C# – Is Async/Await using Task.Run starting a new thread asynchronously

async-awaitcmultithreading

I have read a lot of articles and still cant get understand this part.

Consider this code :

    private async void button1_Click(object sender, EventArgs e)
    {
        await Dosomething();
    }

    private async Task<string> Dosomething()
    {
        await Task.Run((() => "Do Work"));
        return "I am done";
    }

First question:

When I click the button, it will Call DoSomething and await a Task that creates a Thread from the threadpool by calling Task.Run ( if I am not mistaken ) and all of this runs asynchronously. So I achieved creating a thread that does my work but doing it asynchronously? But consider that I don't need any result back, i just want the work to be done without getting any result back, is there really a need to use async/await , and if so, how?

Second question:

When running a thread asynchronously, how does that work? Is it running on the main UI but on a separate thread or is it running on a separate thread and separate is asynchronously inside that method?

Best Answer

The purpose of creating Async methods is so you can Await them later. Kind of like "I'm going to put this water on to boil, finish prepping the rest of my soup ingredients, and then come back to the pot and wait for the water to finish boiling so I can make dinner." You start the water boiling, which it does asynchronously while you do other things, but eventually you have to stop and wait for it. If what you want is to "fire-and-forget" then Async and Await are not necessary.

Simplest way to do a fire and forget method in C#?

Starting a new task queues that task for execution on a threadpool thread. Threads execute in the context of the process (eg. the executable that runs your application). If this is a web application running under IIS, then that thread is created in the context of the IIS worker process. That thread executes separately from the main execution thread, so it goes off and does its thing regardless of what your main execution thread is doing, and at the same time, your main execution thread moves on with its own work.

Related Solutions

C# – Cross-thread operation not valid: Control accessed from a thread other than the thread it was created on

As per Prerak K's update comment (since deleted):

I guess I have not presented the question properly.

Situation is this: I want to load data into a global variable based on the value of a control. I don't want to change the value of a control from the child thread. I'm not going to do it ever from a child thread.

So only accessing the value so that corresponding data can be fetched from the database.

The solution you want then should look like:

UserContrl1_LOadDataMethod()
{
    string name = "";
    if(textbox1.InvokeRequired)
    {
        textbox1.Invoke(new MethodInvoker(delegate { name = textbox1.text; }));
    }
    if(name == "MyName")
    {
        // do whatever
    }
}

Do your serious processing in the separate thread before you attempt to switch back to the control's thread. For example:

UserContrl1_LOadDataMethod()
{
    if(textbox1.text=="MyName") //<<======Now it wont give exception**
    {
        //Load data correspondin to "MyName"
        //Populate a globale variable List<string> which will be
        //bound to grid at some later stage
        if(InvokeRequired)
        {
            // after we've done all the processing, 
            this.Invoke(new MethodInvoker(delegate {
                // load the control with the appropriate data
            }));
            return;
        }
    }
}

C# – HttpClient.GetAsync(…) never returns when using await/async

You are misusing the API.

Here's the situation: in ASP.NET, only one thread can handle a request at a time. You can do some parallel processing if necessary (borrowing additional threads from the thread pool), but only one thread would have the request context (the additional threads do not have the request context).

This is managed by the ASP.NET SynchronizationContext.

By default, when you await a Task, the method resumes on a captured SynchronizationContext (or a captured TaskScheduler, if there is no SynchronizationContext). Normally, this is just what you want: an asynchronous controller action will await something, and when it resumes, it resumes with the request context.

So, here's why test5 fails:

Test5Controller.Get executes AsyncAwait_GetSomeDataAsync (within the ASP.NET request context).
AsyncAwait_GetSomeDataAsync executes HttpClient.GetAsync (within the ASP.NET request context).
The HTTP request is sent out, and HttpClient.GetAsync returns an uncompleted Task.
AsyncAwait_GetSomeDataAsync awaits the Task; since it is not complete, AsyncAwait_GetSomeDataAsync returns an uncompleted Task.
Test5Controller.Get blocks the current thread until that Task completes.
The HTTP response comes in, and the Task returned by HttpClient.GetAsync is completed.
AsyncAwait_GetSomeDataAsync attempts to resume within the ASP.NET request context. However, there is already a thread in that context: the thread blocked in Test5Controller.Get.
Deadlock.

Here's why the other ones work:

(test1, test2, and test3): Continuations_GetSomeDataAsync schedules the continuation to the thread pool, outside the ASP.NET request context. This allows the Task returned by Continuations_GetSomeDataAsync to complete without having to re-enter the request context.
(test4 and test6): Since the Task is awaited, the ASP.NET request thread is not blocked. This allows AsyncAwait_GetSomeDataAsync to use the ASP.NET request context when it is ready to continue.

And here's the best practices:

In your "library" async methods, use ConfigureAwait(false) whenever possible. In your case, this would change AsyncAwait_GetSomeDataAsync to be var result = await httpClient.GetAsync("http://stackoverflow.com", HttpCompletionOption.ResponseHeadersRead).ConfigureAwait(false);
Don't block on Tasks; it's async all the way down. In other words, use await instead of GetResult (Task.Result and Task.Wait should also be replaced with await).

That way, you get both benefits: the continuation (the remainder of the AsyncAwait_GetSomeDataAsync method) is run on a basic thread pool thread that doesn't have to enter the ASP.NET request context; and the controller itself is async (which doesn't block a request thread).

More information:

My async/await intro post, which includes a brief description of how Task awaiters use SynchronizationContext.
The Async/Await FAQ, which goes into more detail on the contexts. Also see Await, and UI, and deadlocks! Oh, my! which does apply here even though you're in ASP.NET rather than a UI, because the ASP.NET SynchronizationContext restricts the request context to just one thread at a time.
This MSDN forum post.
Stephen Toub demos this deadlock (using a UI), and so does Lucian Wischik.

Update 2012-07-13: Incorporated this answer into a blog post.

Best Answer

Related Solutions

C# – Cross-thread operation not valid: Control accessed from a thread other than the thread it was created on

C# – HttpClient.GetAsync(…) never returns when using await/async

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