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A Multithreaded Singleton Implementation The singleton implementation is multithreaded and manages the updates as necessary, and the full implementation is as follows: public class Singleton< singletonupdate, type> where singletonupdate: ISingletonBuilder< type>, new() where type : class { private static Singleton< singletonupdate, type> _instance; private ISingletonBuilder< type> _builder; private type _data; private ReaderWriterLock _lock; Thread _thread; private Singleton() { _builder = new singletonupdate(); _lock = new ReaderWriterLock(); _thread = new Thread( new ThreadStart( SingletonThread)); _thread.Start(); } private void SingletonThread() { while( _builder.KeepPolling) { try { _lock.AcquireReaderLock( -1); if( _data != null && !_builder.IsValid( _data)) { _lock.UpgradeToWriterLock( -1); _data = null; } } finally { _lock.ReleaseLock(); } Thread.Sleep( _builder.SleepTime); } } public static type Instance( object obj) { if( _instance == null) { lock( typeof( Singleton< singletonupdate,type>)) { if( _instance == null) { _instance = new Singleton< singletonupdate, type>(); } } } type retval = null; try {
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_instance._lock.AcquireReaderLock( -1); if( _instance._data == null) { _instance._lock.UpgradeToWriterLock(-1); if( _instance._data == null) { _instance._data = _instance._builder.NewObject( obj); } } retval = _instance._data; } finally { _instance._lock.ReleaseLock(); } return retval; } } The Singleton<> class is a Generic class that has two generic parameters: singletonupdate and type. The type is an implementation of the ISingletonBuilder<> interface. It s specified as a template parameter because of the nature of the singleton. Unfortunately, the singleton is a static type that is instantiated without knowing any reference information. Using a singleton within the singleton is a futile endeavor, as that would create a recursion, and attempting to reference a configuration file creates an unwanted dependency. The only solution is to define the type as a generic parameter. The generic parameter type defines the singleton data, which in the example of Figure 7-4 was SingletonData. Singleton<> has two methods: SingletonThread and Instance. The method Instance functions identically to the classical singleton implementation of the method Instance. The method SingletonThread is a threaded method that scans and queries whether the singleton data needs to be updated. The best way to understand Singleton<> is to explain the order of events when a client executes the following source code (it s assumed that the Singleton<> class with the specified types has never been referenced in the application): Singleton< MySingletonBuilder, SingletonData>.Instance( null).Buffer = "hello"; The .NET compiler will convert the Generic type Singleton<> into a unique type based on the generic parameters MySingletonBuilder and SingletonData. The method Singleton<>.Instance is called with a null parameter indicating that there is no constructor information. Shifting focus to the implementation of the Instance method, the first check is to see whether the data member _instance has been instantiated. If _instance equals a null, which means no instantiation, then _instance is assigned an instance of Singleton<>. When Singleton<> is instantiated, the constructor instantiates an ISingletonBuilder<> instance and assigns it to the data member _builder. After the _builder variable has been assigned, a ReaderWriterLock is instantiated and assigned to the data member _lock. The variable _lock will be discussed in more detail in a moment. The last thing the constructor does is create a thread and execute the method SingletonThread. The method SingletonThread is a polling thread that periodically checks for the validity of the singleton data. What is different about the polling is the while loop that doesn t lock the data, but uses something called a reader-writer lock.
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The idea behind the reader-writer lock is that when a piece of data is being read, it s being considered immutable. And immutability means allowing multiple readers at the same time poses no problem. Where the problem begins is if a thread wants to write data. Then only one thread can write, while all of the others have to wait. Relating this back to the method SingletonThread, this means that it s possible to read the data member _data, but not manipulate it. To acquire a read-only lock, the method AcquireReaderLock is called. The if block that calls _builder.IsValid DelegatePredicate<> tests whether the singleton data needs to be updated. If the predicate returns true, then _lock is upgraded to a writer lock using the method UpgradeWriterLock. Then the _data variable is assigned a value of null, indicating that the client needs to instantiate a new instance before using the singleton data. The reason why _data is assigned a null and not instantiated is because that is the task of the method Instance. The lock is released using the method ReleaseLock. Once the constructor and thread have finished, the Instance method will continue and also acquire a read-only lock to the data member _data. If _data is null, then the read lock will be upgraded to a write lock and the method _instance._builder.NewObject is called. The NewObject method is responsible for instantiating a new instance of the singleton data. In both SingletonThread and Instance, the try-finally construct is used. This is done on purpose so that if within the try block an exception is generated, the lock will be released and not cause a deadlock scenario. Doing otherwise could result in threads waiting for a lock to be released that never will be released. Building a Full Implementation Putting all of this together, what results is a singleton implementation that periodically checks for any updates to the singleton data. If there are updates, then the object is newly instantiated. It s important to understand that singleton data isn t updated on the singleton, but through some other technique. The singleton data is considered a snapshot of the data that should be considered global data. Following is a class definition that represents the singleton data: [Serializable] class SingletonData { public readonly string FileData; public SingletonData( string filedata) { FileData = filedata; } } Notice how SingletonData is both serializable ([Serializable]) and immutable (uses only readonly properties). Doing both of these things makes it very easy to serialize SingletonData across a network or AppDomain. Singleton<> requires an ISingletonBuilder<> instance, and it s defined as follows: class SingletonBuilder : BaseSingletonDelegation< SingletonData> { private string _path; public SingletonBuilder() { //_path = some configuration file entry }
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