C#知识体系（一） --- 常用的LInq 与lambda表达式

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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