Async ing your way to a successful app with net
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Async’ing Your Way to a Successful App with .NET. Stephen Toub Visual Studio 3-301. Agenda. 6 Tips for Building Better Apps. #1. Wrap Events with Tasks to Improve Control Flow. public async Task RunStoryboardAsync ( this Storyboard sb ) {

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Async ing your way to a successful app with net

Async’ing Your Way to a Successful App with .NET

Stephen Toub

Visual Studio




  • 6Tips for Building Better Apps

Wrap events with tasks to improve control flow


Wrap Events with Tasks to Improve Control Flow



vartcs = newTaskCompletionSource<bool>();

EventHandler<object> handler = (s,e) => tcs.SetResult(true);

sb.Completed += handler; sb.Begin();


sb.Completed -= handler;


publicTask<T> RunAsync(Func<T> func)


vartcs = newTaskCompletionSource<T>();

ThreadPool.QueueUserWorkItem(delegate {

try { tcs.SetResult(func()); }

catch (Exception e) { tcs.SetException(e); }




public Task Delay(int milliseconds)


vartcs = newTaskCompletionSource<bool>();

new Timer(_ => tcs.SetResult(true)).Change(milliseconds, -1);



publicasyncTaskWaitForClickAsync(thisButton button)


vartcs = newTaskCompletionSource<bool>();

RoutedEventHandlerhandler = (s,e) => tcs.SetResult(true);

button.Click += handler;


button.Click -= handler;


publicTask<T> FooAsync()


vartcs = newTaskCompletionSource<T>();

// schedule something asynchronously

// to invoke tcs.Set*



Library Support


Can be used to wrap arbitrary operations as a Task

Once operations are exposed as Tasks, you can compose over them as you would any Task


Wrapping events with tasks to improve control flow

Wrapping Events with Tasks to Improve Control Flow

  • Awaiting tasks makes it easy to compose sequences or graphs of operations.

Use cancellation to improve responsiveness


Use Cancellation to Improve Responsiveness











Library Support

CancellationTokenas a composable cancellation mechanism

varcts = new CancellationTokenSource();




Using cancellation to improve responsiveness

Using Cancellation to Improve Responsiveness

  • Responsiveness isn’t just about keeping the UI available for interaction. More broadly, it’s about doing what the user wants when the user wants.

Be wary of async void


Be Wary of “Async Void”

Language Support

asyncvoid, async Task, & async Task<T>

async Task & async Task<T> return handles to the outstanding work

asyncvoid is a “fire-and-forget” mechanism


Use async void methods only for top-level entry points (e.g. UI event handlers)

(Use and await asyncTask-returning methods everywhere else)

Avoid passing async lambdas to void-returning delegates

(Don’t pass async lambdas to WinRT methods unless you really know what you’re doing)

Avoiding async void

Avoiding “async void”

  • Many problems in async code can be traced back to using “async void” incorrectly.

Use configureawait for perf responsiveness


Use ConfigureAwait for Perf & Responsiveness

await task; // force continuation back to the current sync context

awaittask.ConfigureAwait(false); // try to continue executing where awaited task completes

SynchronizationContext represents a target for work

DispatcherSynchronizationContext (Post: Dispatcher.BeginInvoke)

WinRTSynchronizationContext (Post: CoreDispatcher.RunAsync)

Await Behavior


Use ConfigureAwait(false) on all awaits in all methods with code that’s context agnostic (which is most libraries)

Using configureawait

Using ConfigureAwait

async void button1_Click(…)


await DoWorkAsync();


async void button1_Click(…)







Console.WriteLine("Done task");





Console.WriteLine("Done task");


Use ConfigureAwait(false)

2. Task.Run schedules work to run on thread pool

1. DoWorkAsync invoked on UI thread

3. Await captures SynchronizationContext and hooks up a continuation to run when task completes

4. UI blocks waiting for DoWorkAsync-returned Task to complete

5. Task.Run task completes on pool & invokes continuation which Posts back to UI thread

6. UI thread still blocked waiting for async operation to complete.


.ConfigureAwait(false) avoids deadlock.

Using configureawait for responsiveness perf

Using ConfigureAwait for Responsiveness & Perf

  • Avoiding using the UI thread unless you actually need to.

Avoid sync over async async over sync in libraries


Avoid “Sync over Async”, “Async over Sync” in Libraries

What does asynchrony really mean

What does asynchrony really mean?


Perform something here and now.

I’ll regain control to execute something else when it’s done.


Initiate something here and now.

I’ll regain control to execute something else “immediately”.

Sync vs async

Sync vs Async

“Pause for 10 seconds, then output 'Hello' to the console.”



publicstatic voidPausePrintAsync(){ThreadPool.QueueUserWorkItem(_ => PausePrint());}

publicstatic TaskPausePrintAsync(){ return Task.Run(() => PausePrint());}

publicstatic voidPausePrint(){var end = DateTime.Now + TimeSpan.FromSeconds(10);while(DateTime.Now < end);Console.WriteLine("Hello");}

Possible scalability problems

“async over sync”

“syncover async”

publicstatic TaskPausePrintAsync(){vartcs = newTaskCompletionSource<bool>(); newTimer(_ => {Console.WriteLine("Hello");tcs.SetResult(true); }).Change(10000, Timeout.Infinite);returntcs.Task;}

publicstatic voidPausePrint(){Task t = PausePrintAsync();t.Wait();}

publicstatic asyncTaskPausePrintAsync(){awaitTask.Delay(10000);Console.WriteLine("Hello");}

Possible responsiveness problems

Avoid sync over async async over sync in libraries1


Avoid “Sync over Async”, “Async over Sync” in Libraries


Be honest! Suffix should help caller to understand implementation.

Define “FooAsync” iffyou’re not thread-bound (with a few notable exceptions).

Don’t just expose FooAsyncas “return Task.Run(() => Foo());”

Define “Foo” iffyou have a faster sync implementation that won’t deadlock.

Don’t just expose Foo as “FooAsync().Wait();”


Methods exposed from .WinMD assemblies

Overrides in some class hierarchies

Use visual studio s async tooling


Use Visual Studio’s async tooling

Visual Studio 2012: Stepping with Async

Step In

Step Over

Step Out

Stepping in over and out with async

Stepping in, over, and out with async

  • Stepping through your code as if it were synchronous.

Debugging async in visual studio 2013


Debugging Async in Visual Studio 2013

Visual Studio 2013: Call Stack Window

Provides a “call stack” across async points.

Visual Studio 2013: Tasks Window

See and navigate from a list all active async operations.

Using the call stack and task windows with async

Using the Call Stack and Task Windows with async

  • Knowing how you got here and what’s going on



  • 6Tips for Building Better Apps

  • Wrap events with tasks to improve control flow

  • Use cancellation to improve responsiveness

  • Be wary of “async void”

  • Use ConfigureAwait to improve perf & responsiveness

  • Avoid exposing “sync over async” or “async over sync”

  • Use Visual Studio’s async tooling

Questions? Comments?



  • Parallel Programming in .NET Blog:


  • Task-based Async Pattern Whitepaper


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