//
// Swap():
//
// If possible, function swaps the courses for two student.
// So, if student s1 was enrolled in course c1, and student s2
// was enrolled in course c2, after succesful execution
// student s1 is enrolled in s2, and student s2 is enrolled in s1.
//
// Return values:
// 0 - failure
// 1 - success
//
private int Swap(int s1, int s2, int c1, int c2)
{
int retries = 0;
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))
{
// both students must be enrolled
if (StudentEnrolled(s1, c1) && StudentEnrolled(s2, c2))
{
db.UnregisterStudent(s1, c1);
db.RegisterStudent(s2, c1);
System.Threading.Thread.Sleep(_delay);
db.UnregisterStudent(s2, c2);
db.RegisterStudent(s1, c2);
db.SubmitChanges();
transaction.Complete();
return(1);
}
else
{
return(0);
}
}
}
catch (SqlException exc)
{
// deadlock
if (exc.Number == 1205)
{
retries++;
}
}
catch (Exception e)
{
MessageBox.Show("Swap(): " + e.Message);
return(0);
}
} // while
}
return(0);
} // Swap()
//
// 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);
} // Drop()