end to-end async and await

End-to-End Async and Await

Vince FabroCardinal SolutionsPractice Manager, Enterprise App Dev@vfabro

Agenda

• What is Async?• Why bother?• How does this work?• Gotchas• Best Practices & More Details• Next Steps

What is Async?

• C# 5.0 and .NET 4.5• async and await keywords • Asynchronous programming for the

masses!

What is Async?

• Asynchronous == Parallel?• Asynchronous == Multithreaded?• Asynchronous == Easy?

Agenda

• What is Async?

• Why bother?• How does this work?• Gotchas• Best Practices & More Details• Next Steps

Why bother?

• Improve app responsiveness• Simplify asynchronous programming

more approachable

Simpler / More approachable than what?

Asynchrony the good old way

• [… as opposed to the much worse older ways]

• APM and EAP

• APM – Asynchronous Programming Model– http://msdn.microsoft.com/en-us/library/ms228963%28v=vs.110%29.aspx– http://msdn.microsoft.com/en-us/magazine/cc163467.aspx

• EAP – Event-based Asynchronous Pattern– http://msdn.microsoft.com/en-us/library/wewwczdw%28v=vs.110%29.aspx

Asynchronous Programming Model

IAsyncResult result = Dns.BeginGetHostEntry(args[0], null, null);

// Poll for completion information. while (result.IsCompleted != true){

…}// EndGetHostByName blocks until complete.IPHostEntry host = Dns.EndGetHostEntry(result);string[] aliases = host.Aliases;

.NET 2.0+

Event-based Asynchronous Patternprivate SoundPlayer player;

private void InitializeSound(){

// Create an instance of the SoundPlayer class.player = new SoundPlayer();

// Listen for the LoadCompleted event.player.LoadCompleted +=

new AsyncCompletedEventHandler(player_LoadCompleted);

player.SoundLocation = filepathTextbox.Text;player.Play();

}

private void player_LoadCompleted(object sender, AsyncCompletedEventArgs e) { }

.NET 2.0+

Supplanted by the TPL

• Both APM and EAP – “This pattern is no longer recommended for

new development”

• TPL Task Parallel Library– http://msdn.microsoft.com/en-us/library/

dd460693%28v=vs.110%29.aspx

.NET 4.0+

Supplanted by the TPL.N

ET 4.0+

Supplanted by the TPL

Parallel.ForEach(sourceCollection, item => Process(item));Parallel.Invoke(() => DoSomeWork(), () => DoSomeOtherWork());

// Create a task and supply a user delegateTask taskA = new Task( () =>

Console.WriteLine("Hello from taskA.")); // Start the task. taskA.Start();

// Output a message from the calling thread. Console.WriteLine("Hello from thread '{0}'.",

Thread.CurrentThread.Name); taskA.Wait();

.NET 4.0+

The New Hotness!

• TAP – Task-based Asynchronous Pattern

• async and await

• So… Why bother?– Simpler than APM and EAP– Simpler than raw TPL

Builds on the TPL

.NET 4.5

Task-based Asynchronous Pattern

private async Task<int> AccessTheWebAsync(){

HttpClient client = new HttpClient();

string urlContents = await client.GetStringAsync(

"http://msdn.microsoft.com");

return urlContents.Length;}

Agenda

• What is Async?• Why bother?

• How does this work?• Gotchas• Best Practices & More Details• Next Steps

How does this work?







1. Before await

2. Await Task

3. Post await

2. “Awaitable”

3. Continuation

0. For compiler

How does this work?

Gimme the Code! void IAsyncStateMachine.MoveNext() { string result = null; try { int num = state;

//if (!Stop) if (num != -3) { TaskAwaiter<string> taskAwaiter; // Machine starts with num=-1 so we enter if (num != 0) { // First (+ initial) state code, run code before await is invoked httpClient = new HttpClient(); Debug.WriteLine("before await"); // A task is invoked taskAwaiter = httpClient.GetStringAsync(url).GetAwaiter();

●●●

Gimme the Code!

Whole Lotta Code!

How does it work?

• Generates a state machine for every async call

• SynchronizationContext

• Continuation Tasks

How does it work?

• SynchronizationContext– Provides a way to queue a unit of work to a

context, not to a specific thread– Keeps a queue and count of work to do– Every thread has a current context– http://msdn.microsoft.com/en-us/magazine/gg598924.aspx

How does it work?

• SynchronizationContext Implementations:– WindowsFormsSynchronizationContext– DispatcherSynchronizationContext

• WPF and SilverLight

– WinRTSynchronizationContext– AspNetSynchronizationContext

• ASP.NET thread pool

– Default SynchronizationContext• ThreadPool

How does it work?

• Continuation Tasks– On completion of one task, invoke another

// The antecedent task. Can also be created with Task.Factory.StartNew.Task<DayOfWeek> taskA = new Task<DayOfWeek>(() => DateTime.Today.DayOfWeek);

// The continuation. Its delegate takes the antecedent task // as an argument and can return a different type. Task<string> continuation = taskA.ContinueWith((antecedent) =>

{ return String.Format("Today is {0}.", antecedent.Result);

});

// Start the antecedent. taskA.Start();

// Use the contuation's result. Console.WriteLine(continuation.Result);







1. Before await

2. Await Task

3. Post await

2. “Awaitable”

3. Continuation

0. For compiler

Agenda

• What is Async?• Why bother?• How does this work?

• Gotchas• Best Practices & More Details• Next Steps

Gotchas…

• Requirements– You must use the async keyword to await– void events may need marked with async

public void Blah public async Task BlahAsync

• Limitations– You cannot declare ref or out parameters on

an async method

Gotchas…

• When you can’t await async methods– Inside catch and finally blocks– Inside properties– Inside a lock block– In an unsafe region– Within most portions of a LINQ query

Gotchas…

• Deadlock and Race conditions, oh my!

var delayTask = DelayAsync();

delayTask.Wait();

Agenda

• What is Async?• Why bother?• How does this work?• Gotchas

• Best Practices & More Details

• Next Steps

Best Practices & More Details

• Name async methods BlahBlahAsync

• End to end– Cascading Async


• Configuring ASP.NET– Increase App Pool’s queue limit– AsyncTimeout

• Async in ASP.NET page lifecycle events%@Page ... Async=“true” %RegisterAsyncTask(

new PageAsyncTask(GetDataAsync));

http://www.asp.net/mvc/tutorials/mvc-4/using-asynchronous-methods-in-aspnet-mvc-4

http://www.asp.net/web-forms/tutorials/aspnet-45/using-asynchronous-methods-in-aspnet-45


• Unit testing– You can test async methods[TestMethod]

public async Task GivenBlah…() {

var result = await testClass.DoSomethingAsync();

}

– You can mock async method callsmockDownloadContent.Setup(x => x.DoSomethingAsync()) .Returns(Task.FromResult<string>(expectedResult));


• Entity Framework 6– ApplicationDbContext

await context.Categories.Include(c =>

c.Products).LoadAsync();int savedCount =

await context.SaveChangesAsync();

– QueryableExtensionsvar employeeCount = await query.CountAsync();var firstEmployee = await query.FirstAsync();

https://entityframework.codeplex.com/wikipage?title=Task-based%20Asynchronous%20Pattern%20support%20in%20EF


• Executing tasks in parallelawait Task1Async();await Task2Async();await Task3Async();

await Task.WhenAll(Task1Async(),

Task2Async(),

Task3Async());


• Death by a thousand cuts– Batch up async calls

• ConfigureAwait(false)http://msdn.microsoft.com/en-us/magazine/hh456402.aspx


• await Task.Yield()http://msdn.microsoft.com/en-us/library/hh873173%28v=vs.110%29.aspx

• Task cancellationhttp://msdn.microsoft.com/en-us/library/jj155759.aspx

• Reporting progresshttp://simonsdotnet.wordpress.com/2013/10/11/updating-your-ui-asynchronously-part-3-reporting-progress/

Agenda

• What is Async?• Why bother?• How does this work?• Gotchas• Best Practices

• Next Steps

References

• Asynchronous Programming Patterns– http://msdn.microsoft.com/en-us/library/wewwczdw%28v=vs.110%29.aspx

• Async and await FAQ• Steven Cleary’s blog• Channel 9• Stephen Toub: The Costs of Async

– http://msdn.microsoft.com/en-us/magazine/hh456402.aspx

Thank You!

end to-end async and await

Technology

new task

task continuation

antecedent task

asynchronous pattern

asynchronous pattern

user delegate task taska

httpclient client

new httpclient debug