Singleton Design Pattern

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Part 1 - Introduction to Design Patters - Text - Slides

In this tutorial we will discuss
1. What is Singleton Design Pattern
2. Singleton as Creational Pattern
3. Implementation Guidelines
4. How do we implement a Singleton class


This is continuation to Part 1 of Design Patterns Tutorial. So please watch Part 1 before proceeding.

Singleton Pattern belongs to Creational type pattern. As discussed in our previous video, Gang of four have defined five design patterns that belongs to creational design type category. Singleton is one among them and the rest are Factory, Abstract Factory, Builder and Prototype patterns. As the name implies, creational design type deals with object creation mechanisms. Basically, to simplify this, creational pattern explain us the creation of objects in a manner suitable to a given situation.


Singleton design pattern is used when we need to ensure that only one object of a particular class is Instantiated. That single instance created is responsible to coordinate actions across the application.



If you look at the illustrated diagram above you will see different objects trying to invoke an object instantiated as singleton. This single instance of the object is responsible to invoke underneath methods or events.

Advantages and Guidelines for Singleton implementation.

Concurrent access to the resource is well managed by singleton design pattern.

As part of the Implementation guidelines we need to ensure that only one instance of the class exists by declaring all constructors of the class to be private.  Also, to control the singleton access we need to provide a static property that returns a single instance of the object.

Singleton Class Implementation Example

Program.cs

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

/// <summary>

/// First version of Singleton demo

/// </summary>

namespace SingletonDemo

{

    class Program

    {

        static void Main(string[] args)

        {

            /*

             * Assuming Singleton is created from employee class

             * we refer to the GetInstance property from the Singleton class

             */

            Singleton fromEmployee = Singleton.GetInstance;

            fromEmployee.PrintDetails("From Employee");

            /*

             * Assuming Singleton is created from student class

             * we refer to the GetInstance property from the Singleton class

             */

            Singleton fromStudent = Singleton.GetInstance;

            fromStudent.PrintDetails("From Student");

            Console.ReadLine();

        }

    }

}

Singleton.cs

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

/// <summary>

/// First Singleton version

/// </summary>

namespace SingletonDemo

{

    /*

     *  Sealed ensures the class being inherited and

     *  object instantiation is restricted in the derived class

     */

    public sealed class Singleton

    {

        private static int counter = 0;

        /*

         * Private property initilized with null

         * ensures that only one instance of the object is created

         * based on the null condition

         */

        private static Singleton instance = null;

       

        /*

         * public property is used to return only one instance of the class

         * leveraging on the private property

         */

        public static Singleton GetInstance

        {

            get

            {

                if (instance == null)

                    instance = new Singleton();

                return instance;

            }

        }

        /*

         * Private constructor ensures that object is not

         * instantiated other than with in the class itself

         */

        private Singleton()

        {

            counter++;

            Console.WriteLine("Counter Value " + counter.ToString());

        }

        /*

         * Public method which can be invoked through the singleton instance

         */

        public void PrintDetails(string message)

        {

            Console.WriteLine(message);

        }

    }

}