Category: .NET

March 29, 2013

Design Patterns : Singleton

A Singleton, in software engineering, is a design pattern that is used when you only want/need to allow one instance of a class. This is specifically useful when the class instantiation of the said class is resource expensive (e.g. database connection, graphics driver, network resource and file streams…).

Below is a UML describing the implementation details of a Singleton.

Here is an example of a Thread-safe Singleton implemented in C#. In this example, we are trying to simulate a DbConnection object. The DbConnection object has a private constructor that blocks users of this class from ‘new-ing’ this object. Instead, the DbConnection object has a property called Instance that allows users creates a copy of DbConnection.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace DesignPatternsWorkbook.SingletonPattern
{
    public class Program
    {
        static void Main(string[] args)
        {
            // DbConnection object gets created on first call
            var connection1 = DbConnection.Instance;
            connection1.ConnectionTimeout = 100;

            // Create another instance to show that our Singleton is working
            var connection2 = DbConnection.Instance;

            //Compare both bobjects and see if they are equal
            if (connection1 == connection2)
                Console.WriteLine("connection1 and connection2 are the same instance\n");

            //We've only set the ConnectionTime property on connection1. Let's check if it propagate to connection2
            Console.WriteLine("Both should have the same connection timeout value.");
            Console.WriteLine(String.Format("connection1.ConnectionTimeout: {0}", connection1.ConnectionTimeout));
            Console.WriteLine(String.Format("connection2.ConnectionTimeout: {0}", connection2.ConnectionTimeout));
            Console.WriteLine("--------------\n");

            //try to do a connection using connection1
            Console.WriteLine("connection1.Connect() is called....");
            connection1.Connect();
            Console.WriteLine("--------------\n");

            //try to do a connection using connection2 and see what happens
            Console.WriteLine("connection2.Connect() is called....");
            connection2.Connect();
            Console.WriteLine("--------------\n");

            //try to disconnect using connection2
            Console.WriteLine("connection2.Disconnect() is called....");
            connection2.Disconnect();
            Console.WriteLine("--------------\n");

            //try to diconnect using connection1 and see what happens
            Console.WriteLine("connection1.Disconnect() is called....");
            connection1.Disconnect();
            Console.WriteLine("--------------\n");

            Console.ReadLine();
        }
    }

    /// <summary>
    /// Our Singleton class implementation
    /// </summary>
    public class DbConnection
    {
        // Make the constructor Private to prevent 'new'
        private DbConnection() {}

        // The instance read only property
        public static DbConnection Instance
        {
            get { return DbConnectionInstance.ConnectionInstance; }
        }

        /// <summary>
        /// A sample property
        /// </summary>
        public int ConnectionTimeout { get; set; }

        /// <summary>
        /// A sample property used to hold or Connection status
        /// </summary>
        public bool Connected { get; private set; }

        /// <summary>
        /// A sample method to simulate a connection
        /// </summary>
        public void Connect()
        {
            //check if the object is currently connected
            if (Connected)
                Console.WriteLine("Can't Connect. There is currently an active connection.");
            else
            {
                this.Connected = true;
                Console.WriteLine("You are now connected.");
            }
        }

        /// <summary>
        /// A sample method to simulate a disconnection
        /// </summary>
        public void Disconnect()
        {
            //check if the object is currently connected
            if (Connected)
            {
                this.Connected = false;
                Console.WriteLine("You are now disconnected!");
            }
            else
                Console.WriteLine("There is no active connection to disconnect");
        }

        // The nested class with the actual DbConnection object
        private class DbConnectionInstance
        {
            //our connection instance
            internal static readonly DbConnection ConnectionInstance = new DbConnection();

            static DbConnectionInstance() {}
        }
    }

}

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using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

namespace DesignPatternsWorkbook.SingletonPattern

{

public class Program

{

static void Main(string[] args)

{

// DbConnection object gets created on first call

var connection1 = DbConnection.Instance;

connection1.ConnectionTimeout = 100;

// Create another instance to show that our Singleton is working

var connection2 = DbConnection.Instance;

//Compare both bobjects and see if they are equal

if (connection1 == connection2)

Console.WriteLine("connection1 and connection2 are the same instance\n");

//We've only set the ConnectionTime property on connection1. Let's check if it propagate to connection2

Console.WriteLine("Both should have the same connection timeout value.");

Console.WriteLine(String.Format("connection1.ConnectionTimeout: {0}", connection1.ConnectionTimeout));

Console.WriteLine(String.Format("connection2.ConnectionTimeout: {0}", connection2.ConnectionTimeout));