private void initDonViTinh()
{
clsDM_DonViTinh dmKho = new clsDM_DonViTinh();
cboDonViTinh.DisplayMember = "Ten_don_vi_tinh";
cboDonViTinh.ValueMember = "ID_Don_vi_tinh";
cboDonViTinh.DataSource = dmKho.GetAll();
cboDonViTinh.SelectedIndex = -1;
}