C# 单例模式的多种实现方式

        /// <summary>
        /// 饿汉式单例模式
        /// </summary>
        public class SingletonEager
        {
            private SingletonEager() { }
            private static readonly SingletonEager _instance = new SingletonEager();
            public static SingletonEager Instance
            {
                get { return _instance; }
            }
            public void DoSomething()
            {
                Console.WriteLine("饿汉式单例模式.");
            }
        }

        /// <summary>
        /// 懒汉式单例模式
        /// </summary>
        public class SingletonLazy
        {
            private SingletonLazy() { }
            private static SingletonLazy? _instance;
            private static readonly object _lockObj = new object();
            public static SingletonLazy Instance
            {
                get
                {
                    if (_instance == null)
                    {
                        lock (_lockObj)
                        {
                            if (_instance == null)
                            {
                                _instance = new SingletonLazy();
                            }
                        }
                    }
                    return _instance;
                }
            }
            public void DoSomething()
            {
                Console.WriteLine("懒汉式单例模式.");
            }
        }

        /// <summary>
        /// 懒加载单例模式
        /// </summary>
        public sealed class SingletonByLazy
        {
            private static readonly Lazy<SingletonByLazy> _lazy = new Lazy<SingletonByLazy>(() => new SingletonByLazy());
            public static SingletonByLazy Instance { get { return _lazy.Value; } }
            private SingletonByLazy() { }
            public void DoSomething()
            {
                Console.WriteLine("懒加载单例模式.");
            }
        }