Async code allows long-running operations like network and file access to execute without blocking the UI thread. There have been several approaches to async programming in .NET including the Async Programming Model (APM), Event-based Async Pattern (EAP), and Task Parallel Library. The newest approach is to use async and await keywords which allow suspending methods until async operations complete and make control flow easier to reason about.

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”