c# - LINQ - 分组和聚合

Question

如果我有这 3 个类:

class Student { int id; string name; }
class Course { int id; string name; }
class Enrolment { int studentId; int courseId; DateTime enrolmentDate; }

然后使用这个:

IEnumerable<Course> FindCoursesForStudent(Student student)
{
    return from enrolment in Enrolments 
           where enrolment.studentId == student.id
           join course in Courses
              on enrolment.courseId equals course.id
           select course;
}

我可以列出学生注册的所有类(class)。

但是，如果学生多次注册一门类(class)，我该如何只显示每门类(class)的最新注册？

我假设我需要按类(class)分组并过滤 enrolmentDate = Max(enrolmentDate) 或类似的东西？

(如果这看起来很眼熟，那是因为它是我的 previous LINQ question 的后续。)

Answer 1

Enumerable<Course> FindCoursesForStudent(Student student)
{
    return from enrolment in Enrolments 
           where enrolment.studentId == student.id
           && enrolment.enrolmentDate == (from ed in Enrolments 
           where ed.enrolmentDate == enrolment.enrolmentDate 
           select ed.enrolmentDate).Max()
           join course in Courses
              on enrolment.courseId equals course.id
           select course;
}