//--- Display instructor coursrs in list
        private void listBox11_SelectedIndexChanged(object sender, EventArgs e)
        {
            Courses.Items.Clear();
            Online_Exame ent = new Online_Exame();

            //--get instractor from combo box
            string insName = Instructors.SelectedItem.ToString();
            var    insId   = (int)(ent.Get_insid_by_inname(insName)).First();

            label7.Text = "of" + " " + insName;
            //--get courses of same instructor
            var items = ent.get_instructor_course(insId);

            foreach (var i in items)
            {
                Courses.Items.Add(i);
            }
        }