C# Linq group by 和 group by into 运用实例

Form：在一个查询中如果需要查询多个数据源则使用多个From

Group：分组，将查询结果按照指定分组条件放入到不同的分组中

Into：使用Into上下文关键字创建一个临时标示符，一遍将group、join或select子句的结果存储到新的标示符中

Let：在查询中增加一个范围变量，可以直接运算来的，也可以调用外部方法来存放外部方法得到的结果集；

List<Student> listStudent = new List<Student>
            {
               new Student{StuNo="11",StuName="李三",ClassNo="1"},
               new Student{StuNo="22",StuName="王五",ClassNo="2"},
               new Student{StuNo="33",StuName="张猛",ClassNo="3"},
               new Student{StuNo="44",StuName="刘刚",ClassNo="1"},
               new Student{StuNo="55",StuName="夏飞",ClassNo="2"},
               new Student{StuNo="66",StuName="胡凯",ClassNo="3"},
            };
            List<ClassInfo> listClassInfo = new List<ClassInfo>() {
                new ClassInfo{ClassNo="1",ClassName="班级一班"},
                new ClassInfo{ClassNo="2",ClassName="班级二班"},
                new ClassInfo{ClassNo="3",ClassName="班级三班"},
            };
            var item = from s in listStudent
                       group s by s.ClassNo;
            listBox1.Items.Clear();
            foreach(IGrouping<string ,Student> ig in item)
            {
                listBox1.Items.Add(ig.Key);
                listBox1.Items.Add("=============================");
                    foreach(Student s in ig)
                    {
                        string info = string.Format("学号：{0} 姓名：{1} 班级：{2} 当前分组：{3}", s.StuNo, s.StuName, s.ClassNo,ig.Key);
                        listBox1.Items.Add(info);
                    }
            }

========================================

List<Student> listStudent = new List<Student>
            {
               new Student{StuNo="11",StuName="李三",ClassNo="1"},
               new Student{StuNo="22",StuName="王五",ClassNo="2"},
               new Student{StuNo="33",StuName="张猛",ClassNo="3"},
               new Student{StuNo="44",StuName="刘刚",ClassNo="1"},
               new Student{StuNo="55",StuName="夏飞",ClassNo="2"},
               new Student{StuNo="66",StuName="胡凯",ClassNo="3"},
            };
            List<ClassInfo> listClassInfo = new List<ClassInfo>() {
                new ClassInfo{ClassNo="1",ClassName="班级一班"},
                new ClassInfo{ClassNo="2",ClassName="班级二班"},
                new ClassInfo{ClassNo="3",ClassName="班级三班"},
            };
            var query_stu = from stu in listStudent
                            group stu by stu.ClassNo into c_table
                            from cl in listClassInfo
                            where cl.ClassNo == c_table.Key
                            select new { classId = c_table.Key, className = cl.ClassName,stus = c_table };
            foreach(var item in query_stu)
            {
                listBox1.Items.Add("=====================" + item.className + "=================");
                foreach(Student s in item.stus)
                {
                    string info = string.Format("学号：{0} 姓名：{1} ", s.StuNo, s.StuName);
                    listBox1.Items.Add(info);
                }