using MoreLinq;

using System;

using System.Collections.Generic;

using System.IO;

using System.Linq;

using Xunit;

​

namespace Creational.Singleton

{

    public interface IDatabase

    {

        int GetPopulation(string cityName);

    }

​

    public class SingletonDatabase : IDatabase

    {

        private Dictionary<string, int> capitals;

​

        private SingletonDatabase()

        {

            // init database

            capitals = File.ReadAllLines("capitals.txt").Batch(2)

                .ToDictionary(l => l.ElementAt(0).Trim(), l => int.Parse(l.ElementAt(1).Trim()));

        }

​

        public int GetPopulation(string cityName)

        {

            return capitals[cityName];

        }

​

        private static Lazy<SingletonDatabase> instance =

            new Lazy<SingletonDatabase>( () => new SingletonDatabase());

        public static SingletonDatabase Instance => instance.Value;

    }

​

    class Program

    {

        static void Main(string[] args)

        {

            var db = SingletonDatabase.Instance;

            db.GetPopulation("Tokyo");

        }

    }

​

    public class SingletonTest

    {

        [Fact]

        public void IsSingletonTest()

        {

            var db = SingletonDatabase.Instance;

            var db2 = SingletonDatabase.Instance;

​

            Assert.Equal(db, db2);

        }

    }

}

​