様々なシングルトンの実装

SingletonTest.cs

using System;

namespace Wandbox
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(Foo.Instance.Name);
            Console.WriteLine(Bar.Instance.Name);
            Console.WriteLine(Baz.Instance.Name);
            Console.WriteLine(Qux.Instance.Name);
        }
    }


    /// <summary>
    /// First implementation of singleton.
    /// </summary>
    public sealed class Foo
    {
        /// <summary>
        /// Get the only instance of <see cref="Foo"/> class.
        /// </summary>
        public static Foo Instance { get; }

        /// <summary>
        /// Create the only instance of <see cref="Foo"/> class.
        /// </summary>
        static Foo()
        {
            Instance = new Foo();
        }

        /// <summary>
        /// Name of this instance.
        /// </summary>
        public string Name { get; }

        /// <summary>
        /// Initializes a new instance of <see cref="Foo"/> class.
        /// </summary>
        private Foo()
        {
            Name = nameof(Foo);
        }
    }  // class Hoge


    /// <summary>
    /// Second test implementation of singleton.
    /// </summary>
    public sealed class Bar
    {
        /// <summary>
        /// The only instance of <see cref="Bar"/> class.
        /// </summary>
        private static Bar _instance;

        /// <summary>
        /// Get the only instance of <see cref="Bar"/> class.
        /// </summary>
        public static Bar Instance => _instance ??= new Bar();

        /// <summary>
        /// Name of this instance.
        /// </summary>
        public string Name { get; }

        /// <summary>
        /// Initializes a new instance of <see cref="Bar"/> class.
        /// </summary>
        private Bar()
        {
            Name = nameof(Bar);
        }
    }  // class Fuga


    /// <summary>
    /// Third implementation of singleton.
    /// </summary>
    public sealed class Baz : SingletonLazy<Baz>
    {
        /// <summary>
        /// Name of this instance.
        /// </summary>
        public string Name { get; set; }

        /// <summary>
        /// Initializes a new instance of <see cref="Baz"/> class.
        /// </summary>
        private Baz()
        {
            Name = nameof(Baz);
        }
    }  //  class Baz


    /// <summary>
    /// Fourth implementation of singleton.
    /// </summary>
    public sealed class Qux : SingletonLazy<Baz>
    {
        /// <summary>
        /// Name of this instance.
        /// </summary>
        public string Name { get; set; }

        /// <summary>
        /// Initializes a new instance of <see cref="Qux"/> class.
        /// </summary>
        private Qux()
        {
            Name = nameof(Qux);
        }
    }  //  class Qux


    /// <summary>
    /// Generic implementation of singleton class.
    /// </summary>
    /// <example>
    /// <code>
    /// public class Foo : Singleton&lt;Foo&gt;
    /// {
    ///     public string Name { get; set; } = "Foo";
    /// }
    /// </code>
    /// </example>
    /// <typeparam name="T">Type of actual singleton class.</typeparam>
    public abstract class Singleton<T>
    where T : class
    {
        /// <summary>
        /// Get the only instance of <typeparamref name="T"/> class.
        /// </summary>
        public static T Instance { get; }

        /// <summary>
        /// Create the only instance of <typeparamref name="T"/> class.
        /// </summary>
        static Singleton()
        {
            Instance = (T)Activator.CreateInstance(typeof(T), true);
        }
    }  // class Singleton<T>


    /// <summary>
    /// Generic implementation of singleton class.
    /// </summary>
    /// <example>
    /// <code>
    /// public class Foo : SingletonLazy&lt;Foo&gt;
    /// {
    ///     public string Name { get; set; } = "Foo";
    /// }
    /// </code>
    /// </example>
    /// <typeparam name="T">Type of actual singleton class.</typeparam>
    public abstract class SingletonLazy<T>
        where T : class
    {
        /// <summary>
        /// The only instance of <typeparamref name="T"/> class.
        /// </summary>
        private static T _instance;

        /// <summary>
        /// Get the singleton instance of <typeparamref name="T"/> class.
        /// </summary>
        public static T Instance => _instance ??= (T)Activator.CreateInstance(typeof(T), true);
    }  // class SingletonLazy<T>
}