Task.Delay unexpected behavior

Question

I have this metod:

public async Task StartAsync(Task process)
{
    if (process is null)
    {
        throw new ArgumentNullException(nameof(process));
    }
    var loading = ...;
    await Task.WhenAll(process, loading).ContinueWith(t => EndProgress());
}

and is called via a command like so:

private async Task TestAsync()
{
    await StartAsync(new Task(async () =>
    {
        //just for testing purpose
        await Task.Delay(15000);
    }));
}
ExecuteDelegate = async param => await TestAsync();

where ExecuteDelegate is an Action<T> delegate used by command.

Why does the await Task.WhenAll line not waiting those 15 seconds from Task.Dalay?

Why not pass just Task.Delay to the StartAsync? You wrap it in yet another task. — Wiktor Zychla, Jun 23 '20 at 14:43
also i would say you need to await for `StartAsync` in `TestAsync` — Guru Stron, Jun 23 '20 at 14:44
Or do `private Task TestAsync() { return StartAsync(Task.Delay(15000)); }` — Clemens, Jun 23 '20 at 14:45
[You should **never** use the task constructor](https://blog.stephencleary.com/2014/05/a-tour-of-task-part-1-constructors.html), and [*almost never* use `ContinueWith`](https://blog.stephencleary.com/2013/10/continuewith-is-dangerous-too.html). — Stephen Cleary, Jun 24 '20 at 01:01

score 1 · Accepted Answer · edited Jun 23 '20 at 14:47

You need to await the call to StartAsync:

private async Task TestAsync()
{
    await StartAsync(new Task(async () =>
    {
        await Task.Delay(15000);
    }));
}

NOTE: You can also simplify your code by not creating the redundant Task:

private async Task TestAsync()
{
    await StartAsync(Task.Delay(15000));
}

Or even simpler:

private Task TestAsync()
{
    return StartAsync(Task.Delay(15000));
}

score 0 · Answer 2 · answered Jun 23 '20 at 14:58

You shouldn't use the constructor to create a Task but the static Task.Run method:

private async Task TestAsync()
{
    await StartAsync(Task.Run(async () =>
    {
        //just for testing purpose
        await Task.Delay(15000);
    }));
}

The task returned by Task.Run can be awaited as expected.

Task.Delay unexpected behavior

2 Answers2