1. Build 2012
1/2/2019
© 2012 Microsoft Corporation. All rights reserved. Microsoft, Windows, and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

2. Async’ing Your Way to a Successful App with .NET
Windows Azure
1/2/2019
Async’ing Your Way to a Successful App with .NET
Stephen Toub
Visual Studio
3-301
© 2011 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries. The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

3. Agenda
6 Tips for Building Better Apps

4. Wrap Events with Tasks to Improve Control Flow#1
Library Support
TaskCompletionSource<T>
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
Pattern
public Task<T> FooAsync() {
var tcs = new TaskCompletionSource<T>();
// schedule something asynchronously
// to invoke tcs.Set*
return tcs.Task; }
public Task Delay(int milliseconds) {
var tcs = new TaskCompletionSource<bool>();
new Timer(_ => tcs.SetResult(true)).Change(milliseconds, -1);
return tcs.Task; }
public async Task RunStoryboardAsync(this Storyboard sb) {
var tcs = new TaskCompletionSource<bool>();
EventHandler<object> handler = (s,e) => tcs.SetResult(true);
sb.Completed += handler; sb.Begin();
await tcs.Task;
sb.Completed -= handler; }
public Task<T> RunAsync(Func<T> func) {
var tcs = new TaskCompletionSource<T>();
ThreadPool.QueueUserWorkItem(delegate {
try { tcs.SetResult(func()); }
catch (Exception e) { tcs.SetException(e); }
});
return tcs.Task; }
public async Task WaitForClickAsync(this Button button) {
var tcs = new TaskCompletionSource<bool>();
RoutedEventHandler handler = (s,e) => tcs.SetResult(true);
button.Click += handler;
await tcs.Task;
button.Click -= handler; }

5. Wrapping Events with Tasks to Improve Control Flow
Build 2012
1/2/2019
Wrapping Events with Tasks to Improve Control Flow
Awaiting tasks makes it easy to compose sequences or graphs of operations.
© 2012 Microsoft Corporation. All rights reserved. Microsoft, Windows, and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

6. Use Cancellation to Improve Responsiveness#2
Library Support
CancellationToken as a composable cancellation mechanism
var cts = new CancellationTokenSource();
SomeMethod(cts.Token);
cts.Cancel();
Pattern
public async Task FooAsync() { …
await SomeDotNetAsync();
await SomeWinRTAsync(); }
public async Task FooAsync(CancellationToken cancellationToken) { …
await SomeDotNetAsync(cancellationToken);
await SomeWinRTAsync().AsTask(cancellationToken); }

7. Using Cancellation to Improve Responsiveness
Build 2012
1/2/2019
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.
© 2012 Microsoft Corporation. All rights reserved. Microsoft, Windows, and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

8. Be Wary of “Async Void” Language Support Guidance #3
async void, async Task, & async Task<T>
async Task & async Task<T> return handles to the outstanding work
async void is a “fire-and-forget” mechanism
Guidance
Use async void methods only for top-level entry points (e.g. UI event handlers)
(Use and await async Task-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)

9. Build 2012
1/2/2019
Avoiding “async void”
Many problems in async code can be traced back to using “async void” incorrectly.
© 2012 Microsoft Corporation. All rights reserved. Microsoft, Windows, and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

10. Use ConfigureAwait for Perf & Responsiveness#4
SynchronizationContext represents a target for work
DispatcherSynchronizationContext (Post: Dispatcher.BeginInvoke)
WinRTSynchronizationContext (Post: CoreDispatcher.RunAsync)
Await Behavior
Guidance
Use ConfigureAwait(false) on all awaits in all methods with code that’s context agnostic (which is most libraries)
await task; // force continuation back to the current sync context
await task.ConfigureAwait(false); // try to continue executing where awaited task completes

11. .ConfigureAwait(false) avoids deadlock.
Using ConfigureAwait
async void button1_Click(…) {
await DoWorkAsync(); }
async void button1_Click(…) {
DoWorkAsync().Wait(); }
async Task DoWorkAsync() {
await Task.Run(…).ConfigureAwait(false);
Console.WriteLine("Done task"); }
async Task DoWorkAsync() {
await Task.Run(…);
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.
Deadlock!
.ConfigureAwait(false) avoids deadlock.

12. Using ConfigureAwait for Responsiveness & Perf
Build 2012
1/2/2019
Using ConfigureAwait for Responsiveness & Perf
Avoiding using the UI thread unless you actually need to.
© 2012 Microsoft Corporation. All rights reserved. Microsoft, Windows, and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

13. Avoid “Sync over Async”, “Async over Sync” in Libraries#5

14. What does asynchrony really mean?
Synchronous
Perform something here and now.
I’ll regain control to execute something else when it’s done.
Asynchronous
Initiate something here and now.
I’ll regain control to execute something else “immediately”.

15. Sync vs Async Synchronous Asynchronous
TechReady 16
1/2/2019
Sync vs Async
“Pause for 10 seconds, then output 'Hello' to the console.”
Synchronous
Asynchronous
public static void PausePrint() { var end = DateTime.Now TimeSpan.FromSeconds(10); while(DateTime.Now < end); Console.WriteLine("Hello"); }
public static void PausePrintAsync() { ThreadPool.QueueUserWorkItem(_ => PausePrint()); }
public static Task PausePrintAsync() { return Task.Run(() => PausePrint()); }
“async over sync”
Possible scalability problems
public static void PausePrint() { Task t = PausePrintAsync(); t.Wait(); }
public static async Task PausePrintAsync() { await Task.Delay(10000); Console.WriteLine("Hello"); }
public static Task PausePrintAsync() { var tcs = new TaskCompletionSource<bool>(); new Timer(_ => { Console.WriteLine("Hello"); tcs.SetResult(true); }).Change(10000, Timeout.Infinite); return tcs.Task; }
“sync over async”
Possible responsiveness problems
© 2013 Microsoft Corporation. All rights reserved. Microsoft, Windows, and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

16. Avoid “Sync over Async”, “Async over Sync” in Libraries#5
Guidance
Be honest! Suffix should help caller to understand implementation.
Define “FooAsync” iff you’re not thread-bound (with a few notable exceptions).
Don’t just expose FooAsync as “return Task.Run(() => Foo());”
Define “Foo” iff you have a faster sync implementation that won’t deadlock.
Don’t just expose Foo as “FooAsync().Wait();”
Exceptions
Methods exposed from .WinMD assemblies
Overrides in some class hierarchies

17. Use Visual Studio’s async tooling#6
Visual Studio 2012: Stepping with Async
Step In
Step Over
Step Out

18. Stepping in, over, and out with async
Build 2012
1/2/2019
Stepping in, over, and out with async
Stepping through your code as if it were synchronous.
© 2012 Microsoft Corporation. All rights reserved. Microsoft, Windows, and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

19. Debugging Async in Visual Studio 2013#7
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.

20. Using the Call Stack and Task Windows with async
Build 2012
1/2/2019
Using the Call Stack and Task Windows with async
Knowing how you got here and what’s going on
© 2012 Microsoft Corporation. All rights reserved. Microsoft, Windows, and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

21. Questions? Comments? Summary 6 Tips 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?

22. Resources Parallel Programming in .NET Blog:
Task-based Async Pattern Whitepaper

23. 1/2/2019 7:34 PM
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© 2013 Microsoft Corporation. All rights reserved. Microsoft, Windows, and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries. The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.