private void button_lienhe_Click(object sender, EventArgs e)
 {
     if (formLienhe == null || formLienhe.IsDisposed)
     {
         formLienhe           = new LienHe();
         formLienhe.MdiParent = this;
         formLienhe.Show();
     }
     else
     {
         formLienhe.Activate();
     }
 }
 private void bar_lienhe_LinkClicked(object sender, DevExpress.XtraNavBar.NavBarLinkEventArgs e)
 {
     if (formLienhe == null || formLienhe.IsDisposed)
     {
         formLienhe           = new LienHe();
         formLienhe.MdiParent = this;
         formLienhe.Show();
     }
     else
     {
         formLienhe.Activate();
     }
 }