//
// Drop():
//
// If possible, function drops the student from the course.
//
// Return values:
// -1 - error
// 0 - dropped, no waitlist
// 1 - dropped, enrolled student from waitlist
// 2 - not enrolled, dropped from waitlist
// 3 - student was not either enrolled or waitlisted
//
private int Drop(int sid, int cid)
{
int retries = 0;
// make sure parameters are valid
if (sid < 0 || cid < 0)
{
return(-1);
}
using (var db = new CoursemoDataContext())
{
while (retries < 3)
{
try
{
var txOptions = new TransactionOptions();
txOptions.IsolationLevel = System.Transactions.IsolationLevel.Serializable;
using (var transaction = new TransactionScope(TransactionScopeOption.Required,
txOptions))
{
// student enrolled, drop course and enroll first student on waitlist
if (StudentEnrolled(sid, cid))
{
db.UnregisterStudent(sid, cid);
// candidate on waitlist exists
int sidFromWaitList = GetFirstStudentFromWaitlist(cid);
System.Threading.Thread.Sleep(_delay);
if (sidFromWaitList != 0)
{
db.RegisterStudent(sidFromWaitList, cid);
db.UnwaitlistStudent(sidFromWaitList, cid);
db.SubmitChanges();
transaction.Complete();
return(1);
}
db.SubmitChanges();
transaction.Complete();
return(0);
}
// student not enrolled, but waitlisted, remove from waitlist
else if (StudentWaitlisted(sid, cid))
{
db.UnwaitlistStudent(sid, cid);
db.SubmitChanges();
transaction.Complete();
return(2);
}
else
{
return(3);
}
}
}
catch (SqlException exc)
{
// deadlock
if (exc.Number == 1205)
{
retries++;
}
}
catch (Exception e)
{
MessageBox.Show("Drop(): " + e.Message);
return(-1);
}
} // while
}
return(-1);
}
//
// Enroll():
//
// If possible, function enrolls the student in the course.
//
// Return values:
// -1 - error
// 0 - Student already enrolled
// 1 - Student enrolled
// 2 - Student already waitlisted
// 3 - Student waitlisted
//
private int Enroll(int sid, int cid)
{
int retries = 0;
// make sure parameters are valid
if (sid < 0 || cid < 0)
{
return(-1);
}
using (var db = new CoursemoDataContext())
{
while (retries < 3)
{
try
{
var txOptions = new TransactionOptions();
txOptions.IsolationLevel = System.Transactions.IsolationLevel.Serializable;
using (var transaction = new TransactionScope(TransactionScopeOption.Required,
txOptions))
{
// check if student is already enrolled
if (StudentEnrolled(sid, cid))
{
return(0);
}
// class is full, add to waitlist
if (GetCourseCapacity(cid) - GetCurrentEnrollment(cid) < 1)
{
// check if student is not on waitlist alread
if (StudentWaitlisted(sid, cid))
{
return(2);
}
db.WaitlistStudent(sid, cid);
db.SubmitChanges();
transaction.Complete();
return(3);
}
// enroll
else
{
System.Threading.Thread.Sleep(_delay);
db.RegisterStudent(sid, cid);
db.SubmitChanges();
transaction.Complete();
return(1);
}
}
}
catch (SqlException exc)
{
// deadlock
if (exc.Number == 1205)
{
retries++;
}
}
catch (Exception e)
{
MessageBox.Show("Enroll(): " + e.Message);
return(-1);
}
} // while
}
return(-1);
} // Enroll()