Update table in linq2sql

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C#

LINQ

Linq2Sql

hi.

i have a database by following diagram.

My Diagram

when i want to update a record from LINK tables Like LinkStudentClass with following code.

private void btnSave_Click(object sender, EventArgs e)
        {
            var LSC = (from l in Program.scdc.LinkStudentClasses
                                    where l.ClassID == Program.SelectedClassID && l.StudentID == Program.SelectedStudentID
                                    select l).Single();

            int ChangedStudentId = Convert.ToInt32(DDLStudents.Text.Split('-')[0]);
            LSC.ClassID = Program.SelectedClassID;
            LSC.StudentID = ChangedStudentId;
            Program.scdc.SubmitChanges();
            MainForm.NeedRefreshForm = true;
            this.Close();
        }

and i get following error.

Error

WHY?

What I have tried:

I realized that I had to delete the record and create a new record like this one.

private void btnSave_Click(object sender, EventArgs e)
        {
            var LSC = (from l in Program.scdc.LinkStudentClasses
                                    where l.ClassID == Program.SelectedClassID && l.StudentID == Program.SelectedStudentID
                                    select l).Single();

            int ChangedStudentId = Convert.ToInt32(DDLStudents.Text.Split('-')[0]);
            Program.scdc.LinkStudentClasses.DeleteOnSubmit(LSC);
            Program.scdc.SubmitChanges();
            LSC = new LinkStudentClass();
            LSC.ClassID = Program.SelectedClassID;
            LSC.StudentID = ChangedStudentId;
            Program.scdc.LinkStudentClasses.InsertOnSubmit(LSC);
            Program.scdc.SubmitChanges();
            MainForm.NeedRefreshForm = true;
            this.Close();
        }

but how can update directly?