Before implementing our asynchronous pattern in WCF, I think it is better to give an introduction to asynchronous pattern and its applications in this section.  Many of you may be partially or completely familiar with this design pattern so you can escape this section and read the next sections directly.

For long running executions in an application, there is a probability that a current thread cannot keep executing because it may stop user interface.  For example, when a Windows application goes to a new state to execute a long running process, windows may freeze and even crash.  One solution is to move this execution to another thread and let it follow there.

Asynchronous pattern comes into play to solve this issue and Microsoft has a built-in mechanism for this pattern in its great .NET Framework.  Microsoft's implementation of this pattern consists of these pieces:

·         Two methods for an asynchronous operation

·         An object which implements IAsyncCallback interface

·         A callback delegate

This way, execution of a method splits into two steps.  In the first step you create the background process and start it and in the second step you listen for changes in process and wait until it finishes.

For example, if we have a method to add two numbers and name it Add() then asynchronous version of this method consists of two methods: BeginAdd() and EndAdd().

If Add() has a signature like this:

int Add(int x, int y);

Then BeginAdd() has this signature:

IAsyncResult BegingAdd(int x, int y, AsyncCallback callback, object state);

As you see, BeginAdd() has two extra parameters in comparison with original Add() method:

·         AsyncCallback: A delegate that gets an IAsyncResult parameter and does not return anything.  It will be called when asynchronous operation is completed.

·         object: It keeps the state of the asynchronous  operation as an IAsyncResult object.

This method also returns an IAsyncResult object rather than the original integer type.

On the other hand, EndAdd() has the following signature:

int EndAdd(IAsyncResult ar);

As you see, this method only has one parameter of IAsyncResult type. This object gives all necessary information about the state of operation and has the following properties.

·         AsynState: Gets an object that provides some information about asynchronous operation.

·         AsycWaitHandler: Gets a WaitHandle to wait for an asynchronous operation to complete.

·         CompletedSynchronously: Indicates if the asynchronous operation has completed synchronously.

·         IsCompleted: Indicates if the asynchronous operation has completed.

Having this background information, we can step into WCF and talk about asynchronous pattern implementation in it which may be a little different than what you have seen in other parts of .NET Framework.