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A critical section is a region of code, that is mutually exclusive. This means that while one thread is executing that region of code no other thread can. Critical sections are created using the lock and SyncLock constructs.
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Locks are a means of creating a critical section. Unlike a Win32 critical section, it is not necessary to create a variable to serve as the key to the critical section. Instead, any instance of an object can be used to control entering the critical section. A common approach is to lock on the instance of the object itself using the me/this statement. Many of the examples you will see use this form of locking. This is the simplest form of synchronization control. It creates a high degree of control over access but at the expense of flexibility. Under some circumstances this is a valid solution. Other times a more granular approach is required.
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How a lock is used to coordinate two threads.
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CONCURRENCY CONTROL
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SyncLock
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SyncLock is a Visual Basic keyword that is used as a synchronization mech-
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anism to create a section of code that only a single thread can access at a time. This is accomplished by acquiring a lock on an object. If the object is currently locked, the thread must wait until the lock is released.
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Suppose you had a class with three data elements in it. One approach would be to restrict access based on the class as a whole. This would mean that only one of the three data elements could be changed at once. If these elements were independent, this might be too restrictive. An alternative would be to have three objects that serve as locks. In order to access one of the data elements the corresponding lock would first be acquired. Figure 7.15 graphically demonstrates this design tradeoff. This introduces the concept of concurrency. Concurrency is a measure of how many things can happen at once. A high degree of concurrency will often produce higher performance than a low degree. The tradeoff is between concurrency and the risk of race conditions, deadlocks, and complexity. In the previous chapter we discussed deadlocks. Deadlocks are a very real problem with SyncLocks. Using the lock/SyncLock statement there is no way to time out a request for a resource. So if a thread monopolizes a resource, all other threads requesting that resource will be in a WaitSleepJoin state until the resource becomes available. To reduce the possibility of deadlock, the lock is released whenever the thread exits the locked region. This is true if an exception is raised or processing completes normally. The design constraints regarding deadlock should always be followed when using the lock/SyncLock statements. If used correctly, lock/SyncLock is a powerful means of controlling synchronization. You may be wondering how lock/SyncLock works; we ll cover that in the next section.
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Figure 7.15 A single lock can be used to guard multiple items or a lock can be used to protect each item independently.
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THE LOCK AND SYNCLOCK KEYWORDS
THE MONITOR CLASS
During grade school there were generally people tasked with the job of hall monitor. A hall monitor attempts to make sure that students and visitors are in the right places at the right time. If you walk into a strange school and start roaming the halls, you re likely to meet a hall monitor. A monitor is something that watches over something else. What the monitor watches might be the flow of students and visitors in the hall of a school or, in the case of multithreaded development, the access to resources by threads. Monitor is a class in the Threading namespace that contains methods for capturing and releasing synchronous locks.
The Enter and Exit methods Monitor.Enter is called when a lock is requested. It blocks and doesn t return until the lock has been granted. If the thread that is calling Enter already has the lock it is requesting, the lock count for that object is incremented and the thread is allowed to pass. If the thread that is calling Enter does not have the lock and another thread does, it will wait until that other thread releases the lock by calling Monitor.Exit. If the lock count for the parameter passed into Enter is zero, the current thread is granted ownership of that lock and the lock count is incremented to one. Compared to synchronous locks We discussed what the lock/SyncLock method does in section 7.3. If you re like me, you want to know how things work. SyncLocks are implemented using the Monitor.Enter and Monitor.Exit methods. Table 7.1 shows two segments of code that produce almost identical MSIL.
Table 7.1 How the Lock Method Is Implemented Using the Monitor Enter and Exit Methods CEnterExit.cs object objLock = new object(); object tmpObject = objLock; Monitor.Enter(tmpObject ); try { objLock =123; } finally { Monitor.Exit(tmpObject ); } These two pieces of code produce virtually identical MSIL. The lock statement is implemented using Monitor.Enter and Monitor.Exit. CLock.cs object o = new object(); lock(o) { o=123; }
There are two things to notice about the code in table 7.1. The first is that the Monitor.Enter instruction is not inside the try block. There are two exceptions that Enter can throw: ArgumentNullException and ArgumentException. ArgumentNullException is thrown whenever the parameter passed to the Enter method is null. In this case calling Exit would be inappropriate. ArgumentException is raised when the parameter to Enter is a value type, for instance an 128
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