Code
    #region 接口

    /// <summary>
    /// 购房者
    /// </summary>
    public interface ICustomer
    {
        //找中介
        void FindProxy(IProxy proxy);
        //看房
        void LookHouse();
        //买房
        void BuyHouse(string id,int money);
    }

    /// <summary>
    /// 房产中介
    /// </summary>
    public interface IProxy
    {
        //代理房产商
        void SetLandAgent(ILandAgent landAgent);
        //提供房产信息
        void ShowHouse();
        void SellHouse(string id,int money);
    }

    /// <summary>
    /// 房产商
    /// </summary>
    public interface ILandAgent
    {
        //添加可售房源
        void AddHouse(IHouse house);
        //显示房产信息
        void ShowInfo();
        void SoldHouse(string id,int money);
    }

    /// <summary>
    /// 房子
    /// </summary>
    public interface IHouse
    {
        //房子信息
        void GetInfo();
        string Property { get; set; }
    }


    #endregion


    /*
     * 在这里房子的代理是房产商
     * 房产商的代理是中介
     * 买家直接找中介买房子,来改变房子的拥有权
     * 
     */ 
    #region 类

    public class House : IHouse
    {
        private string info;
        private string property;

        public House(string info,string id)
        {
            this.info = info;
            Property = id;

        }
        #region IHouse Members

        public void GetInfo()
        {
            Console.WriteLine(string.Format("房产信息:{0},房产ID:{1}", info,Property));
        }

        public string Property
        {
            get{ return property; }
            set{ property = value; }
        }

        #endregion
    }

    public class LandAgent : ILandAgent
    {
        private List<IHouse> houses;

        public LandAgent()
        {
            houses = new List<IHouse>();
        }

        #region ILandAgent Members

        public void AddHouse(IHouse house)
        {
            houses.Add(house);
        }

        public void ShowInfo()
        {
            foreach (IHouse house in houses)
            {
                house.GetInfo();
            }
                
        }

        public void SoldHouse(string id, int money)
        {
            IHouse soldHouse=new House("","");
            foreach (IHouse house in houses)
            {
                if (house.Property == id)
                {
                    soldHouse = house;
                    break;
                }
            }
            if (soldHouse.Property != "")
            {
                houses.Remove(soldHouse);
                Console.WriteLine(string.Format("我卖出了房子{0}通过中介获得了购房款{1}", id, money));
                soldHouse.Property = "已售出";
                soldHouse.GetInfo();
            }
            else
                Console.WriteLine("没有找到这房子 @_@");
        }

        #endregion
    }

    public class Proxy : IProxy
    {
        ILandAgent landAgent;

        #region IProxy Members

        public void SetLandAgent(ILandAgent landAgent)
        {
            if (landAgent != null)
                this.landAgent = landAgent;
        }

        public void ShowHouse()
        {
            if (landAgent != null)
                landAgent.ShowInfo();
            else
                Console.WriteLine("我还没获得代理房产商的代理权呢");
        }

        public void SellHouse(string id, int money)
        {
            Console.WriteLine(string.Format("我卖出了房子:{0},获得了5%的中介费:{1}", id, money / 20));
            landAgent.SoldHouse(id, money / 20 * 19);
        }

        #endregion
    }

    public class Customer : ICustomer
    {
        private IProxy proxy;

        #region ICustomer Members

        public void FindProxy(IProxy proxy)
        {
            if (proxy != null)
                this.proxy = proxy;
        }

        public void LookHouse()
        {
            proxy.ShowHouse();
        }

        public void BuyHouse(string id,int money)
        {
            proxy.SellHouse(id, money);
        }

        #endregion
    }

    #endregion

    class Program
    {

        static void Main(string[] args)
        {
            ICustomer customer = new Customer();
            IProxy zhongyuandichan = new Proxy();
            ILandAgent wanke = new LandAgent();

            IHouse jinsewanli = new House("万科金色万里1楼101室,230平米超大私人空间,售价200万", "001");
            IHouse huayuanxiaocheng = new House("万科花园小城13楼602室,104平米二室,售价120万", "002");
            IHouse wankejinse = new House("万科金色万里6楼603室,98平米一室户,售价98万", "003");
            IHouse wankeyangguang = new House("万科阳光10楼1008室,156平米三室两厅,售价132万", "004");

            //房子通过房产商来改变拥有属性
            wanke.AddHouse(jinsewanli);
            wanke.AddHouse(huayuanxiaocheng);
            wanke.AddHouse(wankejinse);
            wanke.AddHouse(wankeyangguang);
            //房子卖不出去了,房产商只好通过销售外包的方式销售房子
            zhongyuandichan.SetLandAgent(wanke);
            //顾客找到了中介
            customer.FindProxy(zhongyuandichan);
            //顾客寻求房产信息
            customer.LookHouse();
            //顾客看中了房子,通过中介对房子的拥有权进行了修改
            customer.BuyHouse("003", 980000);
            //顾客再次看房
            customer.LookHouse();
            Console.ReadKey();
        }
    }