linq声明不起作用

Question

我的数据库中有一些数据如下：

我写了一个linq语句来获取在提供teacher_Id之后按courseId分组的student_ID列表：

 foreach (var item4 in TeacherName)
            {

                var studentInthisClassId = from s in DB.ClassMembers
                    where s.Teacher_Id == item4.Key
                    group s.Student_Id by s.CourseId
                    into thisClass
                    select new {Id = thisClass.Key, sid = thisClass.ToList()};

我写了一个循环来使用上面的查询，但它跳出来，甚至没有运行循环一次。这是循环的代码：

int j = 0;
                foreach (var studentId in studentInthisClassId)
                {
                    string sfName =
                        DB.Students.Where(n => n.Student_Id == studentId.sid[j]).Select(p => p.StudentName).First();
                    string slName =
                        DB.Students.Where(n => n.Student_Id == studentId.sid[j])
                            .Select(p => p.StudentLastName)
                            .First();
                    string studentsFulName = string.Format("{0},{1}", sfName, slName);
                    StudentName.Add(studentsFulName);
                    if (!StudentsByTeacherName.ContainsKey(item4.Value))
                    {
                        StudentsByTeacherName.Add(item4.Value, StudentName);

                    }
                    j++;
                }

这是来自linq的答案：

studentInthisClassId = {SELECT 
    [Project3].[C1] AS [C1], 
    [Project3].[CourseId] AS [CourseId], 
    [Project3].[C2] AS [C2], 
    [Project3].[Student_Id] AS [Student_Id]
    FROM ( SELECT 
        [Distinct1].[CourseId] AS [CourseId], 
        1 AS [C1], 
        ...