private void navBarItemQLTangca_LinkClicked(object sender, DevExpress.XtraNavBar.NavBarLinkEventArgs e)
        {
            frmQltangca frm = new frmQltangca();

            foreach (System.Windows.Forms.Form _mdi_child in this.MdiChildren)
            {
                if (frm.Name == _mdi_child.Name)
                {
                    _mdi_child.Close();
                }
            }
            frm.MdiParent = this;
            frm.Show();
        }
        private void btNvTangca_ItemClick(object sender, ItemClickEventArgs e)
        {
            frmQltangca frm = new frmQltangca();

            foreach (System.Windows.Forms.Form _mdi_child in this.MdiChildren)
            {
                if (frm.Name == _mdi_child.Name)
                {
                    _mdi_child.Close();
                }
            }
            frm.MdiParent = this;
            frm.Show();
        }