Waiting for AutoResetEvent in C#

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Listing 1514 Waiting for AutoResetEvent
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using System; using SystemThreading; public classAutoResetEventSample { static AutoResetEvent ResetEvent = new AutoResetEvent(false); public static void DoWork() { ConsoleWriteLine("DoWork() started"); ThreadSleep(1000); ConsoleWriteLine("DoWork() ending"); // Signal to ResetEventWaitOne() ResetEventSet(); } public static void Main() { ConsoleWriteLine("Application started"); Thread thread = new Thread(DoWork); ConsoleWriteLine("Starting thread"); threadStart(); // Block until ResetEventSet() is called ResetEventWaitOne(); ConsoleWriteLine("Application shutting down"); } }
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Output 1511
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Application started Starting thread DoWork() started DoWork() ending Application shutting down
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Listing 1514 begins by starting a new thread Following tHReadStart(), it calls ResetEventWaitOne() This causes the thread executing Main() to suspend and wait for the AutoResetEvent called ResetEvent to be set The thread running DoWork() continues, however Inside DoWork() is a call to ResetEventSet(), and once this method has been called, the call to ResetEventWaitOne() back in Main() is signaled, meaning it is allowed to continue As a result, DoWork() started and DoWork() ending appear before Application shutting down in spite of the fact that DoWork() includes a call to ThreadSleep() and DoWork() is running on a different thread Calling a reset event's WaitOne() method blocks the calling thread until another thread signals and allows the blocked thread to continue Instead of blocking indefinitely, WaitOne() includes 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 The only difference between SystemThreadingAutoResetEvent and SystemThreadingManualResetEvent is the fact that AutoResetEvent will automatically switch to an unsignaled state after calling Set() As a result, a second call to WaitOne() will automatically be blocked until another call to Set() occurs Given this behavior, it is possible for two different threads to call WaitOne() simultaneously, and only one will be allowed to continue with each call to Set() In contrast, ManualResetEvent will require a call to Reset() before it will block any additional threads The remainder of this chapter, and 16, use a call to an AutoResetEvent's Set() method within the worker thread's implementation In addition, AutoResetEvent's WaitOne() method blocks on Main()'s thread until Set() has been called In this way, it demonstrates that the worker thread executes before Main() exits Although not exactly the same, SystemThreadingMonitor includes Wait() and Pulse() methods that provide similar functionality to reset events in some circumstances
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ThreadStaticAttribute
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In some cases, using synchronization locks can lead to unacceptable performance and scalability restrictions In other instances, providing synchronization around a particular data element may be too complex, especially when it is added after the original coding One alternate solution to synchronization is thread local storage Thread local storage creates a new instance of a static field for every thread This provides each thread with its own instance; as a result, there is no need for synchronization, as there is no point in synchronizing data that occurs within only a single thread's context Decorating a field with a ThreadStaticAttribute, as in Listing 1515, designates it as one instance per thread
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Listing 1515 Using the THReadStaticAttribute
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class ThreadStaticData { const int _Total = shortMaxValue; [ThreadStatic] static long _Count = 0; public static void Main() { Thread thread = new Thread(Decrement); threadStart(); // Increment for (int i = 0; i < _Total; i++) { _Count++; } threadJoin(); ConsoleWriteLine("Count = {0}", _Count); } static void Decrement() { for (int i = 0; i < _Total; i++) { _Count--; } } }
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The results of Listing 1515 appear in Output 1512
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Output 1512
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Count = 32767
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As Output 1512 demonstrates, the value of _Count for the thread executing Main() is never decremented by the thread executing Decrement() Since _Count is decorated by the THReadStaticAttribute, the thread running Main() and the thread running Decrement() are operating on entirely different instances of _Count There is one important caveat to the ThreadStaticAttribute If the value of _Count is assigned during declarationprivate int _Count = 42 , for example then only the thread static instance associated with the thread running the constructor will be initialized In Listing 1515, only the thread executing Main() will have a thread local storage variable of _Count that is initialized The value of _Count that Decrement() decrements will never be initialized Similarly, if a constructor initializes a thread local storage field, only the constructor calling thread will have an initialized thread local storage instance For this reason, it is a good practice to initialize a thread local storage field within the method that is initially called by each thread The decision to use thread local storage requires some degree of cost-benefit analysis For example, consider using thread local storage for a database connection Depending on the database management system, database connections are relatively expensive, so creating a connection for every thread could be costly Similarly, locking a connection so that all database calls are synchronized places a significantly lower ceiling on scalability Each pattern has its costs and benefits,
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and the correct choice depends largely on the individual implementation Another reason to use ThreadStatic is to make commonly needed context information available to other methods without explicitly passing the data via parameters If multiple methods in the call stack require user security information, for example, you can pass the data using thread local storage fields instead of as parameters This keeps APIs cleaner while still making the information available to methods in a thread-safe manner
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