If an operation is pending, this method calls Abort on the underlying WebRequest. When you call CancelAsync, your application still receives the completion event associated with the operation. For example, when you call CancelAsync to cancel a DownloadStringAsync(Uri) operation, if you have specified an event handler for the DownloadStringCompleted event, your event handler receives notification that the operation has ended. To learn whether the operation completed successfully, check the Cancelled property on the base class of DownloadStringCompletedEventArgs in the event data object passed to the event handler. If no asynchronous operation is in progress, this method does nothing.