195: Asynchronous Results with Completed Notification in Visual C#

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Listing 195: Asynchronous Results with Completed Notification
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using using using using using System; SystemIO; SystemRuntimeRemotingMessaging; SystemThreading; SystemIO;
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public class FindFilesWithNotifications
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{ // DisplayHelp() method // delegate string[] GetFilesHandler( string searchPattern, bool recurseSubdirectories); public static void Main(string[] args) { string searchPattern; bool recurseSubdirectories = false; IAsyncResult result = null; // Assign searchPattern & recurseSubdirectories // GetFilesHandler asyncGetFilesHandler = GetFiles; ConsoleWriteLine("Searching: {0}", args[0]); if (recurseSubdirectories) { ConsoleWriteLine("\trecursive"); } ConsoleWriteLine("Push ENTER to cancel/exit"); result = asyncGetFilesHandlerBeginInvoke( args[0], recurseSubdirectories, SearchCompleted, null); ConsoleReadLine(); } public static string[] GetFiles( string searchPattern, bool recurseSubdirectories) { string[] files = null; // Search for files matching the pattern // See Listing 192 // return files; } public static void SearchCompleted(IAsyncResult result) { AsyncResult asyncResult = (AsyncResult)result; GetFilesHandler handler = (GetFilesHandler)asyncResultAsyncDelegate; string[] files = handlerEndInvoke(result);
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19: Multithreading Patterns
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foreach (string file in files) { ConsoleWriteLine(PathGetFileName(file)); } } }
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OUTPUT 193:
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Searching: C:\Samples\*cs recursive Push ENTER to cancel/exit AsyncResultPatternIntroductioncs FindFilesWithoutNotificationOrStatecs FindFilesWithNotificationcs FindFilescs AutoResetEventSamplecs RunningASeparateThreadcs
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Callback notification when the worker thread completes provides a key benefit of using the asynchronous results pattern over manual thread manipulation For example, it allows developers to display a widget to indicate that a task has completed As already demonstrated, EndInvoke() can be called from within Main() using the delegate instance and the IAsyncResult reference returned from BeginInvoke() However, EndInvoke() will block until the asynchronous call completes As a result, it is preferable to call EndInvoke() from within the callback To accomplish this, the callback function casts its IAsyncResult parameter, but the cast is unintuitive The data type is AsyncResult, but the namespace is the unintuitive SystemRuntimeRemotingMessaging SearchCompleted() demonstrates the code for calling EndInvoke() from within the thread completion callback Listing 196 shows the fully qualified call
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Listing 196: Calling EndInvoke() from within Asynchronous Callback
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public static void SearchCompleted(IAsyncResult result) { SystemRuntimeRemotingMessagingAsyncResult asyncResult =(SystemRuntimeRemotingMessagingAsyncResult)result; GetFilesHandler handler = (GetFilesHandler)asyncResultAsyncDelegate; string[] files = handlerEndInvoke(result); }
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Passing Arbitrary State The last parameter in the BeginInvoke() call is of type object, and provides a mechanism for passing arbitrary data to the callback method (SearchCompleted()) Consider a situation in which multiple threads were started one after the other, and in each case the callback was to the same AsyncCallback delegate instance The problem is that from within the async callback delegate you don t correlate which completed call to GetFiles() corresponds to which call that initiated the GetFiles() method For example, you cannot print out which search results correspond to which searchPattern Fortunately, this is the purpose of the last parameter in BeginInvoke() Listing 197 starts the GetFiles() method for each search pattern passed in the command line In each call, you pass the search pattern (arg) twice to asyncGetFilesHandlerBeginInvoke(): once as a parameter to the GetFiles() method that is to run asynchronously, and once as the last parameter to be accessed from inside SearchCompleted()
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Listing 197: Asynchronous Results with Completed Notification
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using using using using System; SystemIO; SystemRuntimeRemotingMessaging; SystemIO;
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public class FindFiles { delegate string[] GetFilesHandler( string searchPattern, bool recurseSubdirectories); public static void Main(string[] args) { bool recurseSubdirectories = true; IAsyncResult[] result = new IAsyncResult[argsLength]; int count = 0; foreach(string arg in args) { if (argTrim()ToUpper() == "/S") { recurseSubdirectories = true; break; } }
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GetFilesHandler asyncGetFilesHandler = GetFiles; ConsoleWriteLine("Searching: {0}", stringJoin(", ", args)); if (recurseSubdirectories) { ConsoleWriteLine("\trecursive"); } ConsoleWriteLine("Push ENTER to cancel/exit"); foreach (string arg in args) { if (argTrim()ToUpper() != "/S") { result[count] = asyncGetFilesHandlerBeginInvoke( arg, recurseSubdirectories, SearchCompleted, arg); } count++; } ConsoleReadLine(); } public static string[] GetFiles( string searchPattern, bool recurseSubdirectories) { string[] files; // Search for files matching the pattern // See Listing 192 // return files; } public static void SearchCompleted(IAsyncResult result) { string searchPattern = (string)resultAsyncState; ConsoleWriteLine("{0}:", searchPattern); AsyncResult asyncResult = (AsyncResult)result; GetFilesHandler handler = (GetFilesHandler)asyncResultAsyncDelegate; string[] files = handlerEndInvoke(result); foreach (string file in files) { ConsoleWriteLine("\t"+ PathGetFileName(file)); } } }
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The results of Listing 197 appear in Output 194
OUTPUT 194:
Searching: C:\Samples\*cs, C:\Samples\*exe recursive Push ENTER to cancel/exit C:\Samples\*cs AsyncResultPatternIntroductioncs FindFilesWithoutNotificationOrStatecs FindFilesWithNotificationcs FindFilescs AutoResetEventSamplecs C:\Samples\*exe FindFilesexe
Since the listing passes arg (the search pattern) in the call to BeginInvoke(), it can retrieve the search pattern from the IAsyncResult parameter of SearchCompleted() To do this, it accesses the IAsyncResultAsyncState property Because it is of type object, the string data type passed during BeginInvoke() needs to be downcast to string In this way, it can display the search pattern above the list of files that meet the search pattern
Asynchronous Results Conclusions One of the key features the asynchronous results pattern offers is that the caller determines whether to call a method asynchronously The called object may choose to provide asynchronous methods explicitly, perhaps even using the asynchronous results pattern internally However, this is not a requirement for the caller to make an asynchronous call Called methods may choose to provide asynchronous APIs explicitly when the called class can implement the asynchronous functionality more efficiently than the caller can, or when it is determined that asynchronous method calls are likely, such as with methods that are relatively slow If it is determined that a method should explicitly provide an asynchronous calling pattern, it is a good practice for the API to follow the asynchronous results design pattern An explicit implementation, therefore, should include methods that correspond to BeginInvoke(), along with events callers can subscribe to in order to be notified when a thread completes For example, starting in NET 20, the SystemNetWebClient class includes asynchronous method calls for downloading files To begin the