private void createClass()
        {
            if (cbSubject.Text == "" || cbSession.Text == "")
            {
                MessageBox.Show("Please fill all the information");
                return;
            }
            this.Close();
            DataManager dataMng = DataManager.GetInstance;

            dataMng.setClassSessionDate(cbSubject.SelectedItem.ToString(), cbSession.SelectedItem.ToString(), datClassDate.Value.ToString("dd-MM-yyyy", System.Globalization.CultureInfo.InvariantCulture));
            Insert_Student_form inStufm = new Insert_Student_form();

            inStufm.Show();
        }
        private void Insert()
        {
            if (dataMng.Subject == null || dataMng.Session == null || dataMng.Date == null)
            {
                MessageBox.Show("Some information of class is missing. Please fill again the information of class");
                dataMng.Subject = Interaction.InputBox("Please fill subject name", "", "");
                dataMng.Session = Interaction.InputBox("Please fill session", "", "");
                dataMng.Date    = Interaction.InputBox("Please fill date with the format dd-mm-yyyy", "", "");
            }
            if (dataMng.Subject == null || dataMng.Session == null || dataMng.Date == null)
            {
                MessageBox.Show("Some information of class is still missing. Please try again");
                return;
            }
            Insert_Student_form inStu = new Insert_Student_form();

            inStu.Show();
        }