private void txtMaSP_TextChanged(object sender, EventArgs e)
 {
     if (SanPhamBUS.DanhSachSPTheoMa(txtMaSP.Text).Rows.Count > 0)
     {
         foreach (DataRow row in SanPhamBUS.DanhSachSPTheoMa(txtMaSP.Text).Rows)
         {
             txtTenSP.Text = row["TenSanPham"].ToString();
         }
     }
     else
     {
         txtTenSP.ResetText();
         txtSoLuong.ResetText();
     }
 }