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
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
|
using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Threading.Tasks;namespace SelectMany操作符{ /// <summary> /// 学生类 /// </summary> public class Student { //姓名 public string Name { get; set; } //成绩 public int Score { get; set; } //构造函数 public Student(string name, int score) { this.Name = name; this.Score = score; } }} |
teacher类：
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
|
using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Threading.Tasks;namespace SelectMany操作符{ /// <summary> /// Teacher类 /// </summary> public class Teacher { //姓名 public string Name { get; set; } //学生集合 public List<Student> Students { get; set; } public Teacher(string name, List<Student> students) { this.Name = name; this.Students = students; } }} |
Program类
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
|
using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Threading.Tasks;namespace SelectMany操作符{ class Program { static void Main(string[] args) { //使用集合初始化器初始化Teacher集合 List<Teacher> teachers = new List<Teacher> { new Teacher("徐老师", new List<Student>(){ new Student("宋江",80), new Student("卢俊义",95), new Student("朱武",45) } ), new Teacher("姜老师", new List<Student>(){ new Student("林冲",90), new Student("花荣",85), new Student("柴进",58) } ), new Teacher("樊老师", new List<Student>(){ new Student("关胜",100), new Student("阮小七",70), new Student("时迁",30) } ) }; //问题：查询Score小于60的学生 //方法1：循环遍历、会有性能的损失 foreach (Teacher t in teachers) { foreach (Student s in t.Students) { if (s.Score < 60) { Console.WriteLine("姓名:" + s.Name + ",成绩:"+s.Score); } } } //查询表达式 //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 //直接返回学生的数据 var query = from t in teachers from s in t.Students where s.Score < 60 select s; foreach (var item in query) { Console.WriteLine("姓名:" + item.Name + ",成绩:"+item.Score); } //只返回老师的数据 var query1 = from t in teachers from s in t.Students where s.Score < 60 select new { t, teacherName=t.Name, student=t.Students.Where(p=>p.Score<60).ToList() }; foreach (var item in query1) { Console.WriteLine("老师姓名:" + item.teacherName + ",学生姓名:" +item.student.FirstOrDefault().Name+ ",成绩:" + item.student.FirstOrDefault().Score); } // 使用匿名类 返回老师和学生的数据 var query2 = from t in teachers from s in t.Students where s.Score < 60 select new { teacherName=t.Name, studentName=s.Name,studentScore=s.Score }; foreach (var item in query2) { Console.WriteLine("老师姓名:" + item.teacherName + ",学生姓名:" + item.studentName + ",成绩:" + item.studentScore); } //使用查询方法 var query3 = teachers.SelectMany(p => p.Students.Where(t=>t.Score<60).ToList()); foreach (var item in query3) { Console.WriteLine("姓名:" + item.Name + ",成绩:" + item.Score); } Console.ReadKey(); } }} |
到此这篇关于LINQ操作符SelectMany的文章就介绍到这了。希望对大家的学习有所帮助，也希望大家多多支持服务器之家。
原文链接：https://www.cnblogs.com/dotnet261010/p/6849980.html
