Why would we want to call a WCF Service asynchronously?

The biggest reason to move from synchronous to asynchronous is performance.  When a synchronous call is made, the current thread locks till the call is completed.  It varies by OS and programming language but most application will have a set number of threads they can pull out of a thread pool.  If to many threads are being utilized to retrieve data synchronously it could cause the entire application to lock up while it waits for a thread to become available.  This is bad when end user usability plays a large part in development decisions today.

Asynchronous calls will call the WCF service and release the thread back into the thread pool.  When the async call is completed a callback method is raised, this will pull out a new thread to consume the returning data.

In order to setup Async calls do the following:

• Right click on the service reference
• Select ‘configure service reference’
• Check ‘Allow generation of asynchronous operations’
• Ensure ‘Generate asynchronous operations’ is selected
• Click OK

Then to make the call in your code you just simple do this

C# version

private void AsyncTest()
{
    WcfClient someWcfClient = new WcfClient();
    //Setup a listener that can wait for the async call to complete
    someWcfClient.SimpleSearchCompleted += DocCallback;
    //call the async method and optionally pass in a parm
    someWcfClient.SimpleSearchAsync(someParm);
}

private void DocCallback(object sender, SinpleSearchEventArgs e)
{
    // do something
}

VB.NET version

Private Sub AsyncTest()
Dim someWcfClient As New WcfClient()
'Setup a listener that can wait for the async call to complete
AddHandler someWcfClient.SimpleSearchCompleted, AddressOf DocCallback
'call the async method and optionally pass in a parm
someWcfClient.SimpleSearchAsync(someParm)
End Sub

Private Sub DocCallback(ByVal sender As Object, ByVal e As SinpleSearchEventArgs)
' do something
End Sub