question on linq select join

I would like to list out all the students who have yet to pay off their course fee i.e. when a student is clicked, list out all pending fees by months.

This is what I have done so far..

These are all the students active courses.

Below are the payments record paid by student to their course.

Now i would like to list out all the pending payment foreach students e.g. last payment to the course is on 2/11/2011. If the datetime now is May, Then i would like to show the pending amount as

Month Amount

March 100

April 200

May 400

This is what I tried..

foreach (var result in activeCourses)
{
    //Sho开发者_JAVA百科w all the pending paid coures from dateJoin till datetime.Now
    DateTime JoinedDate = Convert.ToDateTime(result.cus.Date);
    for (DateTime d = JoinedDate; d.Month <= DateTime.Now.Month; d = d.AddMonths(1))
    {
        Payment payment = new Payment();

        payment.Course.Id = result.c.Id;
        payment.Course.Name = result.c.Name;
        payment.Course.Fee = result.c.Fee;
        payment.CourseByStudent.CourseUserStatus.Date = result.cu.Date;
        payment.CourseByTutor.TutorId = result.t.TutorId;
        payment.User.Name = result.u.Name;

        payment.MonthId = d.Month;
        payment.MonthToPay = d.ToString("MMM");
        output.Add(payment);
    }
}

The logic given above does not seem to be efficient in case the student does not pay anything for his courses, then I have to check the pending payment since his first JoinedDate. OtherWise I need to check the pending payment from the DateIssue in the Payment table according to that particular course..please advice thanks

i will use back the same approach to show the late payment.

  for (DateTime d = lastPaidDate.AddMonths(1); d.Month <= DateTime.Now.Month; d = d.AddMonths(1)) 

Thanks