How Asynchronous method invocation pattern in c# works and implemented?

Question

I am moving lots of code loosely based on asynchronous method invocation. How is it typically implemented (preferably in production)?

• How does it work?
• How is it used?
• How is it implemented?

Answer 1

The pattern is usually as follows:

• You have a BeginXXX method which receives all in and ref arguments plus a AsyncCallback delegate (may be null) and a state object reference (may also be null) and returns an IAsyncResult. This method is invoked to start the call, and it returns more or less immediately.
• Then you have a EndXXX method which takes the IAsyncResult returned by the BeginXXX operation plus any ref and out arguments which is invoked when the call is to be completed (either after callback, waiting on event in the IAsyncResult or by blocking). The EndXXX will return the method result (if not a void method) or throw an exception (if the method threw an exception).

Have a look at the Stream class or on any delegate, they provide you with examples of those signatures. Also, there is a complete description of async calls on MSDN.

Answer 2

The pattern is usually implemented via .NET's IOCompletion port. This allows a small pool of threads to service many IO operations completing, with the BeginX calling the appropriate Win32 API to initiate an asynchronous operation, the implementation of IAsyncResult holding any state (associated with the underlying operation via the OVERLAPPED instance), and EndX picking up the result.

The details (and the other approaches) vary widely depending on the resource in question.