31天重构指南的最后一个重构来自于Fowlers的重构目录，你可以在这里查看。
这里展示了面向对象编程的基础之一“多态性”， 有时你需要检查对象的类型，并根据类型进行一些操作，在这种情况下将算法封装到类中，并利用多态性进行抽象调用是一个好主意。
1: public abstract class Customer
2: {3: }
4:
5: public class Employee : Customer
6: {7: }
8:
9: public class NonEmployee : Customer
10: {11: }
12:
13: public class OrderProcessor
14: {15: public decimal ProcessOrder(Customer customer, IEnumerable<Product> products)
16: {17: // do some processing of order
18: decimal orderTotal = products.Sum(p => p.Price);
19:
20: Type customerType = customer.GetType();
21: if (customerType == typeof(Employee))
22: {23: orderTotal -= orderTotal * 0.15m;
24: }
25: else if (customerType == typeof(NonEmployee))
26: {27: orderTotal -= orderTotal * 0.05m;
28: }
29:
30: return orderTotal;
31: }
32: }
在上面的代码中，我们违反了SRP(Single Responsibility Principle)职责，要想对上面的代码进行得构，我们只需要将百分率移除到特定的customer子类中，
1: public abstract class Customer
2: { 3: public abstract decimal DiscountPercentage { get; }4: }
5:
6: public class Employee : Customer
7: {8: public override decimal DiscountPercentage
9: { 10: get { return 0.15m; }11: }
12: }
13:
14: public class NonEmployee : Customer
15: {16: public override decimal DiscountPercentage
17: { 18: get { return 0.05m; }19: }
20: }
21:
22: public class OrderProcessor
23: {24: public decimal ProcessOrder(Customer customer, IEnumerable<Product> products)
25: {26: // do some processing of order
27: decimal orderTotal = products.Sum(p => p.Price);
28:
29: orderTotal -= orderTotal * customer.DiscountPercentage;
30:
31: return orderTotal;
32: }
33: }
