1. Visug
Async in Real Life Applications
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2. http://proq.blogspot.com
@GitteTitter
http://www.qframe.be
@qframedotnet
2

3. Overview
• Async
– The basics
– Exceptions
– Cancellation
– JS
– ...
• In Real Life Applications
• Recommendations
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4. .NET: Where are we?
C# 5.0 + VB 11.0 Windows Runtime + Async
Async CTP
VS11 Beta
C# 4.0 + VB 10.0
Dynamic
C# 3.0 + VB 9.0
Language Integrated Query
C# 2.0 + VB 8.0
Generics
C# 1.0 + VB 7.0
Managed Code
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5. Why Async?
var
var
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6. Why Async
Fast & Fluid
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7. Can we already do this?
Yes, we can!
... But it’s ugly.
... And hard to do.
Begin... – End... Method pairs
Callbacks
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8. public IAsyncResult BeginCopyStreamToStream(
Stream source, Stream destination, AsyncCallback callback, object state)
Can we already do this?
{
var tcs = new TaskCompletionSource<object>(state);
if (callback != null) tcs.Task.ContinueWith(_ => callback(tcs.Task));
var buffer = new byte[0x1000];
Action<IAsyncResult> readWriteLoop = null;
readWriteLoop = iar => {
try {
for (bool isRead = iar == null; ; isRead = !isRead) {
public void CopyStreamToStream(Stream source, Stream destination) switch (isRead) {
case true:
{ iar = source.BeginRead(buffer, 0, buffer.Length,
readResult => {
byte[] buffer = new byte[0x1000]; if (readResult.CompletedSynchronously) return;
readWriteLoop(readResult);
int numRead; }, null);
if (!iar.CompletedSynchronously) return;
while ((numRead = source.Read(buffer, 0, buffer.Length)) != 0) break;
{ case false:
int numRead = source.EndRead(iar);
if (numRead == 0) {
destination.Write(buffer, 0, numRead); tcs.TrySetResult(null);
return;
} }
iar = destination.BeginWrite(buffer, 0, numRead,
} writeResult => {
if (writeResult.CompletedSynchronously) return;
destination.EndWrite(writeResult);
readWriteLoop(null);
}, null);
if (!iar.CompletedSynchronously) return;
destination.EndWrite(iar);
break;
}
}
}
catch (Exception e) {
tcs.TrySetException(e);
}
};
readWriteLoop(null);
return tcs.Task;
}
public void EndCopyStreamToStream(IAsyncResult asyncResult) {
((Task)asyncResult).Wait();
}
8

9. So, what’s new?
C# and Visual Basic let you do
asynchronous programming without
callbacks
9

10. How do we do this?
public void CopyStreamToStream(Stream source, Stream destination)
{
byte[] buffer = new byte[0x1000];
int numRead;
while ((numRead = source.Read(buffer, 0, buffer.Length)) != 0)
{
destination.Write(buffer, 0, numRead);
}
}
public async Task CopyStreamToStreamAsync(Stream source, Stream destination)
{
byte[] buffer = new byte[0x1000];
int numRead;
while ((numRead = await source.ReadAsync(buffer, 0, buffer.Length)) != 0)
{
await destination.WriteAsync(buffer, 0, numRead);
}
} 10

11. What is Async – The Basics
11

12. What is Async – The Basics
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13. What is Async – The Basics
public async Task<XElement> GetXmlAsync(string url) {
var client = new HttpClient();
var response = await client.GetAsync(url);
var text = response.Content.ReadAsString();
return XElement.Parse(text);
}
public Task<XElement> GetXmlAsync(string url) {
var tcs = new TaskCompletionSource<XElement>();
var client = new HttpClient();
client.GetAsync(url).ContinueWith(task => {
var response = task.Result;
var text = response.Content.ReadAsString();
tcs.SetResult(XElement.Parse(text));
});
return tcs.Task;
}
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14. DEMO
Some basic async
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15. So it actually gives me Task or Task<T>
Yes. With different states.
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16. What are those states?
Created Start Started Completed Completed Close Closed
Error
Error
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17. How do we handle errors?
Just use try - catch
try {
FileOpenPicker p = new FileOpenPicker();
p.FileTypeFilter.Add(".jpg");
MyButton.Content =
(await p.PickSingleFileAsync()).FileName;
} catch(Exception e) {}
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18. DEMO
Exception Handling
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19. Are there still other states?
Yes. There is one more.
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20. What are those states?
Cancelled
Cancel
Created Start Started Completed Completed Close Closed
Error
Error
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21. And there’s also a way to
communicate progress.
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22. What are those states?
Cancelled
Cancel
Created Start Started Completed Completed Close Closed
Error
Error
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23. How do we do this?
await FooAsync(…, cancel, progress);
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24. DEMO
Cancellation & Progress
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25. So, it’s always Task<T>?
Erm, no, there’s also void
(... And some other types, but we’ll
get back to those)
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26. DEMO
The problem with void
26

27. So what should I do with void then?
async Task async void
FooAsync(…); Foo_Click(…);
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28. What about those Task methods?
• Yielding control
await Task.Delay(5000);
await Task.Yield();
• Background running
var result = await Task.Run(() => { … work … });
• Parallel composition
Task first = await Task.WhenAny(task1, task2);
await Task.WhenAll(task1, task2);
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29. 29

30. Except for those other
return types that is...
... But I still don’t want to tell you
about those.
First let’s ask another question.
30

31. Are there other languages
I can use this in?
Sure there are.
VB C++
JavaScript
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32. JavaScript Promises
var start = new Date();
Windows.Graphics.Imaging.BitmapDecoder.createAsync(stream)
.then(
function (decoder) {
var timeLabel = document.getElementById("TimeLabel");
timeLabel.innerText = new Date()- start + "ms";
}
);
32

33. DEMO
Async in JavaScript
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34. Was that a library
I saw you using in that demo?
Indeed you did.
34

35. I did have to alter some code,
though...
... And use another return type.
Task IAsyncAction
Task<T> IAsyncOperation<T>
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36. DEMO
Async in a WinMD class library
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37. 37

38. To sum up, we have some
recommendations for you.
38

39. TIP 1
Don’t forget to cancel your awaits.
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40. TIP 2
It’s easy to forget
the ‘Async’ extension.
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41. TIP 3
Awaiting a local variable in
JavaScript gives you funky results.
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42. TIP 4
Awaiting a void method reacts
different than awaiting a non-void
method.
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43. TIP 5
Don’t use dialogs in your async
methods.
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44. TIP 6
Don’t await in a catch clause.
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45. TIP 7
You end up with a whole lot of async
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46. TIP 8
.Wait, WaitAll
= synchronously waiting
await, .ContinueWith
= asynchronously waiting
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47. 47

48. Let us know what you think
http://proq.blogspot.com
@GitteTitter
http://www.qframe.be
@qframedotnet
info@QFrame.be
gitte.vermeiren@QFrame.be
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