How to Implement a service operation asynchronously

In your service contract, declare an asynchronous method pair according to the .NET asynchronous design guidelines. The Begin method takes a parameter, a callback object, and a state object, and returns a System.IAsyncResult and a matching End method that takes a System.IAsyncResult and returns the return value. For more information about asynchronous calls, see Asynchronous Programming Design Patterns.

Mark the Begin method of the asynchronous method pair with the System.ServiceModel.OperationContractAttribute attribute and set the System.ServiceModel.OperationContractAttribute.AsyncPattern property to true.