private void frmMain_Load(object sender, EventArgs e)
        {
            this.v_ds_phanmanhTableAdapter.Fill(this.ds_phanmanh.v_ds_phanmanh);
            Program.bds_dspm = bds_ds_phanmanh;
            frmDangNhap f = new frmDangNhap();

            f.MdiParent     = this;
            f.StartPosition = FormStartPosition.CenterParent;
            f.Show();
        }
        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();
            }
        }