第LINQ操作符SelectMany的用法SelectMany操作符提供了將多個(gè)from子句組合起來的功能，相當(dāng)于數(shù)據(jù)庫中的多表連接查詢，它將每個(gè)對象的結(jié)果合并成單個(gè)序列。

示例：

student類：

usingSystem;

usingSystem.Collections.Generic;

usingSystem.Linq;

usingSystem.Text;

usingSystem.Threading.Tasks;

namespaceSelectMany操作符

///summary

///學(xué)生類

////summary

publicclassStudent

//姓名

publicstringName{get;set;}

//成績

publicintScore{get;set;}

//構(gòu)造函數(shù)

publicStudent(stringname,intscore)

this.Name=name;

this.Score=score;

}

teacher類：

usingSystem;

usingSystem.Collections.Generic;

usingSystem.Linq;

usingSystem.Text;

usingSystem.Threading.Tasks;

namespaceSelectMany操作符

///summary

///Teacher類

////summary

publicclassTeacher

//姓名

publicstringName{get;set;}

//學(xué)生集合

publicListStudentStudents{get;set;}

publicTeacher(stringname,ListStudentstudents)

this.Name=name;

this.Students=students;

}

Program類

usingSystem;

usingSystem.Collections.Generic;

usingSystem.Linq;

usingSystem.Text;

usingSystem.Threading.Tasks;

namespaceSelectMany操作符

classProgram

staticvoidMain(string[]args)

//使用集合初始化器初始化Teacher集合

ListTeacherteachers=newListTeacher{

newTeacher("徐老師",

newListStudent(){

newStudent("宋江",80),

newStudent("盧俊義",95),

newStudent("朱武",45)

newTeacher("姜老師",

newListStudent(){

newStudent("林沖",90),

newStudent("花榮",85),

newStudent("柴進(jìn)",58)

newTeacher("樊老師",

newListStudent(){

newStudent("關(guān)勝",100),

newStudent("阮小七",70),

newStudent("時(shí)遷",30)

//問題：查詢Score小于60的學(xué)生

//方法1：循環(huán)遍歷、會有性能的損失

foreach(Teachertinteachers)

foreach(Studentsint.Students)

if(s.Score60)

Console.WriteLine("姓名:"+s.Name+",成績:"+s.Score);

//查詢表達(dá)式

//方法2：使用SelectMany延遲加載：在不需要數(shù)據(jù)的時(shí)候，就不執(zhí)行調(diào)用數(shù)據(jù)，能減輕程序和數(shù)據(jù)庫的交互，可以提供程序的性能，執(zhí)行循環(huán)的時(shí)候才去訪問數(shù)據(jù)庫取數(shù)據(jù)

//直接返回學(xué)生的數(shù)據(jù)

varquery=fromtinteachers

fromsint.Students

wheres.Score60

selects;

foreach(variteminquery)

Console.WriteLine("姓名:"+item.Name+",成績:"+item.Score);

//只返回老師的數(shù)據(jù)

varquery1=fromtinteachers

fromsint.Students

wheres.Score60

selectnew{

teacherName=t.Name,

student=t.Students.Where(p=p.Score60).ToList()

foreach(variteminquery1)

Console.WriteLine("老師姓名:"+item.teacherName+",學(xué)生姓名:"+item.student.FirstOrDefault().Name+",成績:"+item.student.FirstOrDefault().Score);

//使用匿名類返回老師和學(xué)生的數(shù)據(jù)

varquery2=fromtinteachers

fromsint.Students

wheres.Score60

selectnew{teacherName=t.Name,studentName=s.Name,studentScore=s.Score};

foreach(variteminquery2)

Console.WriteLine("老師姓名:"+item.teacherName+",學(xué)生姓名:"+item.studentName+",成績:"+item.studentScore);

//使用查詢方法

varquery3=teachers.SelectMany(p=p.Students.Where(t=t.Score60).ToList());

foreach(variteminquery