private void btnLogin_Click(object sender, EventArgs e)
        {
            BLL.tb_login_user        u = new BLL.tb_login_user();
            List <BLL.tb_login_user> l = u.getUser(txtUsername.Text, txtPassword.Text);

            if (l.Count > 0)
            {
                frmMain m = new frmMain();
                this.Hide();
                m.ShowDialog();
            }
        }