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