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13. Threading and Synchronization
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Using the Interlocked class introduces tidier code and removes the need to use typeof to protect static variables. The variable must be annotated with the ref keyword to ensure that the target variable is modified. Omitting the annotation causes the value of the instance to be copied and modified without affecting the original instance, in which case simultaneous calls to Interlocked will be dealing with different values.
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WaitHandle
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The WaitHandle class is used as a base class for synchronization classes that interoperate with the native operating system. The class defines a signaling mechanism that is used to indicate that a shared resource has been acquired or released. The significance of this class comes from the static methods that allow callers to block until one or more instances of WaitHandle are signaled. Three methods are available:
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WaitOne Waits for a single WaitHandle to be signaled.
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WaitAny Static method; waits for any one of an array of WaitHandle objects to be signaled.
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WaitAll Static method; waits for all of the WaitHandles in an array to be signaled.
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The three classes in the namespace that are derived from WaitHandle are discussed in the following sections.
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Mutex
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The Mutex class can be used to provide synchronization between processes. Here's a simple example of using the Mutex class:
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Mutex x_mutex = new Mutex(false, "shared"); Console.WriteLine("Created mutex"); while (true) { string x_str = Console.ReadLine(); if (x_str.Equals("a")) { Console.WriteLine("Attempting to acquire..."); x_mutex.WaitOne(); Console.WriteLine("Acquired the mutex"); } else if (x_str.Equals("r")) { Console.WriteLine("Releasing"); x_mutex.ReleaseMutex(); Console.WriteLine("Released"); } }
13. Threading and Synchronization
Running two instances of this code on a machine demonstrates a shared Mutex. Typing a causes the class to attempt to acquire the lock. Typing r will release the lock. If a process dies when holding the lock, the resource becomes available for acquisition. The example illustrates a named Mutex. Processes that know the name of the Mutex can share named instances. Instances can also be created without names. With an unnamed Mutex, an object reference must be passed to enable another thread or process to synchronize against it.
Any process that knows the name of the instance can share a named Mutex. Choose names that are project specific. Be aware that other programmers may attempt to acquire the lock for reasons that were not intended, leading to deadlocks.
AutoResetEvent and ManualResetEvent
The final classes that we'll cover in this chapter are AutoResetEvent and ManualResetEvent. Threads calling the waitOne method of these classes will block until the event is signaled through a call to the Set method. The AutoResetEvent is constructed with a single argument that dictates the initial signal state. Using true as the argument will result in the first call to waitOne not blocking the thread. This class provides a simpler approach to using the Monitor class in order to coordinate access to shared resources driven by external events. In the following example, once started the thread will block on the waitOne method until the user types the s key, when the default thread will set the event signal. This releases the thread, which writes a message to the console. The AutoResetEvent resets automatically hence the name and subsequent calls to waitOne will also block:
class AutoEvent { AutoResetEvent o_event = new AutoResetEvent(false); public AutoEvent() { Thread x_thread = new Thread(new ThreadStart(run)); x_thread.Start(); while (true) { if (Console.Read() == 's') { o_event.Set(); } } } public void run() { while (true) { o_event.WaitOne(); Console.WriteLine("Event signalled"); } }
13. Threading and Synchronization
There is also a ManualResetEvent class that remains signaled after the Set method is used (meaning that calls to waitOne will not block). To reset the signal state, the Reset method must be used, causing waitOne calls to block until the event is signaled again.
Summary
This chapter illustrates that the Java programmer has nothing to fear from the threading and synchronization support in .NET. Most of the constructs and approaches available in Java have at least one analog in .NET. We end the chapter with a note of caution. When Java was first released, all of the calls in the java.lang.Thread class were available for use. Over the years, several have been deprecated because they were found to be unsafe. C# and the CLR are new products, and it's possible that over time the same thing will happen. We think that this is less likely than with Java because the CLR threading model is so closely based on Windows.
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