private void comboBox1_MouseClick(object sender, MouseEventArgs e) { SachBUS objSach = new SachBUS(); if (objSach.GetNhayNhap() != null) { cbNgaynhap.DataSource = objSach.GetNhayNhap(); cbNgaynhap.DisplayMember = "ngaynhap"; } else { MessageBox.Show("Lỗi dữ liệu"); } }