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Addressing a Simple Yet Common C# Async/Await Misconception

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19 Feb 2018CPOL2 min read 73.1K   181   65   54
I regularly come across developers who hold the misconception that code in a method will continue to be executed, in parallel to code in an awaited method call. So I'm going to demonstrate the behaviour we should expect in this article.

Async/await has been part of C# since C# 5.0 yet many developers have yet to explore how it works under the covers, which I would recommend for any syntactic sugar in C#. I won’t be going into that level of detail now, nor will I explore the subtleties of IO and CPU bound operations.

The Common Misconception

That awaited code executes in parallel to the code that follows.

i.e. in the following code, LongOperation() is called and awaited, then while this is executing and before it has completed, the code ‘do other things’ will start being executed.

C++
void DemoAsyncAwait()
{
    WithAwaitAtCallAsync().Wait();
}

async Task WithAwaitAtCallAsync()
{
    await LongOperation();

    // do other things
}

This is not how it behaves.

In the above code, what actually happens is that the await operator causes WithAwaitAtCallAsync() to suspend at that line and returns control back to DemoAsyncAwait() until the awaited task, LongOperation(), is complete.

When LongOperation() completes, then ‘do other things’ will be executed.

And If We Don't Await When We Call?

Then you do get that behaviour some developers innocently expect from awaiting the call, where LongOperation() is left to complete in the background while continuing on with WithoutAwaitAtCallAsync() in parallel, 'doing other things':

C#
void DemoAsyncAwait()
{
    WithoutAwaitAtCallAsync().Wait(); 
} 

async Task WithoutAwaitAtCallAsync() 
{ 
    Task task = LongOperation(); 

    // doing other things 
   
    await task;

    // more things to do
}

However, if LongOperation() is not complete when we reach the awaited Task it returned, then it yields control back to DemoAsyncAwait(), as above. It does not continue to complete 'more things to do' - not until the awaited task is complete.

Complete Console Application Example

Some notes about this code:

  • Always use await over Task.Wait() to retrieve the result of a background task (outside of this demo) to avoid blocking. I've used Task.Wait() in my demonstrations to force blocking and prevent the two separate demo results overlapping in time (I know in hindsight, Winforms would have been a better demo).
  • I have intentionally not used Task.Run() as I don't want to confuse things with new threads. Let's just assume LongOperation() is IO-bound.
  • I used Task.Delay() to simulate the long operation. Thread.Sleep() would block the thread.
C#
private static void Main(string[] args)
{
    // Demo 1
    Console.WriteLine(" Demo 1: Awaiting call to long operation:");
    Task withAwaitAtCallTask = WithAwaitAtCallAsync();
    withAwaitAtCallTask.Wait();

    // Demo 2
    Console.WriteLine(" Demo 2: NOT awaiting call to long operation:");
    Task withoutAwaitAtCallTask = WithoutAwaitAtCallAsync();
    withoutAwaitAtCallTask.Wait();

    Console.ReadKey();
}

private static async Task WithAwaitAtCallAsync()
{   
    Console.WriteLine(" WithAwaitAtCallAsync() entered.");

    Console.WriteLine(" Awaiting when I call LongOperation().");
    await LongOperation();

    Console.WriteLine(" Pretending to do other work in WithAwaitAtCallAsync().");
}

private static async Task WithoutAwaitAtCallAsync()
{
    Console.WriteLine(" WithoutAwaitAtCallAsync() entered.");

    Console.WriteLine(" Call made to LongOperation() with NO await.");
    Task task = LongOperation();

    Console.WriteLine(" Do some other work in WithoutAwaitAtCallAsync() after calling LongOperation().");

    await task;
}

private static async Task LongOperation()
{
     Console.WriteLine(" LongOperation() entered.");

     Console.WriteLine(" Starting the long (3 second) process in LongOperation()...");
     await Task.Delay(4000);
     Console.WriteLine(" Completed the long (3 second) process in LongOperation()...");
}

This is what happens when the code is executed (with colouring):

GiF of example

Summary

If you use the await keyword when calling an async method (from inside an async method), execution of the calling method is suspended to avoid blocking the thread and control is passed (or yielded) back up the method chain. If, on its journey up the chain, it reaches a call that was not awaited, then code in that method is able to continue in parallel to the remaining processing in the chain of awaited methods until it runs out of work to do, and then needs to await the result, which is inside the Task object returned by LongOperation().

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


Written By
Software Developer (Senior)
United Kingdom United Kingdom
Ben is the Principal Developer at a gov.uk and .NET Foundation foundation member. He previously worked for over 9 years as a school teacher, teaching programming and Computer Science. He enjoys making complex topics accessible and practical for busy developers.


Comments and Discussions

 
GeneralRe: Will this work with a UI Thread? Pin
George Swan13-Feb-18 7:47
mveGeorge Swan13-Feb-18 7:47 
GeneralRe: Will this work with a UI Thread? Pin
Stephen Russell AAI MCT14-Feb-18 1:35
professionalStephen Russell AAI MCT14-Feb-18 1:35 
GeneralRe: Will this work with a UI Thread? Pin
George Swan14-Feb-18 3:06
mveGeorge Swan14-Feb-18 3:06 
QuestionOK couple of things Pin
Sacha Barber11-Feb-18 10:43
Sacha Barber11-Feb-18 10:43 
AnswerRe: OK couple of things Pin
Ben Hall (failingfast.io)12-Feb-18 1:46
professionalBen Hall (failingfast.io)12-Feb-18 1:46 
GeneralRe: OK couple of things Pin
wkempf13-Feb-18 3:50
wkempf13-Feb-18 3:50 
GeneralRe: OK couple of things Pin
Ben Hall (failingfast.io)13-Feb-18 6:36
professionalBen Hall (failingfast.io)13-Feb-18 6:36 
QuestionGood Tip Pin
Dirk Bahle11-Feb-18 7:14
Dirk Bahle11-Feb-18 7:14 
Hi Ben,

I really like your tip - You got my 5 - I would go even a bit further and present some sample codes with UML sequence diagrams to make sure its understood Smile | :) I am writing this because I am wondering if you have an opinion on a comman task situation I find myself running into from time to time:

The problem:
I have an application that has the potential to run multiple background tasks
but only one such task should be running at any time. I was able to resolve one
such situation in the FilterTreeView application with a:

SemaphoreSlim and a
_Queue = new Dictionary<string, cancellationtokensource="">(); of CancelToken

in order to be able to cancel tasks later on.

My question is, would you handle mutliple tasks, cancellation, and processing of preferable one task in a similar way or would you do this completely different?

I am facing a similar but much more complex situation with a re-design of my Explorer tool that I currently develop in my MLib project:
GitHub - Dirkster99/MLib: MLib is a set of WPF theming libraries based on MahApps.Metro and MUI

Relevent demo projects are:
- FileListViewTest
- FsContentDialogDemo

You can find the relevant parts in:
- FolderBrowserTreeView.xaml.cs
- DialogBaseViewModel.cs
- TreeListControllerViewModel.cs
- ListControllerViewModel.cs

OK, thats a lot of stuf to look at but the project is coming along really nice and all thats missing now is a clever way of coordinating the browsing tasks - as I mentioned above, the browsing should be cancelable and I do not want more than one task at any time - do you happen to know a pattern for this situation?
AnswerRe: Good Tip Pin
Ben Hall (failingfast.io)12-Feb-18 8:17
professionalBen Hall (failingfast.io)12-Feb-18 8:17 
AnswerRe: Good Tip Pin
wkempf13-Feb-18 3:58
wkempf13-Feb-18 3:58 
GeneralRe: Good Tip Pin
Dirk Bahle14-Feb-18 5:20
Dirk Bahle14-Feb-18 5:20 
GeneralRe: Good Tip Pin
Dirk Bahle18-Feb-18 12:22
Dirk Bahle18-Feb-18 12:22 

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