.NET Interview Questions by Abhishek Goenka - HTML preview

Singleton

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

/// <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 Object _Lock = new Object();

        private static Singleton _instance;

        // Constructor is 'protected'

        protected Singleton()

        {

        }

        public static Singleton Instance()

        {

            // Uses lazy initialization.

            // Note: this not thread safe.

            if (_instance == null)

            {

                lock (_Lock)

                {

                    if(_instance == null)

                        _instance = new Singleton();

                }

            }

            return _instance;

        }

    }