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 why I would need this method.

Can somebody give a good example where Yield() is required?

Answer 1

When you use async/await, there is no guarantee that the method you call when you do await FooAsync() will actually run asynchronously. The internal implementation is free to return using a completely synchronous path.

If you're making an API where it's critical that you don't block and you run some code asynchronously, and there's a chance that the called method will run synchronously (effectively blocking), using await Task.Yield() will force your method to be asynchronous, and return control at that point. The rest of the code will execute at a later time (at which point, it still may run synchronously) on the current context.

This can also be useful if you make an asynchronous method that requires some "long running" initialization, ie:

 private async void button_Click(object sender, EventArgs e)
 {
      await Task.Yield(); // Make us async right away

      var data = ExecuteFooOnUIThread(); // This will run on the UI thread at some point later

      await UseDataAsync(data);
 }

Without the Task.Yield() call, the method will execute synchronously all the way up to the first call to await.

Answer 2

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<bool>();
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<Task> 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?

Answer 3

One use of Task.Yield() is to prevent a stack overflow when doing async recursion. Task.Yield() prevents syncronous continuation. Note, however, that this can cause an OutOfMemory exception (as noted by Triynko). Endless recursion is still not safe and you're probably better off rewriting the recursion as a loop.

private static void Main()
    {
        RecursiveMethod().Wait();
    }

    private static async Task RecursiveMethod()
    {
        await Task.Delay(1);
        //await Task.Yield(); // Uncomment this line to prevent stackoverlfow.
        await RecursiveMethod();
    }

Answer 4

Task.Yield() is like a counterpart of Thread.Yield() in async-await but with much more specific conditions. How many times do you even need Thread.Yield()? I will answer the title "when would you use Task.Yield()" broadly first. You would when the following conditions are fulfilled:

• want to return the control to the async context (suggesting the task scheduler to execute other tasks in the queue first)
• need to continue in the async context
• prefer to continue immediately when the task scheduler is free
• do not want to be cancelled
• prefer shorter code

The term "async context" here means "SynchronizationContext first then TaskScheduler". It was used by Stephen Cleary.

Task.Yield() is approximately doing this (many posts get it slightly wrong here and there):

await Task.Factory.StartNew( 
    () => {}, 
    CancellationToken.None, 
    TaskCreationOptions.PreferFairness,
    SynchronizationContext.Current != null?
        TaskScheduler.FromCurrentSynchronizationContext(): 
        TaskScheduler.Current);

If any one of the conditions is broken, you need to use other alternatives instead.

If the continuation of a task should be in Task.DefaultScheduler, you normally use ConfigureAwait(false). On the contrary, Task.Yield() gives you an awaitable not having ConfigureAwait(bool). You need to use the approximated code with TaskScheduler.Default.

If Task.Yield() obstructs the queue, you need to restructure your code instead as explained by noseratio.

If you need the continuation to happen much later, say, in the order of millisecond, you would use Task.Delay.

If you want the task to be cancellable in the queue but do not want to check the cancellation token nor throw an exception yourself, you need to use the approximated code with a cancellation token.

Task.Yield() is so niche and easily dodged. I only have one imaginary example by mixing my experience. It is to solve an async dining philosopher problem constrained by a custom scheduler. In my multi-thread helper library InSync, it supports unordered acquisitions of async locks. It enqueues an async acquisition if the current one failed. The code is here. It needs ConfigureAwait(false) as a general purpose library so I need to use Task.Factory.StartNew. In a closed source project, my program needs to execute significant synchronous code mixed with async code with

• a high thread priority for semi-realtime work
• a low thread priority for some background work
• a normal thread priority for UI

Thus, I need a custom scheduler. I could easily imagine some poor developers somehow need to mix sync and async code together with some special schedulers in a parallel universe (one universe probably does not contain such developers); but why wouldn't they just use the more robust approximated code so they do not need to write a lengthy comment to explain why and what it does?

Answer 5

Task.Yield() may be used in mock implementations of async methods.