C# in a Nutshell, Second Edition

Name

IAsyncResult

Synopsis

This interface is used in asynchronous programming to act as a placeholder for the result of the async call. It is most commonly used when an instance of a delegate type is fired using the BeginInvoke method. (This idiom is used extensively throughout the .NET Framework Class Library.)

Asynchronous method calls can be harvested in a number of ways. A programmer can poll the call by checking the IsCompleted property of the IAsyncResult object to see if the call has completed yet. This, while perhaps the simplest approach, is also likely the most wasteful, as the caller needs to be in some sort of spin loop, repeatedly checking the property until a true is received.

A variant of the polling spin loop is to use the AsyncWaitHandle property of IAsyncResult. This is a standard Win32 handle that can be used in some of the synchronization primitives provided in the System.Threading namespace. Specifically, this property is a System.Threading.WaitHandle instance, meaning that the programmer can call any of the Wait methods: WaitOne( ), WaitAny( ), or WaitAll( ). The net effect is the same—put the calling thread to sleep until the async call completes.

Although not formally part of the IAsyncResult interface, a corresponding EndInvoke method is supported by delegates. The EndInvoke method blocks the calling thread until the async call completes. Alternatively, at the point of the async delegate call, a programmer can specify a callback delegate to call when the async ...