protected void btn_Subirnota_Click(object sender, EventArgs e)
    {
        if (ddl_alumno.SelectedValue == "0" || ddl_materia.SelectedValue == "0" || ddt_curso.SelectedValue == "0")
        {
            L_Error.Text = "Falta seleccionar";
        }
        else
        {
            DaoUser datos = new DaoUser();
            EUser   enc   = new EUser();

            enc.Id_estudiante = ddl_alumno.SelectedValue;
            enc.Materia       = ddl_materia.SelectedValue;
            enc.Curso         = ddt_curso.SelectedValue;
            DataTable registros = datos.obtenerNota(enc);

            enc.IdNota = registros.Rows[0]["id_nota"].ToString();
            Double n1 = Convert.ToDouble(tb_nt.Text);
            Double n2 = Convert.ToDouble(tb_nt2.Text);
            Double n3 = Convert.ToDouble(tb_nt3.Text);

            Double nd = (n1 + n2 + n3) / 3.0;

            enc.Nota1 = n1.ToString();
            enc.Nota2 = n2.ToString();
            enc.Nota3 = n3.ToString();

            enc.Notadef        = nd.ToString();
            tb_denifitiva.Text = nd.ToString();

            datos.insertarNota(enc);
            ButtonVerNota.Visible = false;
            btn_Subirnota.Visible = true;
        }
    }