When would I use Task.Yield()?

Question

I\'m using async/await and Task a lot but have never been using Task.Yield() and to be honest even with all the explanations I do not understand wh

Answer 1

Internally, await Task.Yield() simply queues the continuation on either the current synchronization context or on a random pool thread, if SynchronizationContext.Current is null.

It is efficiently implemented as custom awaiter. A less efficient code producing the identical effect might be as simple as this:

var tcs = new TaskCompletionSource();
var sc = SynchronizationContext.Current;
if (sc != null)
    sc.Post(_ => tcs.SetResult(true), null);
else
    ThreadPool.QueueUserWorkItem(_ => tcs.SetResult(true));
await tcs.Task;

Task.Yield() can be used as a short-cut for some weird execution flow alterations. For example:

async Task DoDialogAsync()
{
    var dialog = new Form();

    Func showAsync = async () => 
    {
        await Task.Yield();
        dialog.ShowDialog();
    }

    var dialogTask = showAsync();
    await Task.Yield();

    // now we're on the dialog's nested message loop started by dialog.ShowDialog 
    MessageBox.Show("The dialog is visible, click OK to close");
    dialog.Close();

    await dialogTask;
    // we're back to the main message loop  
}

That said, I can't think of any case where Task.Yield() cannot be replaced with Task.Factory.StartNew w/ proper task scheduler.

See also:

• "await Task.Yield()" and its alternatives

• Task.Yield - real usages?