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27 I/O-Bound Asynchronous Operations
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It frees you from having to worry about your application s threading model (as discussed in the Applications and their Threading Models section later in this chapter) . For example, in a GUI application, the AsyncEnumerator invokes your code on the GUI thread by default so you can update UI controls . For an ASP .NET Web Form or XML Web Service application, the AsyncEnumerator automatically ensures that your code is running under the client s culture and identity . It has rich error-handling support . If your iterator returns and an asynchronous operation later completes, the AsyncEnumerator class throws an exception notifying you that there is no way for the EndXxx method to be called and therefore your application is leaking resources . It has rich debugging support . Typically, an application will have many AsyncEnumerator objects in memory . This is especially true of server applications . In a debugger, you can query the AsyncEnumerator s static GetInProgressList method, which returns a list of all the AsyncEnumerator objects currently in existence . This list is sorted with the object that has been waiting the longest for an operation to complete at the top . If your application appears to be hung, looking at the item at the top of the list will usually take you right to the line in your code that is waiting for an operation to complete . In addition, when you look at an individual AsyncEnumerator object in the debugger, it displays a user-definable tag identifying the operation, the timestamp of the last asynchronous operation it performed, which operations have completed, and which operations have not completed yet . It also shows you the source code file and line within that file where the asynchronous operations were initiated .
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Here is what the pipe server code looks like when implemented for use with my AsyncEnumerator class 4:
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private static IEnumerator<Int32> PipeServerAsyncEnumerator(AsyncEnumerator ae) { // Each server object performs asynchronous operations on this pipe using (var pipe = new NamedPipeServerStream( "Echo", PipeDirection.InOut, -1, PipeTransmissionMode.Message, PipeOptions.Asynchronous | PipeOptions.WriteThrough)) { // Asynchronously accept a client connection pipe.BeginWaitForConnection(ae.End(), null); yield return 1; // A client connected, let's accept another client var aeNewClient = new AsyncEnumerator(); aeNewClient.BeginExecute(PipeServerAsyncEnumerator(aeNewClient), aeNewClient.EndExecute); // Accept the client connection pipe.EndWaitForConnection(ae.DequeueAsyncResult()); // Asynchronously read a request from the client Byte[] data = new Byte[1000];
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The sample code for this book contains the pipe client code re-implemented to use AsyncEnumerator . This version of the code follows the same structure shown here .
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Part V
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Threading
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pipe.BeginRead(data, 0, data.Length, ae.End(), null); yield return 1; // The client sent us a request, process it. Int32 bytesRead = pipe.EndRead(ae.DequeueAsyncResult()); // My sample server just changes all the characters to uppercase // But, you can replace this code with any compute-bound operation data = Encoding.UTF8.GetBytes( Encoding.UTF8.GetString(data, 0, bytesRead).ToUpper().ToCharArray()); // Asynchronously send the response back to the client pipe.BeginWrite(data, 0, data.Length, ae.End(), null); yield return 1;
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// The response was sent to the client, close our side of the connection pipe.EndWrite(ae.DequeueAsyncResult()); } // Close happens in a finally block now! }
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There are several things to notice about this new version of the code:
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All the code is in one method as opposed to lots of methods spread out within a class . Since there is no class, there are no fields; all the variables are local variables . Where I would have had a method separation, I now have a yield return 1 statement . This allows the thread to return to where it came from so that it can do more work . To every BeginXxx method, I pass ae.End(); this method returns a delegate referring to a method inside the AsyncEnumerator object . When the operation completes, the thread pool thread notifies the AsyncEnumerator object, which in turn continues executing your iterator method after the yield return 1 statement . I always pass null as the last argument to every BeginXxx method . Therefore, I never need to call IAsyncResult s AsyncState property and cast its return value to the right type; I just use the local variables directly . To every EndXxx method, I pass the result of calling ae.DequeueAsyncResult() . This method returns the IAsyncResult object that was passed to the AsyncEnumerator object by the thread pool thread when the asynchronous operation completed . And last, but not least, notice that now I can use C# s using statement to control the lifetime of the NamedPipeServerStream . This also means that the NamedPipeServerStream object will be closed within a finally block should any other code throw an unhandled exception .
This gives you a good introduction as to what the AsyncEnumerator class can do for you and how it simplifies asynchronous programming . The code above doesn t even show some of the more exciting features . If you re interested in learning more about it, I encourage you to visit the Wintellect Web site mentioned earlier .
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