再谈单例模式

class1:

using System;
using System.Collections.Generic;
using System.Text;

namespace WindowsApplication1
{
    public abstract class Class1
    {
        private string owner;
        private string name;
        private static   Class1 _instance = null;

        protected static  Class1 Instance
        {
            get
            {
                //if (Instance == null)
                //Instance = new aaa(location, name);
                //return Instance;
                return _instance;
            }
            set
            {
                _instance = value;
            }
        }

        //public static Class1 CreateInstance(string location, string name)
        //{
        //    if (Instance == null)
        //        Instance = new aaa(location, name);
        //    return Instance;
        //}

        public string Owner
        {
            get { return owner; }
            set { owner = value; }
        }

        public string Name       
        {
            get{ return this.name;  }
            set{ this.name = value; }
        }

        /// <summary>
        ///
        /// </summary>
        /// <param name="owner"></param>
        public Class1(string owner)
        {
            this.owner = owner;
        }

        public abstract string GetYear();

    }

    public class aaa : Class1
    {
        private string location;

        public string Location
        {
            get { return location; }
            set { location = value; }
        }

        public static Class1 CreateInstance(string location, string name)
        {
            if (Instance == null)
                Instance = new aaa(location, name);
            return Instance;
        }
       
        private aaa(string location,string owner):base(owner)
        {
            this.location = location;
        }

        public override string GetYear()
        {
            return DateTime.Now.Year.ToString();
        }
    }

}

class2:

using System;
using System.Collections.Generic;
using System.Text;

namespace WindowsApplication1
{
    public abstract class BaseClass
    {
        public static BaseClass instance = new Subclass();

        //protected BaseClass() { }

        public abstract string GetYear();
    }

    public class Subclass : BaseClass
    {

        public static void main(string[] args)
        {
            string str = instance.GetYear();
            Console.WriteLine(str);
        }

        public override string GetYear()
        {
            return System.DateTime.Now.ToString();
        }

    }

}