private void buttonItem2_Click(object sender, EventArgs e)
 {
     if (PhongBanfrm == null || PhongBanfrm.IsDisposed)
     {
         PhongBanfrm           = new frmPhongBan();
         PhongBanfrm.MdiParent = this;
         PhongBanfrm.Show();
     }
     else
     {
         PhongBanfrm.Activate();
     }
 }
        private void btnPhong_Click(object sender, EventArgs e)
        {
            frmPhongBan PB = new frmPhongBan();

            PB.ShowDialog();
        }