Synchronized Method Using the lock Statement in .NET

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Listing 3-2 Synchronized Method Using the lock Statement
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public void Foo() { lock (this) { if (val == 5) { val++; // Do something because of val == 6 // } } }
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Behind the scenes, the C# lock statement uses the Monitor class and generatesMonitorEnter and MonitorExit statements to lock the object
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NOTE: Synchronization Within NET Framework Classes
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With the classes of the Microsoft NET Framework, usually all static methods are synchronized and thus thread-safe, whereas all instance methods are not synchronized To be on the safe side, you always have to check the MSDN documentation Thread safety is documented with every class With classes that may be accessed from multiple threads, you have to be aware of the locking issue, and you have to synchronize access to the methods of the class where state of the object is accessed
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Implementing locking is not the only issue that you must pay attention to; you also have to concentrate on how to do locking correctly Too many unnecessary locks will decrease the performance of the application, because threads would wait longer than necessary Using locking the wrong way can also lead to deadlocks Figure 3-1 shows a deadlock scenario If thread 1 issues a lock to object A that is accepted, and while it has the lock for object A it issues a lock for object B, another thread might do it the other way around: first locking B, and then locking A Now a deadly situation can happen: The lock to A from the first thread is accepted, and then the next thread gets a chance to get the CPU and issues a lock to B Now thread 1 waits for object B, while thread 2 waits for object A This is a typical deadlock situation, which can happen at random depending on the thread scheduling of the operating system A deadlock will occur if two threads wait for each other to give up a lock
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Figure 3-1 Deadlock situation
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This might look obvious in a simple method, but the locks can be hidden far into the call stack To resolve such an issue, you should always do the locks in the same order
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TIP: Reality CheckMultithreading Issues
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With multithreaded applications, you must carefully pay attention to threading issues If you do not pay attention from the beginning, threading issues are hard to fix because problems occur only once in a while If you are using a multi-CPU system, threading problems happen more often because multiple threads of a single process run concurrently
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The simple increment operation val++ is not thread-safe! If the variable val is accessed by multiple threads, the access to it must be synchronized With simple increment and decrement operations, it is not necessary to place a lock on an object You can use the utility class Interlocked instead This class offers the static methods Increment, Decrement, Exchange, and CompareExchange These methods are thread-safe Instead of writing val++, you can do this: SystemThreadingInterlockedIncrement(ref val);
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Using the Interlocked class would not help in the previous example with the method Foo where the code was dependent on the value returned from the increment operation, because the lock is already given up as soon as the Increment method completes
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More Synchronization Options
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Instead of the lock statement, you can also use the [Synchronization] attribute from the namespace SystemRuntimeRemotingContexts, as discussed in 2, "Object Activation and Contexts" But now you need to be aware of the features offered by Enterprise Services that lead you to the [Synchronization] attribute in the namespace SystemEnterpriseServices With COM components, Microsoft invented apartments as a synchronization boundary, and they still play an important role with serviced components Even more important are activities, which are discussed after apartments in this chapter
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