16 '20 at 19:05
Are you on a console application? By default the continuations would be background threads on the thread pool. They do not prevent the application from ending. — Theraot, Feb 16 '20 at 19:19
@Yogi: Well, I'd be very surprised if somehow the C# compiler was emitting the wrong code. I think it's *far* more likely that you're misinterpreting the results - I've seen far, far more developers doing that (including myself) than times where the compiler is wrong. If you could provide a [mcve] we could validate that, of course. — Jon Skeet, Feb 16 '20 at 20:21

Isma · Accepted Answer · 2020-02-17T20:31:36.580

2

You could try something like this if you don't want to await within the loop:

public async Task HitEndPointsAsync()
{
    var tasks = new List<Task>();

    for (int i = 0; i < _urls.Length; i++)
    {
        tasks.Add(HitOneEndPointsAsync(i));
    }

    await Task.WhenAll(tasks);
}

Note that this may add extra threads due to the calls to HitEndPointAsync being wrapped in a Task so I am not sure it has any advantages over the original version, maybe you could add more context to your question to know exactly what you are trying to do.

edited Feb 17 '20 at 20:31

answered Feb 16 '20 at 18:48

Isma

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This Task will start multiple threads. I believe it would work but it changes the design. Multiple thread asynchronous instead of single thread asynchronous. Thank you though. Will discuss with the team about this option. – Yogi Feb 16 '20 at 19:00
2

Replace `new Task` with `Task.Factory.StartNew` instead! – Alb Feb 16 '20 at 19:11
2

Thanks, I changed it, Task.Run is the preferred way though – Isma Feb 16 '20 at 21:33
@Isma, thank you for your suggestion. I had to modify a bit to make it work, like this: tasks.Add(HitOneEndPointsAsync(i)); (Without Task.Run) otherwise it errored out on me. Thanks again – Yogi Feb 17 '20 at 20:19
Hi, I updated the answer, I'm glad you solved it ;) – Isma Feb 17 '20 at 20:31

C# Async await in a function called without await

1 Answers1