概念：本文中的“使用策略类” 是指用设计模式中的策略模式来替换原来的switch case和if else语句，这样可以解开耦合，同时也使维护性和系统的可扩展性大大增强。

正文：如下面代码所示，ClientCode 类会更加枚举State的值来调用ShippingInfo 的不同方法，但是这样就会产生很多的判断语句，如果代码量加大，类变得很大了的话，维护中改动也会变得很大，每次改动一个地方，都要对整个结构进行编译（假如是多个工程），所以我们想到了对它进行重构，剥开耦合。

namespace LosTechies.DaysOfRefactoring.SwitchToStrategy.Before{    public class ClientCode    {        public decimal CalculateShipping()        {            ShippingInfo shippingInfo = new ShippingInfo(); return shippingInfo.CalculateShippingAmount(State.Alaska); } } public enum State { Alaska, NewYork, Florida } public class ShippingInfo { public decimal CalculateShippingAmount(State shipToState) { switch (shipToState) { case State.Alaska: return GetAlaskaShippingAmount(); case State.NewYork: return GetNewYorkShippingAmount(); case State.Florida: return GetFloridaShippingAmount(); default: return 0m; } } private decimal GetAlaskaShippingAmount() { return 15m; } private decimal GetNewYorkShippingAmount() { return 10m; } private decimal GetFloridaShippingAmount() { return 3m; } } }

重构后的代码如下所示，抽象出一个IShippingCalculation 接口，然后把ShippingInfo 类里面的GetAlaskaShippingAmount、GetNewYorkShippingAmount、GetFloridaShippingAmount三个方法分别提炼成三个类，然后继承自IShippingCalculation 接口，这样在调用的时候就可以通过IEnumerable<IShippingCalculation> 来解除之前的switch case语句，这和IOC的做法颇为相似。

using System;using System.Collections.Generic;using System.Linq;namespace LosTechies.DaysOfRefactoring.SwitchToStrategy.After_WithIoC{    public interface IShippingInfo { decimal CalculateShippingAmount(State state); } public class ClientCode { [Inject] public IShippingInfo ShippingInfo { get; set; } public decimal CalculateShipping() { return ShippingInfo.CalculateShippingAmount(State.Alaska); } } public enum State { Alaska, NewYork, Florida } public class ShippingInfo : IShippingInfo { private IDictionary
    
      ShippingCalculations { get; set; } public ShippingInfo(IEnumerable
     
       shippingCalculations) { ShippingCalculations = shippingCalculations.ToDictionary(calc => calc.State); } public decimal CalculateShippingAmount(State shipToState) { return ShippingCalculations[shipToState].Calculate(); } } public interface IShippingCalculation { State State { get; } decimal Calculate(); } public class AlaskShippingCalculation : IShippingCalculation { public State State { get { return State.Alaska; } } public decimal Calculate() { return 15m; } } public class NewYorkShippingCalculation : IShippingCalculation { public State State { get { return State.NewYork; } } public decimal Calculate() { return 10m; } } public class FloridaShippingCalculation : IShippingCalculation { public State State { get { return State.Florida; } } public decimal Calculate() { return 3m; } } }