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