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。