LINQ操作符二：SelectMany-白红宇

LINQ操作符二：SelectMany

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发布时间：2019-06-24

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SelectMany操作符提供了将多个from子句组合起来的功能，相当于数据库中的多表连接查询，它将每个对象的结果合并成单个序列。

示例：

student类：

1 using System; 2 using System.Collections.Generic; 3 using System.Linq; 4 using System.Text; 5 using System.Threading.Tasks; 6  7 namespace SelectMany操作符 8 { 9     /// 10     /// 学生类11     /// 
12     public class Student13     {14         //姓名15         public string Name { get; set; }16         //成绩17         public int Score { get; set; }18         //构造函数19         public Student(string name, int score)20         {21             this.Name = name;22             this.Score = score;23         }24     }25 }

teacher类：

1 using System; 2 using System.Collections.Generic; 3 using System.Linq; 4 using System.Text; 5 using System.Threading.Tasks; 6  7 namespace SelectMany操作符 8 { 9     /// 10     /// Teacher类11     /// 
12     public class Teacher13     {14         //姓名15         public string Name { get; set; }16         //学生集合17         public List
     
       Students { get; set; }18 19         public Teacher(string name, List
      
        students)20         {21             this.Name = name;22             this.Students = students;23         }24     }25 }

Program类

1 using System; 2 using System.Collections.Generic; 3 using System.Linq; 4 using System.Text; 5 using System.Threading.Tasks; 6  7 namespace SelectMany操作符 8 { 9     class Program10     {11         static void Main(string[] args)12         {13             //使用集合初始化器初始化Teacher集合14             List
     
       teachers = new List
      
        { 15                new Teacher("徐老师",16                new List
       
        (){17                  new Student("宋江",80),18                 new Student("卢俊义",95),19                 new Student("朱武",45)20                }21                ),22                 new Teacher("姜老师",23                new List
        
         (){24                  new Student("林冲",90),25                 new Student("花荣",85),26                 new Student("柴进",58)27                }28                ),29                 new Teacher("樊老师",30                new List
         
          (){31                  new Student("关胜",100),32                 new Student("阮小七",70),33                 new Student("时迁",30)34                }35                )36             };37 38             //问题：查询Score小于60的学生39             //方法1：循环遍历、会有性能的损失40             foreach (Teacher t in teachers)41             {42                 foreach (Student s in t.Students)43                 {44                     if (s.Score < 60)45                     {46                         Console.WriteLine("姓名:" + s.Name + ",成绩:"+s.Score);47                     }48                 }49             }50 51             //查询表达式52             //方法2：使用SelectMany  延迟加载：在不需要数据的时候，就不执行调用数据，能减轻程序和数据库的交互，可以提供程序的性能，执行循环的时候才去访问数据库取数据            53             //直接返回学生的数据54             var query = from t in teachers55                         from s in t.Students56                         where s.Score < 6057                         select s;58             foreach (var item in query)59             {60                 Console.WriteLine("姓名:" + item.Name + ",成绩:"+item.Score);61             }62             //只返回老师的数据63             var query1 = from t in teachers64                          from s in t.Students65                          where s.Score < 6066                          select new { 67                             t,68                             teacherName=t.Name,69                             student=t.Students.Where(p=>p.Score<60).ToList()70                          };71             foreach (var item in query1)72             {73                 Console.WriteLine("老师姓名:" + item.teacherName + ",学生姓名:" +item.student.FirstOrDefault().Name+ ",成绩:" + item.student.FirstOrDefault().Score);74             }75             // 使用匿名类 返回老师和学生的数据76             var query2 = from t in teachers77                          from s in t.Students78                          where s.Score < 6079                          select new { teacherName=t.Name, studentName=s.Name,studentScore=s.Score };           80             foreach (var item in query2)81             {82                 Console.WriteLine("老师姓名:" + item.teacherName + ",学生姓名:" + item.studentName + ",成绩:" + item.studentScore);83             }84 85             //使用查询方法86             var query3 = teachers.SelectMany(p => p.Students.Where(t=>t.Score<60).ToList());87             foreach (var item in query3)88             {89                 Console.WriteLine("姓名:" + item.Name + ",成绩:" + item.Score);90             }91             Console.ReadKey();92 93         }94     }95 }