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Definition

Ensure a class has only one instance and provides a global point of access to it.

Participants

    The classes and/or objects participating in this pattern are:

• Singleton (LoadBalancer)
  • Defines an Instance operation that lets clients access its unique instance. Instance is a class operation.
  • Responsible for creating and maintaining its own unique instance.

Sample Code in C#

This structural code demonstrates the Singleton pattern which assures only a single instance (the singleton) of the class can be created.

// --------------------------------------------------------------------------------------------------------------------// <copyright company="Chimomo's Company" file="Program.cs">
 
// Respect the work.// </copyright>// <summary>// Structural Singleton Design Pattern.// </summary>// --------------------------------------------------------------------------------------------------------------------namespace CSharpLearning{    using System;    /// <summary>    /// Startup class for Structural Singleton Design Pattern.    /// </summary>    internal static class MainApp    {        #region Methods        /// <summary>        /// Entry point into console application.        /// </summary>        private static void Main()        {            // Constructor is protected -- cannot use new            Singleton s1 = Singleton.Instance();            Singleton s2 = Singleton.Instance();            // Test for same instance            if (s1 == s2)            {                Console.WriteLine("Objects are the same instance");            }        }        #endregion    }    /// <summary>    /// The 'Singleton' class    /// </summary>    internal class Singleton    {        #region Static Fields        /// <summary>        /// The _instance.        /// </summary>        private static Singleton instance;        #endregion        // Constructor is 'protected'        #region Constructors and Destructors        /// <summary>        /// Initializes a new instance of the <see cref="Singleton"/> class.        /// </summary>        protected Singleton()        {        }        #endregion        #region Public Methods and Operators        /// <summary>        /// The instance.        /// </summary>        /// <returns>        /// The <see cref="Singleton"/>.        /// </returns>        public static Singleton Instance()        {            // Uses lazy initialization.            // Note: this is not thread safe.            if (instance == null)            {                instance = new Singleton();            }            return instance;        }        #endregion    }}// Output:/*Objects are the same instance*/

This real-world code demonstrates the Singleton pattern as a LoadBalancing object. Only a single instance (the singleton) of the class can be created because servers may dynamically come on-line or off-line and every request must go through the one object that has knowledge about the state of the (web) farm.

// --------------------------------------------------------------------------------------------------------------------// <copyright company="Chimomo's Company" file="Program.cs">// Respect the work.// </copyright>// <summary>// Real-World Singleton Design Pattern.// </summary>// --------------------------------------------------------------------------------------------------------------------namespace CSharpLearning{    using System;    using System.Collections.Generic;    /// <summary>    /// Startup class for Real-World Singleton Design Pattern.    /// </summary>    internal static class MainApp    {        #region Methods        /// <summary>        /// Entry point into console application.        /// </summary>        private static void Main()        {            LoadBalancer b1 = LoadBalancer.GetLoadBalancer();            LoadBalancer b2 = LoadBalancer.GetLoadBalancer();            LoadBalancer b3 = LoadBalancer.GetLoadBalancer();            LoadBalancer b4 = LoadBalancer.GetLoadBalancer();            // Same instance?            if (b1 == b2 && b2 == b3 && b3 == b4)            {                Console.WriteLine("Same instance\n");            }            // Load balance 15 server requests            LoadBalancer balancer = LoadBalancer.GetLoadBalancer();            for (int i = 0; i < 15; i++)            {                string server = balancer.Server;                Console.WriteLine("Dispatch Request to: " + server);            }        }        #endregion    }    /// <summary>    /// The 'Singleton' class    /// </summary>    internal class LoadBalancer    {        #region Static Fields        /// <summary>        /// The instance.        /// </summary>        private static LoadBalancer instance;        /// <summary>        /// The sync lock.        /// </summary>        private static object syncLock = new object();        #endregion        #region Fields        /// <summary>        /// The random.        /// </summary>        private Random random = new Random();        /// <summary>        /// The servers.        /// </summary>        private List<string> servers = new List<string>();        #endregion        // Constructor (protected)        #region Constructors and Destructors        /// <summary>        /// Initializes a new instance of the <see cref="LoadBalancer"/> class.        /// </summary>        protected LoadBalancer()        {            // List of available servers            this.servers.Add("ServerI");            this.servers.Add("ServerII");            this.servers.Add("ServerIII");            this.servers.Add("ServerIV");            this.servers.Add("ServerV");        }        #endregion        // Simple, but effective random load balancer        #region Public Properties        /// <summary>        /// Gets the server.        /// </summary>        public string Server        {            get            {                int r = this.random.Next(this.servers.Count);                return this.servers[r];            }        }        #endregion        #region Public Methods and Operators        /// <summary>        /// Get load balancer.        /// </summary>        /// <returns>        /// The <see cref="LoadBalancer"/>.        /// </returns>        public static LoadBalancer GetLoadBalancer()        {            // Support multi-threaded applications through 'Double-Check Locking' pattern which (once the instance exists) avoids locking each time the method is invoked.            if (instance == null)            {                lock (syncLock)                {                    if (instance == null)                    {                        instance = new LoadBalancer();                    }                }            }            return instance;        }        #endregion    }}// Output:/*Same instanceDispatch Request to: ServerIIDispatch Request to: ServerVDispatch Request to: ServerIIDispatch Request to: ServerVDispatch Request to: ServerIIDispatch Request to: ServerIIDispatch Request to: ServerIVDispatch Request to: ServerVDispatch Request to: ServerIIDispatch Request to: ServerVDispatch Request to: ServerIIIDispatch Request to: ServerIIIDispatch Request to: ServerIIDispatch Request to: ServerIIDispatch Request to: ServerIV*/

This .NET optimized code demonstrates the same code as above but uses more modern, built-in .NET features.
Here an elegant .NET specific solution is offered. The Singleton pattern simply uses a private constructor and a static read only instance variable that is eagerly initialized. Thread safety is guaranteed by the compiler.

// --------------------------------------------------------------------------------------------------------------------// <copyright company="Chimomo's Company" file="Program.cs">// Respect the work.// </copyright>// <summary>// .NET optimized Singleton Design Pattern.// </summary>// --------------------------------------------------------------------------------------------------------------------namespace CSharpLearning{    using System;    using System.Collections.Generic;    /// <summary>    /// Startup class for .NET optimized Singleton Design Pattern.    /// </summary>    internal static class MainApp    {        #region Methods        /// <summary>        /// Entry point into console application.        /// </summary>        private static void Main()        {            LoadBalancer b1 = LoadBalancer.GetLoadBalancer();            LoadBalancer b2 = LoadBalancer.GetLoadBalancer();            LoadBalancer b3 = LoadBalancer.GetLoadBalancer();            LoadBalancer b4 = LoadBalancer.GetLoadBalancer();            // Confirm these are the same instance            if (b1 == b2 && b2 == b3 && b3 == b4)            {                Console.WriteLine("Same instance\n");            }            // Next, load balance 15 requests for a server            LoadBalancer balancer = LoadBalancer.GetLoadBalancer();            for (int i = 0; i < 15; i++)            {                string serverName = balancer.NextServer.Name;                Console.WriteLine("Dispatch request to: " + serverName);            }        }        #endregion    }    /// <summary>    /// The 'Singleton' class    /// </summary>    internal sealed class LoadBalancer    {        // Static members are 'eagerly initialized', that is, immediately when class is loaded for the first time.        // .NET guarantees thread safety for static initialization.        #region Static Fields        /// <summary>        /// The instance.        /// </summary>        private static readonly LoadBalancer Instance = new LoadBalancer();        #endregion        // Type-safe generic list of servers        #region Fields        /// <summary>        /// The random.        /// </summary>        private Random random = new Random();        /// <summary>        /// The servers.        /// </summary>        private List<Server> servers;        #endregion        // Note: constructor is 'private'        #region Constructors and Destructors        /// <summary>        /// Prevents a default instance of the <see cref="LoadBalancer"/> class from being created.        /// </summary>        private LoadBalancer()        {            // Load list of available servers            this.servers = new List<Server>                                            {                                            new Server { Name = "ServerI", IP = "120.14.220.18" },                                            new Server { Name = "ServerII", IP = "120.14.220.19" },                                            new Server { Name = "ServerIII", IP = "120.14.220.20" },                                            new Server { Name = "ServerIV", IP = "120.14.220.21" },                                            new Server { Name = "ServerV", IP = "120.14.220.22" },                                            };        }        #endregion        // Simple, but effective load balancer        #region Public Properties        /// <summary>        /// Gets the next server.        /// </summary>        public Server NextServer        {            get            {                int r = this.random.Next(this.servers.Count);                return this.servers[r];            }        }        #endregion        #region Public Methods and Operators        /// <summary>        /// Get load balancer.        /// </summary>        /// <returns>        /// The <see cref="LoadBalancer"/>.        /// </returns>        public static LoadBalancer GetLoadBalancer()        {            return Instance;        }        #endregion    }    /// <summary>    /// Represents a server machine    /// </summary>    internal class Server    {        #region Public Properties        /// <summary>        /// Gets or sets the ip.        /// </summary>        public string IP { get; set; }        /// <summary>        /// Gets or sets the name.        /// </summary>        public string Name { get; set; }        #endregion    }}// Output:/*Same instanceDispatch request to: ServerVDispatch request to: ServerIIDispatch request to: ServerIIDispatch request to: ServerIIIDispatch request to: ServerIDispatch request to: ServerVDispatch request to: ServerVDispatch request to: ServerVDispatch request to: ServerIDispatch request to: ServerVDispatch request to: ServerIVDispatch request to: ServerIDispatch request to: ServerIIIDispatch request to: ServerIVDispatch request to: ServerIV*/

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