private void btnNhomHang_Click(object sender, EventArgs e)
        {
            frmNhomHang NhomHang = new frmNhomHang();

            NhomHang.ShowDialog();
            KhoiTaoComboNhomHang();
        }
        public frmNhomHang showNhomHang()
        {
            frmNhomHang F = (frmNhomHang)getForm("frmNhomHang");

            if (F == null)
            {
                F = new frmNhomHang();
                F.Show();
            }
            F.MdiParent   = this;
            F.WindowState = FormWindowState.Maximized;
            F.Activate();
            return(F);
        }