1. Asynchronous Programming in .NET
Mads Torgersen
C# Language PM
Microsoft

2. C# Evolution Asynchrony Dynamic LINQ Generics Managed Code C# 5.0

3. The Future is the Future
Asynchrony is becoming the norm
Responsiveness
Scalability
Futures: Modern abstractions
.NET Tasks
JS Promises
WinRT Async Operations

4. Futures and Promises Tasks
Objects that separate initiation of activities from consumption of their outcome
Tasks

5. Synchronous vs. Asynchronous
var data = DownloadData(...);
ProcessData(data);
STOP
DownloadData
ProcessData
var future = DownloadDataAsync(...);
future.ContinueWith(data => ProcessData(data));
DownloadDataAsync
ProcessData

6. Synchronous vs. Asynchronous
var data = DownloadData(...);
ProcessData(data);
STOP
STOP
DownloadData
ProcessData
var future = DownloadDataAsync(...);
future.ContinueWith(data => ProcessData(data));
DownloadDataAsync
ProcessData

7. Asynchronous Responsive UI
11/21/2018 1:11 AM
Asynchronous Responsive UI
demo
© 2007 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.

8. Unifying Asynchrony Asynchronous Methods Task CPU Network I/O
Composite

9. How Does it Work? async Task<XElement> GetRssAsync(string url) {
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
var text = await task;
var xml = XElement.Parse(text);
return xml; }

10. How Does it Work? (Sort of)
async Task<XElement> GetRssAsync(string url) {
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
return task.ContinueWith(t => {
var text = t.Result;
var xml = XElement.Parse(text);
return xml;
}); }

11. How Does it Work? (Really)
async Task<XElement> GetRssAsync(string url) {
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
var text = await task;
var xml = XElement.Parse(text);
return xml; }

12. How Does it Work? (Really)
Task<XElement> GetRssAsync(string url) {
var $builder = AsyncTaskMethodBuilder<XElement>.Create();
var $state = 0;
TaskAwaiter<string> $a1;
Action $resume = delegate {
try {
if ($state == 1) goto L1;
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
$state = 1;
$a1 = task.GetAwaiter();
if (!$a1.IsCompleted) return;
$a1.OnCompleted($resume);
L1: var text = $a1.GetResult();
var xml = XElement.Parse(text);
$builder.SetResult(xml); }
catch (Exception $ex) { $builder.SetException($ex); } };
$resume();
return $builder.Task;
async Task<XElement> GetRssAsync(string url) {
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
var text = await task;
var xml = XElement.Parse(text);
return xml; }

13. Asynchronous Methods
No more callbacks!

14. © 2011 Microsoft Corporation. All rights reserved
© 2011 Microsoft Corporation. All rights reserved. Microsoft, Visual Studio, the Visual Studio logo, and [list other trademarks referenced] are trademarks of the Microsoft group of companies.
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.

15. Asynchronous Control Flow
async void DoWorkAsync() {
var t1 = ProcessFeedAsync("
var t2 = ProcessFeedAsync("
await Task.WhenAll(t1, t2);
DisplayMessage("Done"); }
async Task ProcessFeedAsync(string url) {
var text = await DownloadFeedAsync(url);
var doc = ParseFeedIntoDoc(text);
await SaveDocAsync(doc);
ProcessLog.WriteEntry(url); }
Message Pump
UI Thread

16. Asynchronous Control Flow
async void DoWorkAsync() {
var t1 = ProcessFeedAsync("
var t2 = ProcessFeedAsync("
await Task.WhenAll(t1, t2);
DisplayMessage("Done"); }
async Task ProcessFeedAsync(string url) {
var text = await DownloadFeedAsync(url);
var doc = ParseFeedIntoDoc(text);
await SaveDocAsync(doc);
ProcessLog.WriteEntry(url); }

17. Asynchronous Control Flow
async void DoWorkAsync() {
var t1 = ProcessFeedAsync("
var t2 = ProcessFeedAsync("
await Task.WhenAll(t1, t2);
DisplayMessage("Done"); }
async Task ProcessFeedAsync(string url) {
var text = await DownloadFeedAsync(url);
var doc = ParseFeedIntoDoc(text);
await SaveDocAsync(doc);
ProcessLog.WriteEntry(url); }

18. Asynchronous Control Flow
async void DoWorkAsync() {
var t1 = ProcessFeedAsync("
var t2 = ProcessFeedAsync("
await Task.WhenAll(t1, t2);
DisplayMessage("Done"); }
async Task ProcessFeedAsync(string url) {
var text = await DownloadFeedAsync(url);
var doc = ParseFeedIntoDoc(text);
await SaveDocAsync(doc);
ProcessLog.WriteEntry(url); }

19. Asynchronous Control Flow
async void DoWorkAsync() {
var t1 = ProcessFeedAsync("
var t2 = ProcessFeedAsync("
await Task.WhenAll(t1, t2);
DisplayMessage("Done"); }
async Task ProcessFeedAsync(string url) {
var text = await DownloadFeedAsync(url);
var doc = ParseFeedIntoDoc(text);
await SaveDocAsync(doc);
ProcessLog.WriteEntry(url); } t1

20. Asynchronous Control Flow
async void DoWorkAsync() {
var t1 = ProcessFeedAsync("
var t2 = ProcessFeedAsync("
await Task.WhenAll(t1, t2);
DisplayMessage("Done"); }
async Task ProcessFeedAsync(string url) {
var text = await DownloadFeedAsync(url);
var doc = ParseFeedIntoDoc(text);
await SaveDocAsync(doc);
ProcessLog.WriteEntry(url); } t1 t2

21. Asynchronous Control Flow
async void DoWorkAsync() {
var t1 = ProcessFeedAsync("
var t2 = ProcessFeedAsync("
await Task.WhenAll(t1, t2);
DisplayMessage("Done"); }
async Task ProcessFeedAsync(string url) {
var text = await DownloadFeedAsync(url);
var doc = ParseFeedIntoDoc(text);
await SaveDocAsync(doc);
ProcessLog.WriteEntry(url); } t1 t2

22. Asynchronous Control Flow
async void DoWorkAsync() {
var t1 = ProcessFeedAsync("
var t2 = ProcessFeedAsync("
await Task.WhenAll(t1, t2);
DisplayMessage("Done"); }
async Task ProcessFeedAsync(string url) {
var text = await DownloadFeedAsync(url);
var doc = ParseFeedIntoDoc(text);
await SaveDocAsync(doc);
ProcessLog.WriteEntry(url); } t1 t2 

23. Asynchronous Control Flow
async void DoWorkAsync() {
var t1 = ProcessFeedAsync("
var t2 = ProcessFeedAsync("
await Task.WhenAll(t1, t2);
DisplayMessage("Done"); }
async Task ProcessFeedAsync(string url) {
var text = await DownloadFeedAsync(url);
var doc = ParseFeedIntoDoc(text);
await SaveDocAsync(doc);
ProcessLog.WriteEntry(url); } t1 t2 

24. Asynchronous Control Flow
async void DoWorkAsync() {
var t1 = ProcessFeedAsync("
var t2 = ProcessFeedAsync("
await Task.WhenAll(t1, t2);
DisplayMessage("Done"); }
async Task ProcessFeedAsync(string url) {
var text = await DownloadFeedAsync(url);
var doc = ParseFeedIntoDoc(text);
await SaveDocAsync(doc);
ProcessLog.WriteEntry(url); } t1 t2  

25. Asynchronous Control Flow
async void DoWorkAsync() {
var t1 = ProcessFeedAsync("
var t2 = ProcessFeedAsync("
await Task.WhenAll(t1, t2);
DisplayMessage("Done"); }
async Task ProcessFeedAsync(string url) {
var text = await DownloadFeedAsync(url);
var doc = ParseFeedIntoDoc(text);
await SaveDocAsync(doc);
ProcessLog.WriteEntry(url); } t1 t2  

26. Asynchronous Control Flow
async void DoWorkAsync() {
var t1 = ProcessFeedAsync("
var t2 = ProcessFeedAsync("
await Task.WhenAll(t1, t2);
DisplayMessage("Done"); }
async Task ProcessFeedAsync(string url) {
var text = await DownloadFeedAsync(url);
var doc = ParseFeedIntoDoc(text);
await SaveDocAsync(doc);
ProcessLog.WriteEntry(url); } t1 t2   

27. Asynchronous Control Flow
async void DoWorkAsync() {
var t1 = ProcessFeedAsync("
var t2 = ProcessFeedAsync("
await Task.WhenAll(t1, t2);
DisplayMessage("Done"); }
async Task ProcessFeedAsync(string url) {
var text = await DownloadFeedAsync(url);
var doc = ParseFeedIntoDoc(text);
await SaveDocAsync(doc);
ProcessLog.WriteEntry(url); } t1 t2    

28. Asynchronous Control Flow
async void DoWorkAsync() {
var t1 = ProcessFeedAsync("
var t2 = ProcessFeedAsync("
await Task.WhenAll(t1, t2);
DisplayMessage("Done"); }
async Task ProcessFeedAsync(string url) {
var text = await DownloadFeedAsync(url);
var doc = ParseFeedIntoDoc(text);
await SaveDocAsync(doc);
ProcessLog.WriteEntry(url); } t1 t2     

29. Asynchronous Control Flow
async void DoWorkAsync() {
var t1 = ProcessFeedAsync("
var t2 = ProcessFeedAsync("
await Task.WhenAll(t1, t2);
DisplayMessage("Done"); }
async Task ProcessFeedAsync(string url) {
var text = await DownloadFeedAsync(url);
var doc = ParseFeedIntoDoc(text);
await SaveDocAsync(doc);
ProcessLog.WriteEntry(url); } t1 t2      

30. Asynchronous Control Flow
async void DoWorkAsync() {
var t1 = ProcessFeedAsync("
var t2 = ProcessFeedAsync("
await Task.WhenAll(t1, t2);
DisplayMessage("Done"); }
async Task ProcessFeedAsync(string url) {
var text = await DownloadFeedAsync(url);
var doc = ParseFeedIntoDoc(text);
await SaveDocAsync(doc);
ProcessLog.WriteEntry(url); } t1 t2       

31. Asynchronous Control Flow
async void DoWorkAsync() {
var t1 = ProcessFeedAsync("
var t2 = ProcessFeedAsync("
await Task.WhenAll(t1, t2);
DisplayMessage("Done"); }
async Task ProcessFeedAsync(string url) {
var text = await DownloadFeedAsync(url);
var doc = ParseFeedIntoDoc(text);
await SaveDocAsync(doc);
ProcessLog.WriteEntry(url); } t1 t2       

32. How Does it Work? async Task<XElement> GetRssAsync(string url) {
var $builder = AsyncTaskMethodBuilder<XElement>.Create();
var $state = 0;
TaskAwaiter<string> $a1;
Action $resume = delegate {
try {
if ($state == 1) goto L1;
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
$state = 1;
$a1 = task.GetAwaiter();
if (!$a1.IsCompleted) return;
$a1.OnCompleted($resume);
L1: var text = $a1.GetResult();
var xml = XElement.Parse(text);
$builder.SetResult(xml); }
catch (Exception $ex) { $builder.SetException($ex); } };
$resume();
return $builder.Task;
async Task<XElement> GetRssAsync(string url) {
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
var text = await task;
var xml = XElement.Parse(text);
return xml; }

33. How Does it Work? async Task<XElement> GetRssAsync(string url) {
var $builder = AsyncTaskMethodBuilder<XElement>.Create();
var $state = 0;
TaskAwaiter<string> $a1;
Action $resume = delegate {
try {
if ($state == 1) goto L1;
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
$state = 1;
$a1 = task.GetAwaiter();
if (!$a1.IsCompleted) return;
$a1.OnCompleted($resume);
L1: var text = $a1.GetResult();
var xml = XElement.Parse(text);
$builder.SetResult(xml); }
catch (Exception $ex) { $builder.SetException($ex); } };
$resume();
return $builder.Task;
async Task<XElement> GetRssAsync(string url) {
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
var text = await task;
var xml = XElement.Parse(text);
return xml; }

34. How Does it Work? async Task<XElement> GetRssAsync(string url) {
var $builder = AsyncTaskMethodBuilder<XElement>.Create();
var $state = 0;
TaskAwaiter<string> $a1;
Action $resume = delegate {
try {
if ($state == 1) goto L1;
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
$state = 1;
$a1 = task.GetAwaiter();
if (!$a1.IsCompleted) return;
$a1.OnCompleted($resume);
L1: var text = $a1.GetResult();
var xml = XElement.Parse(text);
$builder.SetResult(xml); }
catch (Exception $ex) { $builder.SetException($ex); } };
$resume();
return $builder.Task;
async Task<XElement> GetRssAsync(string url) {
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
var text = await task;
var xml = XElement.Parse(text);
return xml; }

35. How Does it Work? async Task<XElement> GetRssAsync(string url) {
var $builder = AsyncTaskMethodBuilder<XElement>.Create();
var $state = 0;
TaskAwaiter<string> $a1;
Action $resume = delegate {
try {
if ($state == 1) goto L1;
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
$state = 1;
$a1 = task.GetAwaiter();
if (!$a1.IsCompleted) return;
$a1.OnCompleted($resume);
L1: var text = $a1.GetResult();
var xml = XElement.Parse(text);
$builder.SetResult(xml); }
catch (Exception $ex) { $builder.SetException($ex); } };
$resume();
return $builder.Task;
async Task<XElement> GetRssAsync(string url) {
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
var text = await task;
var xml = XElement.Parse(text);
return xml; }

36. How Does it Work? async Task<XElement> GetRssAsync(string url) {
var $builder = AsyncTaskMethodBuilder<XElement>.Create();
var $state = 0;
TaskAwaiter<string> $a1;
Action $resume = delegate {
try {
if ($state == 1) goto L1;
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
$state = 1;
$a1 = task.GetAwaiter();
if (!$a1.IsCompleted) return;
$a1.OnCompleted($resume);
L1: var text = $a1.GetResult();
var xml = XElement.Parse(text);
$builder.SetResult(xml); }
catch (Exception $ex) { $builder.SetException($ex); } };
$resume();
return $builder.Task;
async Task<XElement> GetRssAsync(string url) {
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
var text = await task;
var xml = XElement.Parse(text);
return xml; }

37. How Does it Work? async Task<XElement> GetRssAsync(string url) {
var $builder = AsyncTaskMethodBuilder<XElement>.Create();
var $state = 0;
TaskAwaiter<string> $a1;
Action $resume = delegate {
try {
if ($state == 1) goto L1;
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
$state = 1;
$a1 = task.GetAwaiter();
if (!$a1.IsCompleted) return;
$a1.OnCompleted($resume);
L1: var text = $a1.GetResult();
var xml = XElement.Parse(text);
$builder.SetResult(xml); }
catch (Exception $ex) { $builder.SetException($ex); } };
$resume();
return $builder.Task;
async Task<XElement> GetRssAsync(string url) {
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
var text = await task;
var xml = XElement.Parse(text);
return xml; }

38. How Does it Work? async Task<XElement> GetRssAsync(string url) {
var $builder = AsyncTaskMethodBuilder<XElement>.Create();
var $state = 0;
TaskAwaiter<string> $a1;
Action $resume = delegate {
try {
if ($state == 1) goto L1;
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
$state = 1;
$a1 = task.GetAwaiter();
if (!$a1.IsCompleted) return;
$a1.OnCompleted($resume);
L1: var text = $a1.GetResult();
var xml = XElement.Parse(text);
$builder.SetResult(xml); }
catch (Exception $ex) { $builder.SetException($ex); } };
$resume();
return $builder.Task;
async Task<XElement> GetRssAsync(string url) {
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
var text = await task;
var xml = XElement.Parse(text);
return xml; }

39. How Does it Work? async Task<XElement> GetRssAsync(string url) {
var $builder = AsyncTaskMethodBuilder<XElement>.Create();
var $state = 0;
TaskAwaiter<string> $a1;
Action $resume = delegate {
try {
if ($state == 1) goto L1;
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
$state = 1;
$a1 = task.GetAwaiter();
if (!$a1.IsCompleted) return;
$a1.OnCompleted($resume);
L1: var text = $a1.GetResult();
var xml = XElement.Parse(text);
$builder.SetResult(xml); }
catch (Exception $ex) { $builder.SetException($ex); } };
$resume();
return $builder.Task;
async Task<XElement> GetRssAsync(string url) {
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
var text = await task;
var xml = XElement.Parse(text);
return xml; }

40. How Does it Work? async Task<XElement> GetRssAsync(string url) {
var $builder = AsyncTaskMethodBuilder<XElement>.Create();
var $state = 0;
TaskAwaiter<string> $a1;
Action $resume = delegate {
try {
if ($state == 1) goto L1;
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
$state = 1;
$a1 = task.GetAwaiter();
if (!$a1.IsCompleted) return;
$a1.OnCompleted($resume);
L1: var text = $a1.GetResult();
var xml = XElement.Parse(text);
$builder.SetResult(xml); }
catch (Exception $ex) { $builder.SetException($ex); } };
$resume();
return $builder.Task;
async Task<XElement> GetRssAsync(string url) {
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
var text = await task;
var xml = XElement.Parse(text);
return xml; }

41. How Does it Work? async Task<XElement> GetRssAsync(string url) {
var $builder = AsyncTaskMethodBuilder<XElement>.Create();
var $state = 0;
TaskAwaiter<string> $a1;
Action $resume = delegate {
try {
if ($state == 1) goto L1;
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
$state = 1;
$a1 = task.GetAwaiter();
if (!$a1.IsCompleted) return;
$a1.OnCompleted($resume);
L1: var text = $a1.GetResult();
var xml = XElement.Parse(text);
$builder.SetResult(xml); }
catch (Exception $ex) { $builder.SetException($ex); } };
$resume();
return $builder.Task;
async Task<XElement> GetRssAsync(string url) {
var client = new WebClient();
var task = client.DownloadStringTaskAsync(url);
var text = await task;
var xml = XElement.Parse(text);
return xml; }

42. Awaiting a Task public class Task<TResult> : Task { ...
{ ...
    public TaskAwaiter<TResult> GetAwaiter(); }
public struct TaskAwaiter<TResult> {
public bool IsCompleted { get; }
public void OnCompleted(Action a);
    public TResult GetResult();

43. Awaiting a Task public class Task<TResult> : Task { ...
{ ...
    public TaskAwaiter<TResult> GetAwaiter(); }
public struct TaskAwaiter<TResult> {
public bool IsCompleted { get; }
public void OnCompleted(Action a)
var sc = SynchronizationContext.Current;
task.ContinueWith(_ => sc.Post(a));
    public TResult GetResult();