Asynchronous Programming in C# 5.0 part two: Whence await?

Asynchronous Programming in C# 5.0 part two: Whence await?

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I want to start by being absolutely positively clear about two things, because our usability research has shown this to be confusing. Remember our little program from last time?

async void ArchiveDocuments(List<Url> urls)
  Task archive = null;
  for(int i = 0; i < urls.Count; ++i)
    var document = await FetchAsync(urls[i]);
    if (archive != null)
      await archive;
    archive = ArchiveAsync(document);

The two things are:

1) The “async” modifier on the method does not mean “this method is automatically scheduled to run on a worker thread asynchronously”. It means the opposite of that; it means “this method contains control flow that involves awaiting asynchronous operations and will therefore be rewritten by the compiler into continuation passing style to ensure that the asynchronous operations can resume this method at the right spot.” The whole point of async methods it that you stay on the current thread as much as possible. They’re like coroutines: async methods bring single-threaded cooperative multitasking to C#. (At a later date I’ll discuss the reasons behind requiring the async modifier rather than inferring it.)

2) The “await” operator used twice in that method does not mean “this method now blocks the current thread until the asynchronous operation returns”. That would be making the asynchronous operation back into a synchronous operation, which is precisely what we are attempting to avoid. Rather, it means the opposite of that; it means “if the task we are awaiting has not yet completed then sign up the rest of this method as the continuation of that task, and then return to your caller immediately; the task will invoke the continuation when it completes.

It is unfortunate that people’s intuition upon first exposure regarding what the “async” and “await” contextual keywords mean is frequently the opposite of their actual meanings. Many attempts to come up with better keywords failed to find anything better. If you have ideas for a keyword or combination of keywords that is short, snappy, and gets across the correct ideas, I am happy to hear them. Some ideas that we already had and rejected for various reasons were:

wait for FetchAsync(…)
yield with FetchAsync(…)
yield FetchAsync(…)
while away the time FetchAsync(…)
hearken unto FetchAsync(…)
for sooth Romeo wherefore art thou FetchAsync(…)

Moving on. We’ve got a lot of ground to cover. The next thing I want to talk about is “what exactly are those ‘thingies’ that I handwaved about last time?”

Last time I implied that the C# 5.0 expression

document = await FetchAsync(urls[i])

gets realized as:

state = State.AfterFetch;
fetchThingy = FetchAsync(urls[i]);
if (fetchThingy.SetContinuation(archiveDocuments))
AfterFetch: ;
document = fetchThingy.GetValue();

what’s the thingy?

In our model for asynchrony an asynchronous method typically returns a Task<T>; let’s assume for now that FetchAsync returns a Task<Document>. (Again, I’ll discuss the reasons behind this "Task-based Asynchrony Pattern" at a later date.) The actual code will be realized as:

fetchAwaiter = FetchAsync(urls[i]).GetAwaiter();
state = State.AfterFetch;
if (fetchAwaiter.BeginAwait(archiveDocuments))
AfterFetch: ;
document = fetchAwaiter.EndAwait();

The call to FetchAsync creates and returns a Task<Document> - that is, an object which represents a “hot” running task. Calling this method immediately returns a Task<Document> which is then somehow asynchronously fetches the desired document. Perhaps it runs on another thread, or perhaps it posts itself to some Windows message queue on this thread that some message loop is polling for information about work that needs to be done in idle time, or whatever. That’s its business. What we know is that we need something to happen when it completes. (Again, I’ll discuss single-threaded asynchrony at a later date.)

To make something happen when it completes, we ask the task for an Awaiter, which exposes two methods. BeginAwait signs up a continuation for this task; when the task completes, a miracle happens: somehow the continuation gets called. (Again, how exactly this is orchestrated is a subject for another day.) If BeginAwait returns true then the continuation will be called; if not, then that’s because the task has already completed and there is no need to use the continuation mechanism.

EndAwait extracts the result that was the result of the completed task.

We will provide implementations of BeginAwait and EndAwait on Task (for tasks that are logically void returning) and Task<T> (for tasks that return a value). But what about asynchronous methods that do not return a Task or Task<T> object? Here we’re going to use the same strategy we used for LINQ. In LINQ if you say

from c in customers where c.City == "London" blah blah blah

then that gets translated into

customers.Where(c=>c.City=="London") …

and overload resolution tries to find the best possible Where method by checking to see if customers implements such a method, or, if not, by going to extension methods. The GetAwaiter / BeginAwait / EndAwait pattern will be the same; we’ll just do overload resolution on the transformed expression and see what it comes up with. If we need to go to extension methods, we will.

Finally: why "Task"?

The insight here is that asynchrony does not require parallelism, but parallelism does require asynchrony, and many of the tools useful for parallelism can be used just as easily for non-parallel asynchrony. There is no inherent parallelism in Task; that the Task Parallel Library uses a task-based pattern to represent units of pending work that can be parallelized does not require multithreading.

As I've pointed out a few times, from the point of view of the code that is waiting for a result it really doesn't matter whether that result is being computed in idle time on this thread, in a worker thread in this process, in another process on this machine, on a storage device, or on a machine halfway around the world. What matters is that it's going to take time to compute the result, and this CPU could be doing something else while it is waiting, if only we let it.

The Task class from the TPL already has a lot of investment in it; it's got a cancellation mechanism and other useful features. Rather than invent some new thing, like some new "IFuture" type, we can just extend the existing task-based code to meet our asynchrony needs.

Next time: How to further compose asynchronous tasks.

  • I vote for "yield for" or "yield while".

  • While we're throwing mad ideas around, how about "return until"? After all, that's precisely what it actually does in practice.

  • "yield until" gets my vote.  thank god some language finally got something like this -- i think every serious async coder around has thought of this, but it's never been implemented until now because it requires too much language support.

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  • "yield while/until" as well as "yield return" - is way to loooong to type it frequently.

    +1 for "await" - it's short and clear, ex:

    async void MegaBossAI() {

     while (IsAlive) {

       var player = FindClosestPlayer();

       await kill(player);  // short & human readable

       yield while kill(player);  // hm....

       yield until kill(player); // hm... :)

       return until kill(player); // return? return where? that player is not even killed yet!

       continue after kill(player); // nice, but even longer then YW

       await (kill(player)) { /* and what? place every routine into separate block, what for? */ }

       when kill(player); // looks like conditional op to me

       future kill(player); // mooom, not it the future, i want to kill him NOW!

       // other async stuff



  • I'd pick 'task' as the keyword for the method modifier. That's what it makes.

    But 'await' is fine for the 2nd keyword. (Though I could live with 'hence'.)

  • Comments don't really let me go into enough details here, so I've blogged about it:

  • Hi there, I am also completely confused by the await keyword. It feels like the code is lying, because "await" has the described "blocking, synchronous" feel to it. The yield option doesn't feel right either. What do you think of the following?

    async void ArchiveDocuments(List<Url> urls)


     Task archive = null;

     for(int i = 0; i < urls.Count; ++i)


       var document = will FetchAsync(urls[i]);

       if (archive != null) will archive;

       archive = ArchiveAsync(document);



    Making a literal transition of the snippet, I was trying to come up with some thing that would not have the same artificial feel of what is going on behind the scenes. The "await" keyword needs a lot of explanation, and so does every combination of yield.

    Ok, sure, my suggestion needs some explanation here, too:

    The simple WILL keyword is an indicator that there is something going on somewhere, but not right now. Synchronus code is more like: HEY, DO IT RIGHT NOW OR I, MYSELF WILL STOP DOING ANYTHING ELSE. Using "will" is more like a child saying to his parents: Yeah, I WILL clean up my room some time this week, but not right now, Pokemon is on.

    The parents are good with that, they want the room to be cleaned up by the end of the week. The care for the result asynchronously.

    Especially the part where it says: will FetchAsync this would work, but I myself am not to sure if "will archive" is the right style to use. Maybe renaming the variable could help, so something like:

      if (haveArchived != null) will haveArchived;

       haveArchived = ArchiveAsync(document);

    What do you think?

    bye, @cessor

  • while "await" is a good choice, I would prefer "join" to make it clear that two paths of execution get together.

  • Why not name the things as they behave?

    Actually, await dispatches the async operation with additional code to run as a task was accomplished. So, what about dispatch [with]/dispathches pair instead of await/async? Or "begin invoke with" - a slightly verbose, but a very recognizable pattern.

    If no, I still prefer something alike "continue" or "foo/bar" pair;)

  • +1 for "return until"

    I think "return" is totally the key point in this stuff, and marks the difference between "block until the async stuff completes" and "return to the caller, and resume the method after the async stuff completes".

  • Let's nip the "return until", or any other "return" variant in the bud, please.   This feature doesn't provide return semantics at all.  

    At the point marked by "await" (or some new keyword) the thread is potentially(*) released to execute other work, but flow of control NEVER returns to the caller from that point.

    (*) See Jon Skeet's blog entry linked above for the explanation about why it only potentially releases the thread.

  • @Blake: from Eric's post (bold text added)

    “if the task we are awaiting has not yet completed then sign up the rest of this method as the continuation of that task, and then RETURN TO YOUR CALLER immediately; the task will invoke the continuation when it completes.”

  • Oh and one more after a quick dictionary lookup.

    give way until

    Perhaps better for non English speakers? I still prefer "defer" though

  • @Filini - yes as far as it goes, but async methods are meant to be composed.

    Just as the async method you are writing uses "await" to call various async  methods in it's body, the caller will be using 'await' to call you.   This is the primary benefit, letting the whole call tree be structured in a simple, easy-to-reason-about fashion.    

    Only at the very bottom (where asyncs are being built out of platform primitives), and at the very top (where an entire async workflow is being kicked off with a TaskEx.Run or such) will this pattern not hold true.

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