重构第四天 : 用多态替换条件语句（if else & switch)

面相对象的一个核心基础就是多态，当你要根据对象类型的不同要做不同的操作的时候，一个好的办法就是采用多态，把算法封装到子类当中去。

重构前代码：

 public abstract class Customer
 {
 }

 public class Employee : Customer
 {
 }

 public class NonEmployee : Customer
 {
 }

 public class OrderProcessor
 {
      public decimal ProcessOrder(Customer customer, IEnumerable<Product> products)
      {
          // do some processing of order
          decimal orderTotal = products.Sum(p => p.Price);

          Type customerType = customer.GetType();
          if (customerType == typeof(Employee))
          {
              orderTotal -= orderTotal * 0.15m;
          }
          else if (customerType == typeof(NonEmployee))
          {
              orderTotal -= orderTotal * 0.05m;
          }

          return orderTotal;
      }
 }

重构后的代码：

 public abstract class Customer
   {
       public abstract decimal DiscountPercentage { get; }
   }

   public class Employee : Customer
   {
       public override decimal DiscountPercentage
       {
           get { return 0.15m; }
       }
   }

   public class NonEmployee : Customer
   {
       public override decimal DiscountPercentage
       {
           get { return 0.05m; }
       }
   }

   public class OrderProcessor
   {
       public decimal ProcessOrder(Customer customer, IEnumerable<Product> products)
       {
           // do some processing of order
           decimal orderTotal = products.Sum(p => p.Price);

           orderTotal -= orderTotal * customer.DiscountPercentage;

           return orderTotal;
       }
   }