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Task.Yield(...), Task.Delay(...)

I think that a lot of person already heard about these new methods. In this post I want to clarify some things about these new methods that I saw that are not very clear.
We will start with Task.Yield(). The book definition of this method is: “Creates an awaitable that asynchronously yields back to the current context when awaited”. Let see what does it means in reality.
Yield gives us the possibility to leave the current async code (method or lambda expression) and allow other code to run in the underlying thread. Usually this is used when we have long running code that is executed in events (on main UI thread for example). In this case we want to allow other code to be executed on the UI thread. For this purpose the Yield method can be called. The rest of the function that need to be executed is posted back and will be executed after other messages that were waiting were executed.
For example we can have a for in an event handler that process items for a list. To permit the UI to execute not only our code we can use Yield.
public async void StartButton_Click(...)
    for( int i=0; i < list.Count; i++)
        await Task.Yield();
We use await in front of the Yield() because we want to wait until the other messages are processed. On the other side, we can use this method when our application uses threads from ThreadPool. If an action execute for long time, you don’t want other thread to block and wait for that current action. In this situation, using Yield() can permit other actions from the queue to be executed. In this way all the actions will be executed.
In background this method resumes the current action. We can compare this method to be something similar to a pause. When Yield() is called, the remaining action is posted back to the current context (it can be the TaskScheduler.Default or SynchronizationContext). After the rest of the code is executed, the rest of our action is resumed.
When you are working with tasks, especially when you want to simulate some behavior you will need a method to put a task to sleep. Thread.Sleep cannot be found anymore (for Metro style app). This method would put the thread on sleep, but we don’t want to freeze the UI thread or block another thread. Other actions could be executed on this thread.
For this purpose Task.Delay(…) was introduce. We can specify a delay time when the task will be suspended. Other tasks will be able to be executed on that read. In this way all the resources will be used at maximum. Optionally, you can specify a cancelation token that will be used if the task is canceled and stop the delay.


  1. Do not rely on await Task.Yield() to keep a UI responsive!


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