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：
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：
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
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编程网。
