private void btnDangNhap_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e) { Form frm = this.CheckExists(typeof(frmDangNhap)); if (frm != null) { frm.Activate(); } else { frmDangNhap f = new frmDangNhap(); f.MdiParent = this; f.Show(); } }