装饰模式（Decorator Pattern）

      装饰模式通过采用组合，而非继承的手法，实现了在运行时动态地扩展对象功能的能力。
      下例通过饮料店中饮料和调料的关系模拟了装饰模式的用法，其中结构图与类对应关系是：
      Beverage = Component
      Espresso(HouseBlend) = ConcreteComponent
      CondimentDecorator = Decorator
      Sugar(Milk/Mocha) = ConcreteDecorator
      

    public class CoffeeStore
    {
        public static void Main()
        {
            Beverage beverage = new Espresso();
            //订了一杯Espresso
            Console.WriteLine("Coffee name:{0} Cost:{1}", beverage.GetDescription(), beverage.Cost());

            //订了一杯HouseBlend，加Sugar，加Milk
            Beverage beverage2 = new HouseBlend();
            beverage2 = new Sugar(beverage2);
            beverage2 = new Milk(beverage2);
            Console.WriteLine("Coffee name:{0} Cost:{1}", beverage2.GetDescription(), beverage2.Cost());

            Console.ReadLine();
        }
    }

    public abstract class Beverage
    {
        protected string description = "unknown";

        public virtual string GetDescription()
        {
            return description;
        }

        public abstract double Cost();
    }

    #region 咖啡
    public class Espresso : Beverage
    {
        public Espresso()
        {
            this.description = "Espresso";
        }

        public override double Cost()
        {
            return 9.9;
        }
    }

    public class HouseBlend : Beverage
    {
        public HouseBlend()
        {
            this.description = "HouseBlend";
        }

        public override double Cost()
        {
            return 6.8;
        }
    }
    #endregion

    #region 调味
    public abstract class CondimentDecorator : Beverage
    {
        public abstract override string GetDescription();
    }

    public class Sugar : CondimentDecorator
    {
        Beverage beverage;
        public Sugar(Beverage beverage)
        {
            this.beverage = beverage;
        }

        public override string GetDescription()
        {
            return beverage.GetDescription() + ",Sugar";
        }

        public override double Cost()
        {
            return beverage.Cost() + 2d;
        }
    }

    public class Milk : CondimentDecorator
    {
        Beverage beverage;
        public Milk(Beverage beverage)
        {
            this.beverage = beverage;
        }

        public override string GetDescription()
        {
            return beverage.GetDescription() + ",Milk";
        }

        public override double Cost()
        {
            return beverage.Cost() + 2.5d;
        }
    }

    public class Mocha : CondimentDecorator
    {
        Beverage beverage;
        public Mocha(Beverage beverage)
        {
            this.beverage = beverage;
        }

        public override string GetDescription()
        {
            return beverage.GetDescription() + ",Mocha";
        }

        public override double Cost()
        {
            return beverage.Cost() + 3.5d;
        }
    }
    #endregion