WaitHandle in Visual C#.NET

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WaitHandle
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The base class for Mutex is a SystemThreadingWaitHandle This is a fundamental synchronization class used by the Mutex, EventWaitHandle, and
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19: Synchronization and More Multithreading Patterns Semaphore synchronization classes The key methods on a WaitHandle
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are the WaitOne() methods These methods block execution until the WaitHandle instance is signaled or set The WaitOne() methods include several overloads allowing for an indefinite wait: void WaitOne(), a millisecond timed wait; bool WaitOne(int milliseconds); and bool WaitOne(TimeSpan timeout), a TimeSpan wait The versions that return a Boolean will return a value of true whenever the WaitHandle is signaled before the timeout In addition to the WaitHandle instance methods, there are two key static members: WaitAll() and WaitAny() Like their instance cousins, the static members also support timeouts In addition, they take a collection of WaitHandles, in the form of an array, so that they can respond to signals coming from any within the collection One last point to note about WaitHandle is that it contains a handle (of type SafeWaitHandle) that implements IDisposable As such, care is needed to ensure that WaitHandles are disposed when they are no longer needed
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Reset Events: ManualResetEvent and ManualResetEventSlim
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One way to control uncertainty about when particular instructions in a thread will execute relative to instructions in another thread is with reset events In spite of the term events, reset events have nothing to do with C# delegates and events Instead, reset events are a way to force code to wait for the execution of another thread until the other thread signals These are especially useful for testing multithreaded code because it is possible to wait for a particular state before verifying the results The reset event types are SystemThreadingManualResetEvent and the NET Framework 4 added lightweight, version SystemThreadingManual ResetEventSlim (As discussed in the Advanced Topic on page 772, there is a third type, SystemThreadingAutoResetEvent, but programmers should avoid it in favor of one of the first two) The key methods on the reset events are Set() and Wait() (called WaitOne() on ManualResetEvent) Calling the Wait() method will cause a thread to block until a different thread calls
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Synchronization Set(), or until the wait period times out Listing 199 demonstrates how this
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works, and Output 196 shows the results
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Listing 199: Waiting for ManualResetEventSlim
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using System; using SystemThreading; using SystemThreadingTasks; public class Program { static ManualResetEventSlim MainSignaledResetEvent; static ManualResetEventSlim DoWorkSignaledResetEvent; public static void DoWork() { ConsoleWriteLine("DoWork() started"); DoWorkSignaledResetEventSet(); MainSignaledResetEventWait(); ConsoleWriteLine("DoWork() ending"); } public static void Main() { using(MainSignaledResetEvent = new ManualResetEventSlim()) using (DoWorkSignaledResetEvent = new ManualResetEventSlim()) { ConsoleWriteLine( "Application started"); ConsoleWriteLine("Starting task"); Task task = TaskFactoryStartNew(DoWork); // Block until DoWork() has started DoWorkSignaledResetEventWait(); ConsoleWriteLine("Thread executing"); MainSignaledResetEventSet(); taskWait(); ConsoleWriteLine("Thread completed"); ConsoleWriteLine( "Application shutting down"); } } }
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19: Synchronization and More Multithreading Patterns
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OUTPUT 196:
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Application started Starting thread DoWork() started Waiting while thread executes DoWork() ending Thread completed Application shutting down
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Listing 199 begins by instantiating and starting a new Task Table 193 shows the execution path in which each column represents a thread In cases where code appears on the same row, it is indeterminate which side executes first
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TABLE 193: Execution Path with ManualResetEvent Synchronization
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Main( )
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ConsoleWriteLine( "Application started"); Task task = new Task(DoWork); ConsoleWriteLine( "Starting thread"); taskStart(); DoWorkSignaledResetEventWait();
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DoWork( )
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ConsoleWriteLine( "DoWork() started"); DoWorkSignaledResetEventSet();
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ConsoleWriteLine( "Thread executing"); MainSignaledResetEventSet(); taskWait();
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MainSignaledResetEventWait();
ConsoleWriteLine( "DoWork() ending");
Synchronization
TABLE 193: Execution Path with ManualResetEvent Synchronization (Continued)
Main( )
ConsoleWriteLine( "Thread completed"); ConsoleWriteLine( "Application exiting");
DoWork( )
Calling a reset event s Wait() method (for a ManualResetEvent it is called WaitOne()) blocks the calling thread until another thread signals and allows the blocked thread to continue Instead of blocking indefinitely, Wait()/WaitOne() overrides include a parameter, either in milliseconds or as a TimeSpan object, for the maximum amount of time to block When specifying a timeout period, the return from WaitOne() will be false if the timeout occurs before the reset event is signaled ManualResetEvent Wait() also includes a version that takes a cancellation token, allowing cancellation requests as discussed in the preceding chapter The difference between ManualResetEventSlim and ManualReset Event is the fact that the latter uses kernel synchronization by default whereas the former is optimized to avoid trips to the kernel except as a last resort Thus, ManualResetEventSlim is more performant even though it could possibly use more CPU cycles Therefore, use ManualResetEventSlim in general unless waiting on multiple events or across processes is required Notice that reset events implement IDisposable, so they should be disposed when they are no longer needed In Listing 199, we do this via a using statement (CancellationTokenSource contains a ManualResetEvent, which is why it too implements IDisposable) Although not exactly the same, SystemThreadingMonitor s Wait() and Pulse() methods provide similar functionality to reset events in some circumstances