聊聊 if else 那些事

从这周起，公司技术部每周五组织一次内部的技术分享，日常工作中，发现大家对if掌握的不是很好，今天先来聊聊if。

一、场景

简历的价格是根据专业的父Id设定的，下面根据简历Id获取简历的价格。

/// <summary>
/// 专业
/// </summary>
public class Subject
{
public int Id { get; set; }
public int Pid { get; set; }
}
/// <summary>
/// 简历
/// </summary>
public class Resume
{
public int Id { get; set; }
public int SubjectId { get; set; }
}

二、if、三元运算符、Null合并运算符

下面的方法用来获取专业实体，如果出现专业数据异常，给一个默认的专业，这里主要为了说明三元运算符和Null合并运算符的用法

public interface ISubjectService
{
Subject GetSubjectById(int id);
}
public interface ISubjectDao
{
Subject GetSubjectById(int id);
}
public class SubjectService : ISubjectService
{
private readonly ISubjectDao _subjectDao;

/// <summary>
/// 1、if判断
/// </summary>
/// <param name="subjectId"></param>
/// <returns></returns>
public Subject GetSubjectById(int subjectId)
{

var subject = _subjectDao.GetSubjectById(subjectId);
if (subject == null)
subject = new Subject() {Id = 1001, Pid = 10};

return subject;
}

/// <summary>
/// 2、三元运算符
/// </summary>
/// <param name="subjectId"></param>
/// <returns></returns>
public Subject GetSubjectById(int subjectId)
{

var subject = _subjectDao.GetSubjectById(subjectId) != null
? _subjectDao.GetSubjectById(subjectId)
: new Subject() { Id = 1001, Pid = 10 };

return subject;
}

/// <summary>
/// 3、null 合并运算符
/// </summary>
/// <param name="subjectId"></param>
/// <returns></returns>
public Subject GetSubjectById(int subjectId)
{
return _subjectDao.GetSubjectById(subjectId) ?? new Subject() { Id = 1001, Pid = 10 };
}
}

三、if else和switch

根据简历Id计算简历价格

public interface IResumeService
{
Resume GetResumeById(int id);
int GetResumePrice(int id);
}
public interface IResumeDao
{
Resume GetResumeById(int id);
}
public class ResumeService
{
private readonly IResumeDao _resumeDao;
private readonly ISubjectService _SubjectService;
/// <summary>
/// 1、if else
/// </summary>
/// <param name="id"></param>
/// <returns></returns>
public int GetResumePrice(int id)
{
var resume = _resumeDao.GetResumeById(id);
var subject = _SubjectService.GetSubjectById(resume.SubjectId);
if (subject.Pid == 1)
{
return 10;
}
else if (subject.Pid == 2)
{
return 20;
}
else if (subject.Pid == 3)
{
return 30;
}

return 10;
}
/// <summary>
/// 2、switch
/// </summary>
/// <param name="id"></param>
/// <returns></returns>
public int GetResumePrice(int id)
{
var resume = _resumeDao.GetResumeById(id);
var subject = _SubjectService.GetSubjectById(resume.SubjectId);
switch (subject.Pid)
{
case 1:
return 10;
case 2:
return 20;
case 3:
return 30;
default:
return 10;
}
}
}

四、if else和[b]卫语句
[/b]

什么是卫语句？

许多程序员觉得函数应该只有一个出口 (return), 结果导致函数中的条件逻辑 (Conditional Logic) 本来完全可以终止下面的代码继续执行 (因为没有必要), 结果却只在函数最后 return, 使人难以看清程序的执行路径.“使用卫语句取代嵌套条件”用来解决这个问题, 它能带给代码可读性的提高, 还有性能上一点点的优化。

public int GetResumePrice(int id)
{
var resume = _resumeDao.GetResumeById(id);
var subject = _SubjectService.GetSubjectById(resume.SubjectId);
if (subject.Pid == 1) return 10;
if (subject.Pid == 2) return 20;
if (subject.Pid == 3) return 30;

return 10;
}

五、if else和策略模式

/// <summary>
/// 抽象一个计算简历价格的接口
/// </summary>
public interface IResumePrice
{
int GetResumePrice();
}
/// <summary>
/// 专业One的简历价格
/// </summary>
public class OneSubjectPrice : IResumePrice
{
public int GetResumePrice()
{
return 10;
}
}
/// <summary>
/// 专业Two的简历价格
/// </summary>
public class TwoSubjectPrice : IResumePrice
{
public int GetResumePrice()
{
return 20;
}
}
/// <summary>
/// 专业Three的简历价格
/// </summary>
public class ThreeSubjectPrice : IResumePrice
{
public int GetResumePrice()
{
return 30;
}
}
public class ResumeService
{
private readonly IResumePrice _resumePrice;
/// <summary>
/// 实例化IResumePrice
/// </summary>
/// <param name="resumePrice"></param>
public ResumeService(IResumePrice resumePrice)
{
_resumePrice = resumePrice;
}
/// <summary>
/// 技术简历价格
/// </summary>
/// <param name="subjectPrice"></param>
/// <returns></returns>
public int GetResumePrice(string subjectPrice)
{
return _resumePrice.GetResumePrice();
}
}

六、if else和状态模式

使用上面定义的ResumePrice的接口和实现，状态模式需要用到反射

public class ResumeService
{
/// <summary>
/// 专业1价格计算类
/// </summary>
public static String OneSubjectPrice = "OneSubjectPrice";
/// <summary>
/// 专业2价格计算类
/// </summary>
public static String TwoSubjectPrice = "TwoSubjectPrice";
/// <summary>
/// 专业3价格计算类
/// </summary>
public static String ThreeSubjectPrice = "ThreeSubjectPrice";

/// <summary>
/// 技术简历价格
/// 利用了反射的伪代码
/// </summary>
/// <param name="subjectPriceType"></param>
/// <returns></returns>
public int GetResumePrice(string subjectPriceType)
{
var dll = Assembly.LoadFrom("assemblyName");
var type = (IResumePrice)dll.GetType(subjectPriceType, true);
return type.GetResumePrice();
}
}

public class Main
{
public void CalPrice()
{
var resumeService = new ResumeService();
var price1 = resumeService.GetResumePrice(ResumeService.OneSubjectPrice);
var price2 = resumeService.GetResumePrice(ResumeService.TwoSubjectPrice);
var price3 = resumeService.GetResumePrice(ResumeService.ThreeSubjectPrice);

}
}