单例模式

 public class Singleton
    {
        private static Singleton instance;

        private Singleton()
        {

        }

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

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

        private Singleton()
        {

        }

        public static Singleton GetInstance()
        {
            if (instance == null)
            {
                lock (_lock)
                {
                    if (instance == null)
                    {
                        instance = new Singleton();
                    }
                }
            }
            return instance;
        }
    }

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

        private Singleton()
        {
        }

        public static Singleton GetInstance()
        {
            return instance;
        }
    }