C# join子句

using System;
using System.Collections.Generic;
using System.Linq;

namespace HelloWorld
{

    class Program
    {
        public class Student
        {
            public int StID;
            public string LastName;
        }
        public class CourseStudent
        {
            public string CourseName;
            public int StID;
        }
        static CourseStudent[] studentsInCourses = new CourseStudent[]{
            new CourseStudent{CourseName="Art", StID=1},
            new CourseStudent{CourseName="Art", StID=2},
            new CourseStudent{CourseName="History", StID=1},
            new CourseStudent{CourseName="History", StID=3},
            new CourseStudent{CourseName="Physics", StID=3},
        };
        static Student[] students = new Student[]{
            new Student{StID=1, LastName="Carson"},
            new Student{StID=2, LastName="Klassen"},
            new Student{StID=3, LastName="Fleming"},
        };


        
        static void Main(string[] args)
        {
            //查找所有选择了历史课的学生的姓氏
            var query = from s in students
                        join c in studentsInCourses on s.StID equals c.StID
                        where c.CourseName == "History"
                        select s.LastName;
            //显示所有选择了历史课的学生的名字
            foreach(var q in query)
            {
                Console.WriteLine("Student taking History: {0}", q);
            }
           
        }
    }
}