0:000> !handle 1fc 8 Handle 1fc Object Specific Information Mutex is Free in .NET framework

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Semaphore
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A semaphore is a kernel mode synchronization object accessible from user mode It is similar to a mutex in the sense that it allows exclusive access to a resource The main difference, however, is that a semaphore employs resource counting, thereby allowing X number of threads access to the resource A great example of when to use a semaphore is in a system that has four USB ports that are accessed by a piece of code Because there are four USB ports, we would like to allow four threads to concurrently use one of the available USB ports To accomplish this, we would create a semaphore with a max resource count of four As threads try to acquire the semaphore, the reference count (initialized to 4) is checked as to whether it is greater than 0, and if it is, it allows the acquisition and decrements the reference count When the reference count reaches 0, a thread trying to acquire the semaphore is put to sleep until a thread releases the semaphore and the reference count is incremented As with events and mutexes, you would use the do command followed by the handle extension command in the debugger to get extended information on a semaphore
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0:000> !do 0x01d159ac Name: SystemThreadingSemaphore MethodTable: 7a76397c EEClass: 7a763904 Size: 24(0x18) bytes (C:\Windows\assembly\GAC_MSIL\System\2000__b77a5c561934e089\Systemdll) Fields: MT Field Offset Type VT Attr Value Name 790fd0f0 400018a 4 SystemObject 0 instance 00000000 __identity 791016bc 40005b3 c SystemIntPtr 1 instance 238 waitHandle
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79112728 40005b4 8 esSafeWaitHandle 7910be50 40005b5 10 SystemBoolean hasThreadAffinity 791016bc 40005b6 994 SystemIntPtr >> Domain:Value 0018fdc0:ffffffff << 79102290 4001066 8c8 SystemInt32 0:000> !handle 238 Handle 238 Type Semaphore 0:000> !handle 238 8 Handle 238 Object Specific Information Semaphore Count 0 Semaphore Limit 3
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0 instance 01d159c4 safeWaitHandle 1 instance 0 1 1 shared static static InvalidHandle 0 MAX_PATH
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Monitor
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A monitor is best viewed as a construct that monitors access to an object and creating a lock on it thereby not allowing other threads to obtain access until the owning thread explicitly leaves (or unlocks) the monitor In contrast with the earlier synchronization primitives discussed, a monitor is not just a wrapper around core Windows synchronization primitives, rather a first class NET citizen To use the monitor, the SystemThreadingMonitor class should be used The Monitor class cannot be instantiated; rather it contains a set of static methods that can be used to acquire a lock The two most common methods are Enter and Exit The Enter method acquires an exclusive lock on the specified object (assuming the object is not locked already) and the Exit method releases an exclusive lock on the specified object For example, the following code snippet locks on object db1:
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MonitorEnter(db1); // Do work MonitorExit(db1);
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6 SYNCHRONIZATION
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In C#, there is a more convenient lock statement that can be used to acquire and release an exclusive lock Even though the lock statement doesn t seem to have anything to do with monitors, in reality, it is just a shortcut form for calling the MonitorEnter and MonitorExit methods This can easily be seen by looking at the intermediate language (IL) that is generated for a lock statement Figure 6-2 shows an example of the IL that is generated from the following code:
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lock (someObj) { ConsoleWriteLine("I'm locked now"); }
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The IL shows that the lock statement is expanded to automatically enter a monitor (as illustrated by the boxed statements) followed by wrapping the protected region of code in a try/finally to ensure that the monitor is released at the end of the scope Because the Monitor class is an object that cannot be instantiated, there is really no state that we can look at in the debugger per se In light of this, who keeps track of which locks are available and the state of each lock The answer is that each object that is locked also keeps the information necessary to maintain the integrity of the locks as part of the object s layout We ll look into the internals of this bookkeeping in the Synchronization Internals section of this chapter
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ReaderWriterLock(Slim)
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The Monitor class previously discussed allows exclusive access to an object from a single thread at a time Although this works great in cases where writes are very frequent, the Monitor suffers in performance where there are more reads than writes and when the lock is heavily contended To address this performance issue, the ReaderWriterLock(Slim) was introduced A ReaderWriterLock allows multiple concurrent threads read access while limiting write operations to a single thread at a time In contrast with the Monitor class, the ReaderWriterLock class itself contains a state that controls access to the lock:
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0:000> !do 0x01e86cdc Name: SystemThreadingReaderWriterLock MethodTable: 79108ba4 EEClass: 79108b40 Size: 44(0x2c) bytes
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