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copied!<h2>Singleton Pattern</h2> <p><strong>Definition:</strong> If a system only needs one instance of a class, and that instance needs to be accessible in many different parts of a system, you control both instantiation and access by making that class a singleton.</p> <p><strong>Example:</strong> A concrete example would be the <code>ISessionFactory</code> API while using NHibernate. Because of the load in memory the <code>ISessionFactory</code> costs to instantiate, due that it involves all of the object-relation mappings and database connection to be loaded in memory all at once, it is advisable to make only one instance, and create the <code>ISession</code> API instances using this unique instance of the <code>ISessionFactory</code> API, that is, the <em>Singleton Pattern</em>.</p> <pre><code>using NHibernate; internal class NhConnectionFactory { private static readonly ISessionFactory _sessionFactory = config.BuildSessionFactory(); internal NhSystemFactory() { } internal ISession OpenSession(string connectionString) { return _sessionFactory.OpenSession(); } // Other methods here... } public static class NhSystemFacade { private static readonly NhConnectionFactory _nhConnectionFactory = new NhConnectionFactory(); public static string DefaultConnectionString { get { return Properties.Settings.Default.DefaultConnectionString; } } public static ISession OpenSession() { return _nhConnectionFactory.OpenSession(DefaultConnectionString); } } </code></pre> <p><strong>Links:</strong></p> <ul> <li><a href="http://c2.com/cgi/wiki?SingletonPattern" rel="nofollow">Description from the Portland Pattern Repository</a></li> <li><a href="http://en.wikipedia.org/wiki/Singleton_pattern" rel="nofollow">Singleton pattern on Wikipedia</a></li> </ul>

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