C#知识体系（一） --- 常用的LInq 与lambda表达式

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LinQ是我们常用的技术之一。因为我们绕不开的要对数据进行一系列的调整，如排序、条件筛选、求和、分组、多表联接等等。

lambda则是我们常用的语法糖，配合linq使用天衣无缝，不知不觉就用上了，主要还是看着代码简洁，逼格够高。。。

接下来我们就来看看常用的有哪些

先来点基础数据

//用户类 public class User { [DataMember] public int ID { get; set; } [DataMember] public string Name { get; set; } [DataMember] public string Address { get; set; } [DataMember] public string Phone { get; set; } [DataMember] public string Sex { get; set; } [DataMember] public int Age { get; set; } [DataMember] public int SchID { get; set; } }

/// <summary> /// 学校类 /// </summary> internal class School { public int SchID { get; set; } public string SchName { get; set; } public School(){} public School(int id,string name) { SchID = id; SchName = name; } } /// <summary> /// 公司类 /// </summary> internal class Company { public string Name { get; set; } public List<User> Users { get; set; } public Company() { } public Company(string name, List<User> list) { Name = name; Users = list; } }

接下来初始化点测试数据

User user1 = new User() { ID = , Name = "张一", Address = "上海市某某区", Phone = ", Age = , Sex = "男", SchID= }; User user2 = new User() { ID = , Name = "李小二", Address = "上海市某某区", Phone = ", Age = , Sex = "女", SchID = }; User user3 = new User() { ID = , Name = "张三", Address = "上海市某某区", Phone = ", Age = , Sex = "男", SchID = }; User user4 = new User() { ID = , Name = "李四", Address = "上海市某某区", Phone = ", Age = , Sex = "男", SchID = }; List<User> userlist = ) { user1, user2, user3, user4 };

List<School> Schlist = new List<School>() {
new School(1,"武汉大学"),new School(2,"华中科技大学"),new School(3,"华中师范大学")
};

统计（求和）Sum

//求和 ; }).Sum(a=>a.ID); Console.WriteLine(sum);

//另一种写法 sum =( select a.ID ).Sum(); Console.WriteLine(sum);

IEnumerable.Where()方法里面的参数数委托类型Func<TSource, bool> 前面为参数最后面一个为返回值

求最大最小值 Max Min

//求最大值 var max = userlist.Max(a => a.ID); Console.WriteLine(max); //最小值 var min = userlist.Min(a => a.ID); Console.WriteLine(min);

循环 ForEach

//循环输出 userlist.ForEach(a => { ) { Console.WriteLine(a.ID); } });

条件筛选 where

//筛选 ).Single(); //筛选所有男性用户 var templist = userlist.Where(a => a.Sex == "男").ToList();

排序

//排序根据ID逆序 templist = userlist.OrderByDescending(a => a.ID).ToList(); //升序 templist = userlist.OrderBy(a => a.ID).ToList();

分组 ToLookup 和 GroupBy

//分组 var lookup = userlist.ToLookup(a => a.Sex); foreach (var item in lookup) { Console.WriteLine(item.Key); foreach (var sub in item) { Console.WriteLine("\t\t"+sub.Name+" "+sub.Age); } } //另一种 var dic = userlist.GroupBy(a => a.Sex); foreach (var item in dic) { Console.WriteLine(item.Key); foreach (var sub in item) { Console.WriteLine("\t\t" + sub.Name + " " + sub.Age); } }

联接（这里只展示内联接）Join

//联接 var temp = from usertemp in userlist join sch in Schlist on usertemp.SchID equals sch.SchID select new { Id = usertemp.ID, Name = usertemp.Name, Age = usertemp.Age,Schname=sch.SchName };

返回的匿名类，也可以指点实体类

类型查找 OfType

类型查找很少用到因为我们都是用的泛型，不过这里还是列出来看看

//类型查找 List<,,Name="xx"}}; IEnumerable<string> query1 = objlist.OfType<string>(); foreach (string fruit in query1) { Console.WriteLine(fruit); }

可以看到把2个字符串类型给查找出来了

查找深层嵌套 SelectMany

selectMany很少人用也比较少人知道，它可以避免多层嵌套循环其使用场景还是挺多的，见下代码

//查找深层嵌套 //初始化数据 Company ChinaMobile = new Company("中国移动",userlist); Company ChinaUnicom = new Company("中国联通", userlist); List<Company> companylist = new List<Company>() { ChinaMobile, ChinaUnicom }; //找出2个公司所有女性成员 var selectlist = companylist.SelectMany(a => a.Users).Where(b => b.Sex == "女"); foreach (var item in selectlist) { Console.WriteLine(item.Name+":"+item.Sex); }

这里可以看到给出的数据源都是同一个list集合然后我们看每个集合中只有一个女性成员

正常情况下要想取得这数据的话要经过2层嵌套循环

foreach (Company c in companylist) { foreach (var item in c.Users) { if (item.Sex == "女") { Console.WriteLine(item.Name + ":" + item.Sex); } } }

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