Understanding of asynchronous model is an essential knowledge for development of quality Windows Store apps. This session dives deep into implementation details of async and await C# keywords and provides information about common pitfalls and mistakes of async pattern use and the ways to avoid them.

1. Deceptive simplicity of async and await
WIN387
Andrei Marukovich
lunarfrog.com
@amarukovich

2. Looks familiar?
Deceptive simplicity of async and await

3. • Application errors
• Using the UI thread for performing long operations
UI unresponsiveness
Deceptive simplicity of async and await

4. Deceptive simplicity of async and await
Synchronous method
private void ButtonClick(object sender, EventArgs e)
{
WebClient client = new WebClient();
var imageData = client.DownloadData(“http://image-url”);
pictureBox.Image = Image.FromStream(new MemoryStream(imageData));
}

5. Deceptive simplicity of async and await
Asynchronous method
private async void ButtonClick(object sender, EventArgs e)
{
WebClient client = new WebClient();
var imageData = await client.DownloadDataTaskAsync(“http://image-url”);
pictureBox.Image = Image.FromStream(new MemoryStream(imageData));
}

6. Deceptive simplicity of async and await
Asynchronous method
private async void ButtonClick(object sender, EventArgs e)
{
WebClient client = new WebClient();
var imageData = await client.DownloadDataTaskAsync(“http://image-url”);
pictureBox.Image = Image.FromStream(new MemoryStream(imageData));
var strData = await client.DownloadStringTaskAsync(“http://str-url”);
// …
}

7. Every API which might take more than 50ms
should be async
WinRT
Deceptive simplicity of async and await

8. Deceptive simplicity of async and await
Demo
WinRT and async

9. Deep dive
Deceptive simplicity of async and await
static void Main(string[] args)
{
DoSomething();
}
static void DoSomething()
{
// method body
}
static void Main(string[] args)
{
DoSomething();
}
async static Task DoSomething()
{
// method body
}

10. Deep dive
Deceptive simplicity of async and await

11. State machine
Deceptive simplicity of async and await
struct DoSomethingStruct : IAsyncStateMachine
{
// Argument and local variables
public Int32 x;
// Compiler-generated variables
public Int32 _state;
public AsyncTaskMethodBuilder _builder;
void MoveNext() { … }
}

12. MoveNext()
Deceptive simplicity of async and await
void MoveNext() {
try {
switch(_state) {
// modified method body
}
}
catch (Exception exception) {
_builder.SetException(exception);
return;
}
_builder.SetResult();
}

13. Async function transformation
Deceptive simplicity of async and await
static Task DoSomething()
{
var stateMachine = new DoSomethingStruct();
stateMachine._builder = AsyncTaskMethodBuilder.Create();
stateMachine._state = -1;
stateMachine._builder.Start(stateMachine);
return stateMachine._builder.Task;
}

14. Using await
Deceptive simplicity of async and await
await DoSomething();
Task task = DoSomething();
INotifyCompletion awaiter = task.GetAwaiter();
if (!awaiter.IsCompleted) {
awaiter.OnCompleted(continue_delegate)
}

15. Bringing it all together
async Task Method1()
{
await Method2();
}
async Task Method2()
{
await _file.CopyAsync(_folder, _name);
}

16. Bringing it all together
Method1
Method2
Task
CopyAsync
Copy
Task Method2
Method1

17. Defining async methods
async Task Method1()
{
}
async Task<StorageFolder> Method2()
{
return ApplicationData.Current.LocalFolder;
}
async void button1_Click(object sender, EventArgs e)
{
}

18. Avoid async void as much as possible
Deceptive simplicity of async and await

19. Demo
async void and error handling
Deceptive simplicity of async and await

20. async lambdas
obj.Handle( async () => { await Task.Delay(1000); } );
Deceptive simplicity of async and await
void Handle(Func<Task> action);
Func<Task> f = async () => { await Task.Delay(1000); }
void Handle(Action action);
Action a = async () => { await Task.Delay(1000); }
IL: AsyncTaskMethodBuilder
IL: AsyncVoidMethodBuilder

21. Demo
More async in WinRT
Deceptive simplicity of async and await

22. Constrains
• Cannot use await inside constructor and property accessor
• Cannot use await inside catch and finally
• Cannot use await inside lock block
• Cannot use await inside Main method
• An async method cannot have a ref or out parameters

23. What to take away
• async/await are not magic
• Always await your async operations
• async void methods are only suited for UI event handlers
• Be careful around async void and exceptions
• Be aware of deferral classes in WinRT

24. Next steps
• Parallel Programming team blog
 http://bit.ly/pfxteam
• Deeper into Async and Await, Kathleen Dollard
 ARC319
• Three Essential Tips for Async (video)
 http://bit.ly/AsyncTips
• Async for .NET 4.0 and Windows Phone (NuGet)
 http://bit.ly/AsyncNet4