重构指南 - 使用多态代替条件判断(Replace conditional with Polymorphism)

多态(polymorphism)是面向对象的重要特性,简单可理解为：一个接口，多种实现。

当你的代码中存在通过不同的类型执行不同的操作，包含大量if else或者switch语句时，就可以考虑进行重构，将方法封装到类中，并通过多态进行调用。

代码重构前：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using LosTechies.DaysOfRefactoring.SampleCode.BreakMethod.After;

namespace LosTechies.DaysOfRefactoring.SampleCode.ReplaceWithPolymorphism.Before
{
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;
}
}
}

代码重构后：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using LosTechies.DaysOfRefactoring.SampleCode.BreakMethod.After;

namespace LosTechies.DaysOfRefactoring.SampleCode.ReplaceWithPolymorphism.After
{
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;
}
}
}

重构后的代码，将变化点封装在了子类中，代码的可读性和可扩展性大大提高。