How to: Call a Service Method Asynchronously
Most methods in Windows Communication Foundation (WCF) services may be called either synchronously or asynchronously. Calling a method asynchronously enables your application to continue to work while the method is being called when it operates over a slow connection.
By default, when a service reference is added to a project it is configured to call methods synchronously. You can change the behavior to call methods asynchronously by changing a setting in the Configure Service Reference dialog box.
This option is set on a per-service basis. If one method for a service is called asynchronously, all methods must be called asynchronously.
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To call a service method asynchronously
In Solution Explorer, select the service reference.
On the Project menu, click Configure Service Reference.
In the Configure Service Reference dialog box, select the Generate asynchronous operations check box.