LINQ 学习路程 -- 查询操作 let into关键字

IList<Student> studentList = new List<Student>() { 
    new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
    new Student() { StudentID = 2, StudentName = "Steve",  Age = 21 } ,
    new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,
    new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
    new Student() { StudentID = 5, StudentName = "Ron" , Age = 21 } 
};

var lowercaseStudentNames = from s in studentList
                            where s.StudentName.ToLower().StartsWith("r")
                            select s.StudentName.ToLower();

var lowercaseStudentNames = from s in studentList
                            let lowercaseStudentName = s.StudentName.ToLower()
                                where lowercaseStudentName.StartsWith("r")
                                select lowercaseStudentName;

foreach (var name in lowercaseStudentNames)
    Console.WriteLine(name);

var teenAgerStudents = from s in studentList
    where s.age > 12 && s.age < 20
    select s
        into teenStudents
        where teenStudents.StudentName.StartsWith("B")
        select teenStudents;