private void button_duan_Click(object sender, EventArgs e)
 {
     if (formDuAn == null || formDuAn.IsDisposed)
     {
         formDuAn           = new QL_DuAn();
         formDuAn.MdiParent = this;
         formDuAn.Show();
     }
     else
     {
         formDuAn.Activate();
     }
 }
 private void DuAn_LinkClicked(object sender, DevExpress.XtraNavBar.NavBarLinkEventArgs e)
 {
     if (formDuAn == null || formDuAn.IsDisposed)
     {
         formDuAn           = new QL_DuAn();
         formDuAn.MdiParent = this;
         formDuAn.Show();
     }
     else
     {
         formDuAn.Activate();
     }
 }