2. What is Async?
 Synchronous
 every line of code is executed in order
 the next line doesn’t execute until the
current one is completed
 Asynchronous
 an operation is started, and then code
continues
 A callback is usually executed when the
operation completes

3. Why Async?
 Essential when executing long running
operations, such as file or network
access
 Especially important when starting
operations on the UI thread – async
prevents the UI from locking up while
the operation completes

4. Async Programming Model
(APM)
 .NET’s first crack at async
 Uses method signatures like Begin* and
End*
 i.e. Stream.BeginRead
 Returns an “IAsyncResult” object to
query status of operation
 Usually executes a callback when
operation is completed

5. Event-based Async Pattern
(EAP)
 2nd attempt at Async
 Uses events to notify when async
operations complete
 i.e. WebClient.OnDownloadStringCompleted
 Operation is started with *Async
 i.e. WebClient.DownloadStringAsync
 No IAsyncResult – hard to query current
status
 Sometimes they have “OnProgress” events

6. Task Async Pattern
 Uses Task Parallel Library to wrap APM
methods
 Task.Factory.FromAsync(Begin*, End*,
args)
 Get access to Task functionality
 ContinueWith to execute code after
operation is complete
 ContinueWhenAll to wait for multiple async
operations

7. The New Hotness – Async &
Await
 In an effort to simplify async even
more, Async and Await keywords
introduced into .net 4.5
 Async – indicates that a method has a
point at which it can be suspended
 Await – suspends the current method
until an operation yields a result

8. Async Control Flow
 Starts an async operation by calling async
method
 Execute whatever other code you’d like
while operation executes
 Call await to suspend method until async
operation returns
 This will return control to the previous method in
the call stack
 Once async operation returns, control is
restored to method and continues as
normal

9. Things To Know
 This is all managed through Tasks
 Await essentially calls Task.Wait() and
automatically returns Task.Result
 Most of this happens on the same thread
 Actually uses time slices interleaved within the
current thread
 Meant to be used to create non-blocking
operations
 By convention, async methods should end
with “Async”