SelectMany操作符提供了將多個from子句組合起來的功能，相當於數據庫中的多表連接查詢，它將每個對象的結果合並成單個序列。

示例：

 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 Program
10     {
11         static void Main(string
 
[] args)
12         {
13             //使用集合初始化器初始化Teacher集合
14             List<Teacher> teachers = new List<Teacher> { 
15                new Teacher("徐老師",
16                new List<Student>(){
17                  new Student("宋江",80),
18                 new Student("盧俊義",95),
19                 new
 
 Student("朱武",45)
20                }
21                ),
22                 new Teacher("姜老師",
23                new List<Student>(){
24                  new Student("林沖",90),
25                 new Student("花榮",85),
26                 new Student("柴進",58)
27                }
28                ),
 
29                 new Teacher("樊老師",
30                new List<Student>(){
31                  new Student("關勝",100),
32                 new Student("阮小七",70),
33                 new Student("時遷",30)
34                }
35                )
36             };
37 
38             //問題：查詢Score小於60的學生
39             //方法1：循環遍歷、會有性能的損失
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 + ",成績:"+s.Score);
47                     }
48                 }
49             }
50 
51             //查詢表達式
52             //方法2：使用SelectMany  延遲加載：在不需要數據的時候，就不執行調用數據，能減輕程序和數據庫的交互，可以提供程序的性能，執行循環的時候才去訪問數據庫取數據            
53             //直接返回學生的數據
54             var query = from t in teachers
55                         from s in t.Students
56                         where s.Score < 60
57                         select s;
58             foreach (var item in query)
59             {
60                 Console.WriteLine("姓名:" + item.Name + ",成績:"+item.Score);
61             }
62             //只返回老師的數據
63             var query1 = from t in teachers
64                          from s in t.Students
65                          where s.Score < 60
66                          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("老師姓名:" + item.teacherName + ",學生姓名:" +item.student.FirstOrDefault().Name+ ",成績:" + item.student.FirstOrDefault().Score);
74             }
75             // 使用匿名類 返回老師和學生的數據
76             var query2 = from t in teachers
77                          from s in t.Students
78                          where s.Score < 60
79                          select new { teacherName=t.Name, studentName=s.Name,studentScore=s.Score };           
80             foreach (var item in query2)
81             {
82                 Console.WriteLine("老師姓名:" + item.teacherName + ",學生姓名:" + item.studentName + ",成績:" + item.studentScore);
83             }
84 
85             //使用查詢方法
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 + ",成績:" + item.Score);
90             }
91             Console.ReadKey();
92 
93         }
94     }
95 }

LINQ操作符二：SelectMany