.net qr code reader Class Libraries and Thread Safety in .NET framework

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Class Libraries and Thread Safety
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Now, I d like to say a quick word about class libraries and thread synchronization . Microsoft s Framework Class Library (FCL) guarantees that all static methods are thread safe . This means that if two threads call a static method at the same time, no data will get corrupted . The FCL had to do this internally because there is no way that multiple companies producing different assemblies could coordinate on a single lock for arbitrating access to the resource . The Console class contains a static field, inside which many of its methods acquire and release to ensure that only one thread at a time is accessing the console . For the record, making a method thread safe does not mean that it internally takes a thread synchronization lock . A thread-safe method means that data doesn t get corrupted if two
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Part V
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Threading
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threads attempt to access the data at the same time . The System.Math class has a static Max method implemented as follows:
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public static Int32 Max(Int32 val1, Int32 val2) { return (val1 < val2) val2 : val1; }
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This method is thread safe even though it doesn t take any lock . Since Int32 is a value type, the two Int32 values passed to Max are copied into it and multiple threads could be calling Max simultaneously, but each thread is working on its own data, isolated from any other thread . On the other hand, the FCL does not guarantee that instance methods are thread safe because adding all the locking code would hurt performance too much . And, in fact, if every instance method acquires and releases a lock, then you ultimately end up having just one thread running in your application at any given time, which hurts performance even more . As mentioned earlier, when a thread constructs an object, only this thread has a reference to the object, no other thread can access that object, and no thread synchronization is required when invoking instance methods . However, if the thread then exposes the reference to the object by placing it in a static field, passing as the state argument to ThreadPool. QueueUserWorkItem or to a Task, and so on then thread synchronization is required if the threads could attempt simultaneous non read-only access . It is recommended that your own class libraries follow this pattern; that is, make all your static methods thread safe and make all your instance methods not thread-safe . There is one caveat to this pattern: if the purpose of the instance method is to coordinate threads, then the instance method should be thread safe . For example, one thread can cancel an operation by calling CancellationTokenSource s Cancel method, and another thread detects that it should stop what it s doing by querying the corresponding CancellationToken s IsCancellationRequested property . These two instance members have some special thread synchronization code inside them to ensure that the coordination of the two threads goes as expected .1
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Primitive user-Mode and Kernel-Mode Constructs
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In this chapter, I explain the primitive thread synchronization constructs . By primitive, I mean the simplest constructs that are available to use in your code . There are two kinds of primitive constructs: user-mode and kernel-mode . Whenever possible, you should use the primitive user-mode constructs because they are significantly faster than the kernel-mode constructs as they use special CPU instructions to coordinate threads . This means that the coordination is occurring in hardware (which is what makes it fast) . But this also means that the Microsoft
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Specifically, the field that both members access is marked as volatile, a concept that will be discussed later in this chapter .
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