private void bar_Login_LinkClicked(object sender, DevExpress.XtraNavBar.NavBarLinkEventArgs e)
 {
     DangNhap_HeThong           = new DangNhap();
     DangNhap_HeThong.MdiParent = this;
     DangNhap_HeThong.Show();
 }
 private void button_dangnhap_Click(object sender, EventArgs e)
 {
     DangNhap_HeThong           = new DangNhap();
     DangNhap_HeThong.MdiParent = this;
     DangNhap_HeThong.Show();
 }