private void frmMain_Load(object sender, EventArgs e)
 {
     QuyenDangNhap(false);
     if (!CheckForm("frmDangNhap"))
     {
         Form frm = new frmDangNhap();
         frm.MdiParent = this;
         frm.Show();
     }
     else
     {
         ActivateForm("frmDangNhap");
     }
 }
 private void đăngNhậpToolStripMenuItem_Click(object sender, EventArgs e)
 {
     if (!CheckForm("frmDangNhap"))
     {
         Form frm = new frmDangNhap();
         frm.MdiParent = this;
         frm.Show();
     }
     else
     {
         ActivateForm("frmDangNhap");
     }
     //đăngNhậpToolStripMenuItem.Enabled = false;
 }