Singleton Design Pattern in .NET

Author : SriHarsha Kalagara (sriharsha.kalagara@gmail.com)

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"The goal of Design Patterns is to address the design problems, developers normally have while designing the applications."

Singleton Design Patters categorized under  Creational Pattern Category. This pattern uses when developers have to create an instance of the class.

Definition:

Singleton Design Pattern allows to create only single instance of the class and provides a global point to access the class instance.

When to use Singleton Design Pattern:

Singleton Design Pattern mainly used, when we have a class and the class properties to be accessed globally through out the application and most of the time these class properties are unique.

In this scenario, normally developer ends with creating multiple instances of the same class in different pages of the application, to get the class property values.

To address the problem of creating multiple instances of the same class, we use Singleton Design Pattern.

Example:

Take a scenario, in an ASP.NET application, developer storing his database connection string and other constant values as properties in the class. Whenever he want the connection string and other constants, he has to instantiate the class each time to access them as properties. 

Using Singleton in this scenario, developers creates a single instance of the class and reuse the same instance where ever he want the class properties.

Steps in Creating Singleton class:

1. Create a class with property variables.
2. Convert the class constructor as private instead of public (default it is public).
3. Create a Static variable of the class (this variable holds class instance for reuse).
4. Create a Static method with in the class, class is the return type  for the static method.(this static method is the global point for accessing the class constructor).
5. In Static method, verify if the class is already instantiated. If the class is already having an instance, then reuse the same instance. if the class doesn't have any instance, then create a new instance of the class and send the instance as return type of the static method.
6. Get the instance of the class using the static method.

A Sample class created using Singleton Design Pattern:

 public class clsSingleTon

    {

        #region Private Variables

        /// <summary>

        /// Static variable holds class instance.

        /// </summary>

        private static clsSingleTon _SingletonInsance = null;

        private string _Server = string.Empty;

        private string _Database = string.Empty;

    

        #endregion

        #region Class Properties

        public string Database

        {

            get { return _Database; }

            set { _Database = value; }

        }

        public string Server

        {

            get { return _Server; }

            set { _Server = value; }

        }

        #endregion

        #region Singleton Constrcutor

        /// <summary>

        /// Convert the constructor of the class as "Private" instead of Public

        /// </summary>

        private clsSingleTon()

        {

        }

        /// <summary>

        /// Static method

        /// Create a Public Static Method, with in this method verify if the class is already instantiated.

        /// If class is instantiated before, return existing instance of the class

        /// or else return new instance of the class

        /// </summary>

        /// <returns></returns>

        public static clsSingleTon GetInstance()

        {

            ///verifying if class is already instantiated

            if (_SingletonInsance == null)

            {

                _SingletonInsance = new clsSingleTon();

            }

            return _SingletonInsance;

        }

        #endregion

    }

 Create the class instance:

Now to access the class properites, we have use the static method of the class as mentioned below.

 clsSingleTon _Instance = clsSingleTon.GetInstance();

This static method always returns the existing instance of the class , if any instance created. or else it will return new instance of the class.