摘要

在C#中，LINQ（Language Integrated Query）是一种强大的查询语言，它允许你以声明式的方式查询和操作数据。其中，Select和SelectMany是两个非常重要的操作符，它们在处理集合时非常有用。

正文

Select操作符用于将一个集合中的每个元素投影（转换）成一个新的形式。这意味着你可以使用Select来改变集合中元素的形状或提取元素的某些部分。Select通常用于执行一对一的转换。

SelectMany操作符用于将集合的集合（或者说是集合中的元素的集合）展平为一个集合。这意味着它可以将多对一或多对多的关系转换为一对一的关系。SelectMany非常适用于处理嵌套集合的场景。

一个例子

C#
private void btnSearch_Click(object sender, EventArgs e)
{
    List<string> words=new List<string>()
    {
        "张飞",
        "关羽",
        "刘备",
        "刘封",
        "刘禅"
    };

    var ret = from it in words
              select it.Substring(0, 1);
    foreach (var item in ret)
    {
        lstMain.Items.Add(item);
    }
}

再来一个例子

C#
class Student
{
    public int Score { get; set; }
    
    public string Name { get; set; }

    public Student(int score, string name)
    {
        this.Score = score;
        Name = name;
    }
}

class Teacher
{
    public string Name { get; set; }

    public List<Student> Students;

    public Teacher(string order, List<Student> students)
    {
        this.Name = order;

        this.Students = students;
    }
}

调用

C#
private void btnSearch_Click(object sender, EventArgs e)
{
    List<Teacher> teachers = new List<Teacher>
    {
        new Teacher("a",new List<Student>{ new Student(100, "A"),new Student(90,"AA"),new Student(30, "1") }),
        new Teacher("b",new List<Student>{ new Student(100, "B"),new Student(90,"AB"),new Student(60, "2") }),
        new Teacher("c",new List<Student>{ new Student(26, "C"),new Student(90,"AC"),new Student(40, "3") }),
        new Teacher("d",new List<Student>{ new Student(100, "D"),new Student(90,"AD"),new Student(60, "4") }),
        new Teacher("e",new List<Student>{ new Student(100, "E"),new Student(90,"AE"),new Student(50, "5") }),
        new Teacher("f",new List<Student>{ new Student(100, "F"),new Student(90,"AF"),new Student(60, "6") }),
        new Teacher("g",new List<Student>{ new Student(100, "G"),new Student(90,"AG"),new Student(60, "7") })
    };

    var list1 = from t in teachers
                from s in t.Students
                where s.Score < 60
                select s;

    foreach (var item in list1)
    {
        lstMain.Items.Add(item.Name+" "+item.Score.ToString());
    }
}

我们用SelectMany

C#
private void btnSearch_Click(object sender, EventArgs e)
{
    List<Teacher> teachers = new List<Teacher>
    {
        new Teacher("a",new List<Student>{ new Student(100, "A"),new Student(90,"AA"),new Student(30, "1") }),
        new Teacher("b",new List<Student>{ new Student(100, "B"),new Student(90,"AB"),new Student(60, "2") }),
        new Teacher("c",new List<Student>{ new Student(26, "C"),new Student(90,"AC"),new Student(40, "3") }),
        new Teacher("d",new List<Student>{ new Student(100, "D"),new Student(90,"AD"),new Student(60, "4") }),
        new Teacher("e",new List<Student>{ new Student(100, "E"),new Student(90,"AE"),new Student(50, "5") }),
        new Teacher("f",new List<Student>{ new Student(100, "F"),new Student(90,"AF"),new Student(60, "6") }),
        new Teacher("g",new List<Student>{ new Student(100, "G"),new Student(90,"AG"),new Student(60, "7") })
    };

    var list1 = teachers.SelectMany(t => t.Students).Where(s => s.Score < 60);

    foreach (var item in list1)
    {
        lstMain.Items.Add(item.Name+" "+item.Score.ToString());
    }
}