懒汉式(多线程不安全，延迟创建)：

public class Singleton
    {
        private static Singleton instance;

        private Singleton()
        {
        }

        public static Singleton Instance 
        {
            get
            {
                if (instance == null)
                    instance = new Singleton();
                return instance;
            }
        }
    }

饿汉式(多线程不安全，立即创建)

  public class Singleton
    {
        private static Singleton instance = new Singleton();

        private Singleton()
        {
        }

        public static Singleton Instance 
        {
            get => instance;
        }
    }

双重锁定(多线程安全，如果对象已经存在了，就不需要加锁来创建对象)

 public class Singleton
    {
        private static Singleton instance;
        private static readonly object accessLock = new object();

        private Singleton()
        {
        }

        public static Singleton Instance 
        {
            get
            {
                if (instance == null)
                {
                    lock (accessLock)
                    {
                        if (instance == null)
                            instance = new Singleton();
                    }
                }
                return instance;
            }
        }
    }