qr code generator vb.net 2010 s_value = new Singleton(); // This is what you'd ideally like to write in .NET

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s_value = new Singleton(); // This is what you'd ideally like to write
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You would expect the compiler to produce code that allocates the memory for a Singleton, calls the constructor to initialize the fields, and then assigns the reference into the s_value field . Making a value visible to other threads is called publishing . But the compiler could do this instead: allocate memory for the Singleton, publish (assign) the reference into s_value, and then call the constructor . From a single thread s perspective, changing the order like this has no impact . But what if, after publishing the reference into s_value and before calling the constructor, another thread calls the GetSingleton method This thread will see that s_value is not null and start to use the Singleton object, but its constructor has not finished executing yet! This can be a very hard bug to track down, especially since it is all due to timing . The call to Interlocked.Exchange fixes this problem . It ensures that the reference in temp can be published into s_value only after the constructor has finished executing . Another way to solve this problem would be to mark the s_value field with C# s volatile keyword . This makes the write to s_value volatile, and again, the constructor has to finish running before the write can happen . Unfortunately, this makes all reads volatile, too, and since there is no need for this, you are hurting your performance with no benefit .
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In the beginning of this section, I mentioned that the double-check locking technique is not that interesting . In my opinion, developers think it is cool, and they use it far more often than they should . In most scenarios, this technique actually hurts efficiency . Here is a much simpler version of the Singleton class that behaves the same as the previous version . This version does not use the double-check locking technique:
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internal sealed class Singleton { private static Singleton s_value = new Singleton(); // Private constructor prevents any code outside this class from creating an instance private Singleton() { // Code to initialize the one Singleton object goes here... } // Public, static method that returns the Singleton object (creating it if necessary) public static Singleton GetSingleton() { return s_value; } }
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Since the CLR automatically calls a type s class constructor the first time code attempts to access a member of the class, the first time a thread queries Singleton s GetSingleton method, the CLR will automatically call the class constructor, which creates an instance of the object . Furthermore, the CLR already ensures that calls to a class constructor are thread safe . I explained all of this in 8 . The one downside of this approach is that the type constructor is called when any member of a class is first accessed . If the Singleton type defined some other static members, then the Singleton object would be created when any one of them was accessed . Some people work around this problem by defining nested classes . Let me show you a third way of producing a single Singleton object:
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internal sealed class Singleton { private static Singleton s_value = null; // Private constructor prevents any code outside this class from creating an instance private Singleton() { // Code to initialize the one Singleton object goes here... } // Public, static method that returns the Singleton object (creating it if necessary) public static Singleton GetSingleton() { if (s_value != null) return s_value; // Create a new Singleton and root it if another thread didn't do it first Singleton temp = new Singleton(); Interlocked.CompareExchange(ref s_value, temp, null); // If this thread lost, then the second Singleton object gets GC'd return s_value; // Return reference to the single object } }
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