c# - If async-await doesn't create any additional threads, then how does it make applications responsive?

Question

Time and time again, I see it said that using async-await doesn't create any additional threads. That doesn't make sense because the only ways that a computer can appear to be doing more than 1 thing at a time is

• Actually doing more than 1 thing at a time (executing in parallel, making use of multiple processors)
• Simulating it by scheduling tasks and switching between them (do a little bit of A, a little bit of B, a little bit of A, etc.)

So if async-await does neither of those, then how can it make an application responsive? If there is only 1 thread, then calling any method means waiting for the method to complete before doing anything else, and the methods inside that method have to wait for the result before proceeding, and so forth.

Answer 1

Actually, async/await is not that magical. The full topic is quite broad but for a quick yet complete enough answer to your question I think we can manage.

Let's tackle a simple button click event in a Windows Forms application:

public async void button1_Click(object sender, EventArgs e)
{
    Console.WriteLine("before awaiting");
    await GetSomethingAsync();
    Console.WriteLine("after awaiting");
}

I'm going to explicitly not talk about whatever it is GetSomethingAsync is returning for now. Let's just say this is something that will complete after, say, 2 seconds.

In a traditional, non-asynchronous, world, your button click event handler would look something like this:

public void button1_Click(object sender, EventArgs e)
{
    Console.WriteLine("before waiting");
    DoSomethingThatTakes2Seconds();
    Console.WriteLine("after waiting");
}

When you click the button in the form, the application will appear to freeze for around 2 seconds, while we wait for this method to complete. What happens is that the "message pump", basically a loop, is blocked.

This loop continuously asks windows "Has anyone done something, like moved the mouse, clicked on something? Do I need to repaint something? If so, tell me!" and then processes that "something". This loop got a message that the user clicked on "button1" (or the equivalent type of message from Windows), and ended up calling our button1_Click method above. Until this method returns, this loop is now stuck waiting. This takes 2 seconds and during this, no messages are being processed.

Most things that deal with windows are done using messages, which means that if the message loop stops pumping messages, even for just a second, it is quickly noticeable by the user. For instance, if you move notepad or any other program on top of your own program, and then away again, a flurry of paint messages are sent to your program indicating which region of the window that now suddenly became visible again. If the message loop that processes these messages is waiting for something, blocked, then no painting is done.

So, if in the first example, async/await doesn't create new threads, how does it do it?

Well, what happens is that your method is split into two. This is one of those broad topic type of things so I won't go into too much detail but suffice to say the method is split into these two things:

1. All the code leading up to await, including the call to GetSomethingAsync
2. All the code following await

Illustration:

code... code... code... await X(); ... code... code... code...

Rearranged:

code... code... code... var x = X(); await X; code... code... code...
^                                  ^          ^                     ^
+---- portion 1 -------------------+          +---- portion 2 ------+

Basically the method executes like this:

1. It executes everything up to await

2. It calls the GetSomethingAsync method, which does its thing, and returns something that will complete 2 seconds in the future

   So far we're still inside the original call to button1_Click, happening on the main thread, called from the message loop. If the code leading up to await takes a lot of time, the UI will still freeze. In our example, not so much

3. What the await keyword, together with some clever compiler magic, does is that it basically something like "Ok, you know what, I'm going to simply return from the button click event handler here. When you (as in, the thing we're waiting for) get around to completing, let me know because I still have some code left to execute".

   Actually it will let the SynchronizationContext class know that it is done, which, depending on the actual synchronization context that is in play right now, will queue up for execution. The context class used in a Windows Forms program will queue it using the queue that the message loop is pumping.

4. So it returns back to the message loop, which is now free to continue pumping messages, like moving the window, resizing it, or clicking other buttons.

   For the user, the UI is now responsive again, processing other button clicks, resizing and most importantly, redrawing, so it doesn't appear to freeze.

5. 2 seconds later, the thing we're waiting for completes and what happens now is that it (well, the synchronization context) places a message into the queue that the message loop is looking at, saying "Hey, I got some more code for you to execute", and this code is all the code after the await.
6. When the message loop gets to that message, it will basically "re-enter" that method where it left off, just after await and continue executing the rest of the method. Note that this code is again called from the message loop so if this code happens to do something lengthy without using async/await properly, it will again block the message loop

There are many moving parts under the hood here so here are some links to more information, I was going to say "should you need it", but this topic is quite broad and it is fairly important to know some of those moving parts. Invariably you're going to understand that async/await is still a leaky concept. Some of the underlying limitations and problems still leak up into the surrounding code, and if they don't, you usually end up having to debug an application that breaks randomly for seemingly no good reason.

• Asynchronous Programming with Async and Await (C# and Visual Basic)
• SynchronizationContext Class
• Stephen Cleary - There is no thread well worth a read!
• Channel 9 - Mads Torgersen: Inside C# Async well worth a watch!

---

OK, so what if GetSomethingAsync spins up a thread that will complete in 2 seconds? Yes, then obviously there is a new thread in play. This thread, however, is not because of the async-ness of this method, it is because the programmer of this method chose a thread to implement asynchronous code. Almost all asynchronous I/O don't use a thread, they use different things. async/await by themselves do not spin up new threads but obviously the "things we wait for" may be implemented using threads.

There are many things in .NET that do not necessarily spin up a thread on their own but are still asynchronous:

• Web requests (and many other network related things that takes time)
• Asynchronous file reading and writing
• and many more, a good sign is if the class/interface in question has methods named SomethingSomethingAsync or BeginSomething and EndSomething and there's an IAsyncResult involved.

Usually these things do not use a thread under the hood.

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OK, so you want some of that "broad topic stuff"?

Well, let's ask Try Roslyn about our button click:

Try Roslyn

I'm not going to link in the full generated class here but it's pretty gory stuff.