private void buttonBackToFDG_Click(object sender, EventArgs e)
        {
            FormDocGia fdg = new FormDocGia();

            this.Hide();
            fdg.ShowDialog();
        }
        private void buttonThem_Click(object sender, EventArgs e)
        {
            try
            {
                SINHVIEN sv = new SINHVIEN()
                {
                    MaSinhVien  = textBoxMaSV.Text,
                    HoTen       = textBoxHoTen.Text,
                    NgaySinh    = dateTimePickerNS.Value,
                    Gioitinh    = textBoxGT.Text,
                    DiaChi      = textBoxDiaChi.Text,
                    SoDienThoai = int.Parse(textBoxSDT.Text),
                };
                db.SINHVIENs.Add(sv);
                db.SaveChanges();

                THETHUVIEN ttv = new THETHUVIEN()
                {
                    MaSinhVien = textBoxMaSV.Text,
                    MaThe      = textBoxMaThe.Text,
                    NgayTaoThe = dateTimePickerNgTao.Value,
                    NgayHetHan = dateTimePickerNgHH.Value,
                    NgayGiaHan = dateTimePickerNgGH.Value
                };
                db.THETHUVIENs.Add(ttv);
                db.SaveChanges();

                FormDocGia fdg = new FormDocGia();
                this.Hide();
                fdg.ShowDialog();

                MessageBox.Show("Them thanh cong!");
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }