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Using the lock Keyword Because of the frequent need for synchronization using Monitor in multithreaded code, and the fact that the try/finally block could easily be forgotten, C# provides a special keyword to handle this locking synchronization pattern Listing 194 demonstrates the use of the lock keyword, and Output 193 shows the results
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Listing 194: Synchronization Using the lock Keyword
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using System; using SystemThreading; using SystemThreadingTasks; class Program { readonly static object _Sync = new object(); const int _Total = intMaxValue; static long _Count = 0; public static void Main() { Task task = TaskFactoryStartNew(Decrement); // Increment for (int i = 0; i < _Total; i++) { lock (_Sync) { _Count++; } } taskWait(); ConsoleWriteLine("Count = {0}", _Count); } static void Decrement() { for (int i = 0; i < _Total; i++) { lock (_Sync) { _Count--; }
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19: Synchronization and More Multithreading Patterns
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} } }
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OUTPUT 193:
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Count = 0
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By locking the section of code accessing _Count (using either lock or Monitor), you make the Main() and Decrement() methods thread-safe, meaning they can be safely called from multiple threads simultaneously (Prior to C# 40 the concept was the same except the compiler-emitted code depended on the MonitorEnter() method without the lockTaken parameter and the MonitorEnter() called was emitted before the try block) Synchronization comes at a cost to performance Listing 194, for example, takes an order of magnitude longer to execute than Listing 191 does, which demonstrates lock s relatively slow execution compared to the execution of incrementing and decrementing the count Even when lock is insignificant in comparison with the work it synchronizes, programmers should avoid indiscriminate synchronization in order to avoid the possibility of deadlocks and unnecessary synchronization on multiprocessor computers that could instead be executing code in parallel The general best practice for object design is to synchronize mutable static state (there is no need to synchronize something that never changes) and not any instance data Programmers who allow multiple threads to access a particular object must provide synchronization for the object Any class that explicitly deals with threads is likely to want to make instances thread-safe to some extent
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Choosing a lock Object Whether or not the lock keyword or the Monitor class is explicitly used, it is crucial that programmers carefully select the lock object In the previous examples, the synchronization variable, _Sync, is declared as both private and read-only It is declared read-only to ensure that the value is not changed between calls to MonitorEnter() and MonitorExit() This allows correlation between entering and exiting the synchronized block
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Synchronization
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Similarly, the code declares _Sync as private so that no synchronization block outside the class can synchronize the same object instance, causing the code to block If the data is public, then the synchronization object may be public so that other classes can synchronize using the same object instance This makes it harder to avoid deadlock Fortunately, the need for this pattern is rare For public data, it is preferable to leave synchronization entirely outside the class, allowing the calling code to take locks with its own synchronization object It s important that the synchronization object not be a value type If the lock keyword is used on a value type, then the compiler will report an error (In the case of accessing the SystemThreadingMonitor class explicitly [not via lock], no such error will occur at compile time Instead, the code will throw an exception with the call to MonitorExit(), indicating there was no corresponding MonitorEnter() call) The issue is that when using a value type, the runtime makes a copy of the value, places it in the heap (boxing occurs), and passes the boxed value to MonitorEnter() Similarly, MonitorExit() receives a boxed copy of the original variable The result is that MonitorEnter() and MonitorExit() receive different synchronization object instances so that no correlation between the two calls occurs
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Why to Avoid Locking on this, typeof(type), and string One common pattern is to lock on the this keyword for instance data in a class, and on the type instance obtained from typeof(type) (for example, typeof(MyType)) for static data Such a pattern provides a synchronization target for all states associated with a particular object instance when this is used, and all static data for a type when typeof(type) is used The problem is that the synchronization target that this (or typeof(type)) points to could participate in the synchronization target for an entirely different synchronization block created in an unrelated block of code In other words, although only the code within the instance itself can block using the this keyword, the caller that created the instance can pass that instance to a synchronization lock The result is that two different synchronization blocks that synchronize two entirely different sets of data could block each other Although perhaps unlikely, sharing the same synchronization target could have an unintended performance impact and, in extreme cases, even cause
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