MSDN Magazine - June 2008 - (Page 119)

JEFFREY RICHTER ConCurrent AffAirs Simplified APM With The AsyncEnumerator In my last column (msdn.microsoft.com/magazine/cc163323), I introduced some concepts about how new C# language features—anonymous methods, lambda expressions, and iterators—could be used to sim plify asynchronous programming. I ended that column showing how asynchronous programming could be accomplished using a synchronous programming model by way of a C# iterator. The ex ample iterator is shown in Figure 1. However, you can’t just use a normal C# foreach statement to execute the iterator’s code because the code would always be executed by the thread calling foreach, and execution would run continuously without pausing for the asynchronous I/O operations to complete. In this column, I introduce my AsyncEnumerator class, which intelligently drives an iterator so that different thread pool threads can execute the code at different times, and I ensure that the itera tor only advances after asynchronous I/O operations complete. I am also going to explain the architecture of my AsyncEnumerator class and how it works. Figure 1 C# Iterator private static IEnumerator ApmPatternWithIterator( AsyncEnumerator ae, String pathname) { using (FileStream fs = new FileStream(pathname, FileMode.Open, FileAccess.Read, FileShare.Read, 8192, FileOptions.Asynchronous)) { Byte[] data = new Byte[fs.Length]; fs.BeginRead(data, 0, data.Length, ae.End(), null); yield return 1; Int32 bytesRead = fs.EndRead(ae.DequeueAsyncResult()); ProcessData(data); } } Using My AsyncEnumerator Class Here is the definition of my AsyncEnumerator class: public class AsyncEnumerator { // Methods called by code outside of the iterator public AsyncEnumerator(); public void Execute(IEnumerator enumerator); // Methods called by code inside the iterator public AsyncCallback End(); public IAsyncResult DequeueAsyncResult(); Enumerator class. So, as each asynchronous operation completes, code in an AsyncEnumerator object is notified. This code then places the IAsyncResult object for the completed operation into a List object, which I refer to as the inbox. In your code, after calling a BeginXxx method, place a yield re turn statement that returns the number of asynchronous opera tions that you’ve queued up. In Figure 1, since I called just one BeginXxx method (BeginRead), I yield return 1. The yield return } Using my AsyncEnumerator class is quite simple. Let’s talk first about how to implement your iterator member and then I’ll ex plain how to invoke it. Define your iterator member to accept any parameters you want and add an additional parameter that is a reference to an Async Enumerator object. Your iterator member must be prototyped as returning a collection of Int32s; in other words, it must have a re turn type of IEnumerator . Then, inside your iterator member, start each asynchronous op eration by calling the appropriate BeginXxx method. As you know, when you call a BeginXxx method, it is important that you pass it the name of a method that should be called when the asynchro nous operation completes. Instead of defining your own method, call the AsyncEnumerator object’s End method. The End method returns an AsyncCallback delegate that identifies a private method defined inside my Async You can’t just use a normal foreach statement to execute the iterator’s code. statement suspends your iterator method and stops anymore code in it from executing. I’ll show other examples later where I yield return values other than 1, but, for many applications, returning 1 will be appropri ate. The number you specify in a yield return statement tells the AsyncEnumerator object how many asynchronous operations need to complete before resuming your iterator. So, while your iterator is suspended, any asynchronous operations you initiat ed start completing. As each completes, an entry is added to the AsyncEnumerator’s inbox. When the count of items in the inbox is Send your questions and comments for Jeffrey to mmsync@microsoft.com. June 2008 119 http://msdn.microsoft.com/magazine/cc163323

Table of Contents Feed for the Digital Edition of MSDN Magazine - June 2008

MSDN Magazine - June 2008 Contents Toolbox CLR Inside Out Cutting Edge Patterns In Practice SAAS Concurrency Robotics Form Filler GUI Library Service Station Foundations Windows With C++ Concurrent Affairs Going Places { End Bracket }

MSDN Magazine - June 2008

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