Singleton.md

Singleton

Definition

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

Frequency of use:

Medium high

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UML class diagram

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Participants

    The classes and 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.

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Structural code in C#

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

using System;

namespace DoFactory.GangOfFour.Singleton.Structural
{
    /// <summary>
    /// MainApp startup class for Structural
    /// Singleton Design Pattern.
    /// </summary>
    class MainApp
    {
        /// <summary>
        /// Entry point into console application.
        /// </summary>
        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");
            }

            // Wait for user
            Console.ReadKey();
        }
    }

    /// <summary>
    /// The 'Singleton' class
    /// </summary>
    class Singleton
    {
        private static Singleton _instance;

        // Constructor is 'protected'
        protected Singleton()
        {
        }

        public static Singleton Instance()
        {
            // Uses lazy initialization.
            // Note: this is not thread safe.
            if (_instance == null)
            {
                _instance = new Singleton();
            }

            return _instance;
        }
    }
}

Output

Objects are the same instance