CSharp: Singleton Pattern in donet core 3

 

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/// <summary>    /// 单例模式 Singleton Pattern    /// geovindu,Geovin Du edit    /// </summary>    public class Database    {          /// <summary>        ///        /// </summary>        private Database()        {        }          /// <summary>        ///        /// </summary>        public static Database Instance { get; } = new Database();    }    /// <summary>    ///    /// </summary>    public class MyDatabase    {          /// <summary>        ///        /// </summary>        private MyDatabase()        {            Console.WriteLine("Initializing database");        }          /// <summary>        ///        /// </summary>        private static Lazy<MyDatabase> instance =          new Lazy<MyDatabase>(() => new MyDatabase());        /// <summary>        ///        /// </summary>        public static MyDatabase Instance => instance.Value;    }    /// <summary>    ///    /// </summary>    public interface IDatabase    {        int GetPopulation(string name);    }        /// <summary>    ///    /// </summary>    public class SingletonDatabase : IDatabase    {          /// <summary>        ///        /// </summary>        private Dictionary<string, int> capitals;          /// <summary>        ///        /// </summary>        private static int instanceCount;          /// <summary>        ///        /// </summary>        public static int Count => instanceCount;        /// <summary>        ///        /// </summary>        private SingletonDatabase()        {            Console.WriteLine("Initializing database");              capitals = File.ReadAllLines(              Path.Combine(                new FileInfo(typeof(IDatabase).Assembly.Location)                  .DirectoryName,                "capitals.txt")              )              .Batch(2)              .ToDictionary(                list => list.ElementAt(0).Trim(),                list => int.Parse(list.ElementAt(1)));        }        /// <summary>        ///        /// </summary>        /// <param name="name"></param>        /// <returns></returns>        public int GetPopulation(string name)        {            return capitals[name];        }          /// <summary>        /// laziness + thread safety        /// </summary>        private static readonly Lazy<SingletonDatabase> instance          = new Lazy<SingletonDatabase>(() =>          {              instanceCount++;              return new SingletonDatabase();          });          public static IDatabase Instance => instance.Value;    }      /// <summary>    ///    /// </summary>    public class SingletonRecordFinder    {          /// <summary>        ///        /// </summary>        /// <param name="names"></param>        /// <returns></returns>        public int TotalPopulation(IEnumerable<string> names)        {            return names.Sum(name => SingletonDatabase.Instance.GetPopulation(name));        }    }    /// <summary>    ///    /// </summary>    public class ConfigurableRecordFinder    {          /// <summary>        ///        /// </summary>        private IDatabase database;        /// <summary>        ///        /// </summary>        /// <param name="database"></param>        public ConfigurableRecordFinder(IDatabase database)        {            this.database = database;        }        /// <summary>        ///        /// </summary>        /// <param name="names"></param>        /// <returns></returns>        public int GetTotalPopulation(IEnumerable<string> names)        {            return names.Sum(name => database.GetPopulation(name));        }    }    /// <summary>    ///    /// </summary>    public class DummyDatabase : IDatabase    {          /// <summary>        ///        /// </summary>        /// <param name="name"></param>        /// <returns></returns>        public int GetPopulation(string name)        {            return new Dictionary<string, int>            {                ["alpha"] = 1,                ["beta"] = 2,                ["gamma"] = 3            }[name];        }    }    /// <summary>    ///    /// </summary>    public class OrdinaryDatabase : IDatabase    {          /// <summary>        ///        /// </summary>        private readonly Dictionary<string, int> cities;        /// <summary>        ///        /// </summary>        public OrdinaryDatabase()        {            Console.WriteLine("Initializing database");              cities = File.ReadAllLines(                Path.Combine(                  new FileInfo(typeof(IDatabase).Assembly.Location)                    .DirectoryName,                  "capitals.txt")              )              .Batch(2)              .ToDictionary(                list => list.ElementAt(0).Trim(),                list => int.Parse(list.ElementAt(1)));        }        /// <summary>        ///        /// </summary>        /// <param name="name"></param>        /// <returns></returns>        public int GetPopulation(string name)        {            return cities[name];        }    }

　　

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/// <summary>    /// 单例模式 Singleton Pattern    /// geovindu,Geovin Du edit    /// </summary>    public sealed class Singleton    {        #region Singleton implementation using static constructor        //The following line is discussed in analysis section.        //private static readonly Singleton Instance = new Singleton();        private static readonly Singleton Instance;                 private static int TotalInstances;        /*         * Private constructor is used to prevent         * creation of instances with 'new' keyword         * outside this class.        */        /// <summary>        ///        /// </summary>        private Singleton()        {            Console.WriteLine("--Private constructor is called.");            Console.WriteLine("--Exit now from private constructor.");                   }          /*         * A static constructor is used  for the following purposes:         * 1. To initialize any static data;         * 2. To perform a specific action only once.         *         * The static constructor will be called automatically before         *         * i. You create the first instance; or         * ii.You refer to any static members in your code.         *         */          /// <summary>        /// Here is the static constructor        /// </summary>        static Singleton()        {            // Printing some messages before you create the instance            Console.WriteLine("-Static constructor is called.");            Instance = new Singleton();            TotalInstances++;            Console.WriteLine($"-Singleton instance is created.Number of instances:{ TotalInstances}");            Console.WriteLine("-Exit from static constructor.");                   }        /// <summary>        ///        /// </summary>        public static Singleton GetInstance        {            get            {                return Instance;            }        }                 /*         * If you like to use expression-bodied read-only         * property, you can use the following line (C# v6.0 onwards).         */        //public static Singleton GetInstance => Instance;        #endregion        /* The following line is used to discuss        the drawback of the approach. */        /// <summary>        ///        /// </summary>        public static int MyInt = 25;    }

　　

调用：

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Console.WriteLine("***单例模式 Singleton Pattern Demonstration.***\n");             /*The following line is used to discuss               the drawback of the approach.*/             //Console.WriteLine($"The value of MyInt is :{Singleton.MyInt}");             // Private Constructor.So,you cannot use the 'new' keyword.              //Singleton s = new Singleton(); // error            Console.WriteLine("Trying to get a Singleton instance, called firstInstance.");             Singleton firstInstance = Singleton.GetInstance;             Console.WriteLine("Trying to get another Singleton instance, called secondInstance.");             Singleton secondInstance = Singleton.GetInstance;             if (firstInstance.Equals(secondInstance))             {                 Console.WriteLine("The firstInstance and secondInstance are the same.");             }             else             {                 Console.WriteLine("Different instances exist.");             }               //             var rf = new SingletonRecordFinder();             var names = new[] { "Seoul", "Mexico City" };             var ci = names[0].ToString();             int tp = rf.TotalPopulation(names);             Console.WriteLine(tp);             var db = new DummyDatabase();             var rf2 = new ConfigurableRecordFinder(db);            int t= rf2.GetTotalPopulation(new[] { "alpha", "gamma" }); ;             Console.WriteLine(t);             db.GetPopulation("gamma");                          var db2 = SingletonDatabase.Instance;               // works just fine while you're working with a real database.             var city = "Tokyo";             Console.WriteLine($"{city} has population {db2.GetPopulation(city)}");             Console.WriteLine($"{names[0]} has population {db2.GetPopulation(names[0])}");             Console.Read();

　　

输出：

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***单例模式 Singleton Pattern Demonstration.***   Trying to get a Singleton instance, called firstInstance. -Static constructor is called. --Private constructor is called. --Exit now from private constructor. -Singleton instance is created.Number of instances:1 -Exit from static constructor. Trying to get another Singleton instance, called secondInstance. The firstInstance and secondInstance are the same. Initializing database 34900000 4 Tokyo has population 33200000 Seoul has population 17500000