Async Programming in C# 5

C# 5.0 Async
Pratik Khasnabis
DDD Melbourne 2012

About Me

Quick background
Lead Developer at Bupa Australia
Developer for 15 years
C# & .Net 8 years
C++ before that

Disclaimer
This presentation is my own and I do not
represent my company.

The need for Async

Responsive UI
UI thread is running as a message pump in a Touch ready Metro Apps
loop In WinRT if an API is likely to take
Waiting on IO or long computation will stop more than 50ms to run the API is
message processing => Frozen UI asynchronous

Scalable Server Applications
A CLR thread is used to service requests
A blocked thread => less scalable
Service thousands of IO bound requests on a
small pool of threads

Async in .Net 4.5

First introduced in PDC
Two new keywords
2010 and the refresh in
async and await
MIX 2011 (VS 2010 SP1)

An unsupported out-of- Built to take advantage of
band release. Updated C# Task and Task<T>
and VB compilers. Introduced in .Net 4.0

AsyncCtpLibrary.dll
Async methods
introduced async
Pervasive in .Net 4.5
extension methods for
common classes.

.NET 4 and Silverlight 5
Async Targeting Pack
Download using NuGet
Microsoft.CompilerServices.AsyncTargetingPack

Differences in Behaviour
Read the release notes
Static helper methods are in TaskEx
class instead of Task
e.g. Task.Run(…) vs TaskEx.Run(…) Windows Phone and Azure
No support yet

Task

Task Awaiters
A Task is promise of a Tasks are the backing
result that will delivered types for async methods
at some time in future in .Net 4.5

Compositional Two types
Tasks can be composed Task
using continuations Task<T>

Awaiters
Cancellable Tasks are the backing
Tasks can be designed to types for async methods
be cancelled easily in .Net 4.5

Async and Await

Keyword: async Keyword: await
Only methods or lamdbas with async Makes the rest of method a
modifier can use the await keyword. continuation
Return type must be void, Task or When the method completes
Task<T> execution resumes where it left off on
The method actually returns void or T, the right synchronisation context
the compile creates the Task object Compiler generates a state machine
Doesn’t make the method
asynchronous

var awaiter = expression.GetAwaiter();
if (awaiter.IsCompleted)
Console.WriteLine (awaiter.GetResult());
else
var result = await expression;
awaiter.OnCompleted (() =>
<code block> {
var result = awaiter.GetResult();
<code block>
);

Task-Based Async Pattern

TAP methods has an async modifier ,
TAP methods should return quickly to
returns a running Task or
caller with a small synchronous
Task<TResult> and ends with an
phase.
“Async” suffix by convention

TAP methods should have the same
parameters as the synchronous one Avoids out and ref parameters
in the same order

Can have CancellationToken
Can have IProgress<T> parameter
parameter

Async in .Net 4.5 BCL

System.Xml.XmlReader
System.Net.Mail
public virtual Task<bool>
public Task SendMailAsync(
ReadAsync()
MailMessage message )

System.Net.Http.HttpClient
System.IO.Stream public Task<string>
public Task CopyToAsync( GetStringAsync( string
Stream destination ) requestUri )

System.Net.WebSockets
public abstract
System.IO.TextReader Task<WebSocketReceiveResult>
public virtual Task<int> ReceiveAsync( ArraySegment<byte>
ReadAsync( char[] buffer, buffer, CancellationToken
int index, int count ) cancellationToken )

Async in WPF/WinForms apps
Caller Void Async Method Task Async Method Awaitable
UI
IOCP
thread
thread
async void
LoadPhotosAsync(string tag)
{ async Task<FlickrPhoto[]>
… GetPhotosAsync(string tag, int page)
var photosTask = GetPhotosAsync(tag, pa {
ge);
WebClient client = new WebClient();

var webTask = client.DownloadStringTaskAsyn webTask
c(…);
var apiResponse = await webTask;

var photos = await photosTask;

var photos = ParseResponse(apiResponse);
return photos;
DisplayPhotos(photos);

}

}

Concurrency without threads

UI Thread

Download Process
Message Pump
Data Data

Process UI Events

Async in Server Apps
Thread Pool Exhaustion

DB
ASP.Net
IIS IIS I/O Thread
Worker
Process
WS

MaxConcurrentRequestsPerCPU

Worker Threads File
Waiting for I/O ops
HTTP.SYS
Kernel
To complete
Queue
HTTP
Requests

ASP.Net Core Services

Asynchronous HTTP modules Asynchronous HTTP handlers
public class MyHttpModule : IHttpModule
{ public class MyAsyncHandler :
private async Task
ScrapeHtmlPage(object caller, EventArgs e) HttpTaskAsyncHandler
{ {
await ...
} public override async Task
public void Init(HttpApplication ProcessRequestAsync(HttpContext
context)
{ context)
EventHandlerTaskAsyncHelper helper = {
new EventHandlerTaskAsyncHelper(ScrapeHtmlPage);
context.AddOnPostAuthorizeRequestAsync( await …
helper.BeginEventHandler, helper.EndEventHandler); }
}
} }

ASP.Net MVC 4
Timeout
Controller [AsyncTimeout(2000)]
Derive from AsyncController ?? [HandleError(ExceptionType =
typeof(TaskCanceledException), View
= "TimedOut")]

Actions
async methods returning Task or
Task<ActionResult>

ASP.Net WebForms
Page Markup
<%@ Page Language="C#"
Async="true" AsyncTimeOut="2"
CodeBehind="AsyncPage.aspx.cs" %>

Page_Load method
Register the async method
The async mehtod will be
asynchronoiusly executed after
PreRender stage in page lifecycle

WCF
Service: Async operations Client: Async proxies
[ServiceContract] Use svcutil or Add Service Reference
public interface IDemoService
{
[OperationContract] var proxy = new
Task<string> GetStockQuoteAsync(string ticker); StockQuoteService.StockQuoteSoapClie
}
nt();
public class DemoService : IDemoService var quote = await
{ proxy.GetQuoteAsync("MSFT");
public async Task<string> GetStockQuoteAsync
(string ticker)
{
await ...
}
}

VS 2012 Unit Tests
Async Test Methods – return Task Test Frameworks
[TestMethod] MS Test – Supports async, Built in test
public async Task UnitTestMethod() provider
{ xUnit – Supports async in v1.9,
await Task.Delay(1000); downloadable test provider
Assert.AreEqual(1,1); NUnit – Doesn’t support async,
} downloadable test provider

Execution Time
1 sec

Metro and WinRT
http://blogs.msdn.com/b/windowsappdev/archive/2012/06/14/exposing-net-tasks-
as-winrt-asynchronous-operations.aspx
http://blogs.msdn.com/b/windowsappdev/archive/2012/03/20/keeping-apps-fast-
and-fluid-with-asynchrony-in-the-windows-runtime.aspx
http://blogs.msdn.com/b/windowsappdev/archive/2012/04/24/diving-deep-with-
winrt-and-await.aspx
Watch the other
sessions on Metro
IAsyncInfo App Development
All async methods in WinRT
implements this interface and 4 other Compiler Magic
IAsyncAction IAsyncInfo has the awaitable pattern
IAsyncOperation<TResult> The C# and VB compiler will convert
IAsyncActionWithProgress<TProgress> WinRT async methods to return Tasks
IAsyncOperationWithProgress<TResult,
TProgress>

Choosing Sync vs Async

Synchronous
Operations are simple or short-running
CPU bound operations
Simplicity is more important

Asynchronous
I/O bound operations (DB, network, disk)
Provide cancellation support
Parallelism is more important

ADO.Net
Tabular Data Stream Protocol
Worker
Thread
reader.NextResult()

i/o “DONE” “DONE”
Result Set Token Result Set Token

reader.Next()

i/o “ROW” “ROW” “ROW”
Token Row Token Row Token

Reader.IsDBNull()
i/o
Column Column Column Column Column
Header Data Header Data Header

pa ck et s

Large Column e.g. varbinary(8000)

ADO.Net
Recommendations

Use
Use
CommandBehavior.SequentialAccess
NextResultAsync() and ReadAsync()
Unless reading large columns

Use
Synchronous column access Use
IsDBNull, GetFieldValue<T> Streaming for massive columns
Unless reading large columns GetStream()
IsDBNullAsync, GetTextReader()
GetFieldValueAsync<T> GetXmlReader()

Resources
• http://msdn.microsoft.com/en-US/async
• VB.Net - http://blogs.msdn.com/b/lucian/
• C# (implementation details) -
https://msmvps.com/blogs/jon_skeet/archive
/tags/Eduasync/default.aspx

Async Programming in C# 5

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