C#•3y ago

Understanding await control flow [Answered]

According to my understanding, when the control flow reaches an await, it would return immediately to the call site. But what would happen if the method you are currently awaiting awaits another method? Let's say I have 3 methods: Main, AsyncMethod1, and AsyncMethod2. If I call AsyncMethod1 in Main, the control flow would immediately return to Main as far as I know. Then, inside AsyncMethod1, it awaits AsyncMethod2. In that case, would the control flow return to AsyncMethod1? But that wouldn't make much sense because AsyncMethod1 is already awaiting something so it cannot continue. Which part of my understanding is wrong? By the way, if I have an async Main, where would the control flow return to when I await in Main? Does it simply not return?

19 Replies

333fred•3y ago

You're misunderstanding await here. Let's put some real code here to make talking about it easier

async Task Main()
{
    await M1();
}

async Task M1()
{
    await M2();
}

async Task M2()
{
    await Task.Sleep(Timespan.FromSeconds(1));
}

async Task Main()
{
    await M1();
}

async Task M1()
{
    await M2();
}

async Task M2()
{
    await Task.Sleep(Timespan.FromSeconds(1));
}

When you say await, you're saying "I would like this method to resume after the Task that is returned from the method that I'm calling has completed" And you return a task yourself, so the caller has ability to call you, and then ignore that task for a while, and then eventually check to see if it has completed But by doing await, you are signaling "Hey, don't resume me until the thing I awaited is done" Let's rewrite the above code a bit (the real lowering is much more complicated than this, but this is a good example):

palapapaOP•3y ago

Can I understand it as "await will return to the call site immediately if and only if the caller did not await this call"?

333fred•3y ago

void CompilerGeneratedMain()
{
    Main().GetAwaiter().GetResult(); // Synchronously wait for Main to finish
}

Task Main()
{
    return M1().ContinueWith(() => Console.WriteLine("Main"));
}

Task M1()
{
    return M2().ContinueWith(() => Console.WriteLine("M1"));
}

Task M2()
{
    return Task.Sleep(Timespan.FromSeconds(1)).ContinueWith(() => Console.WriteLine("M2"));
}

void CompilerGeneratedMain()
{
    Main().GetAwaiter().GetResult(); // Synchronously wait for Main to finish
}

Task Main()
{
    return M1().ContinueWith(() => Console.WriteLine("Main"));
}

Task M1()
{
    return M2().ContinueWith(() => Console.WriteLine("M1"));
}

Task M2()
{
    return Task.Sleep(Timespan.FromSeconds(1)).ContinueWith(() => Console.WriteLine("M2"));
}

palapapaOP•3y ago

So does the control flow "return immediately" at all?

333fred•3y ago

Ish Let's just take M2 It calls Task.Sleep() (which returns a task), schedules a continuation after it, and returns that task Now, to be clear, this isn't precisely how it works. The actual generated code is a good deal more complicated

palapapaOP•3y ago

From this example, I understood it as "await will only continue after the thing it's awaiting and everything that that thing awaits and so on all completes"

333fred•3y ago

Right, that's the effect All the code knows, though, is that its waiting for that one thing And however long it takes for those to finish is however long it takes

palapapaOP•3y ago

So if I call an async method in a non-async method, is the only way to retrive the return value to block the thread and wait?

333fred•3y ago

Correct That's why async is considered "viral"

palapapaOP•3y ago

So when I await something, will the current thread return to the thread pool and be ready to do anything that the OS wants it to do, instead of blocking and do nothing?

333fred•3y ago

Maybe Depends on whether the thing you await actually defers work and yields the thread It's possible it completes synchronously

palapapaOP•3y ago

But in the case where it's done multithreadedly, that's what happens right?

333fred•3y ago

Uh... mulithreadedly isn't an adequate description for me to answer

palapapaOP•3y ago

What do you mean? Or should I say asynchronously?

333fred•3y ago

I mean that "mulithreadedly" isn't a description of anything It's not even a word Asynchronously is also not a description of anything

palapapaOP•3y ago

Thank you, I got it !solved

333fred•3y ago

$close

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Accord•3y ago

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Understanding await control flow [Answered]