protected void btnModificar_Click(object sender, EventArgs e)
        {
            List<Matricula> lstMatri = new List<Matricula>();

            if (Session["Matri"] != null)
            {
                lstMatri = ((List<Matricula>)Session["Matri"]);
            }

            Matricula MatriculaEditada = lstMatri.SingleOrDefault(s => s.Codigo == Convert.ToInt32(txtCodigo.Text));
            int indice = lstMatri.IndexOf(MatriculaEditada);

            MatriculaEditada.NombreEstud = txtNombre.Text;
            MatriculaEditada.Valor = Convert.ToInt32(txtValor.Text);

            lstMatri[indice] = MatriculaEditada;

            GridMatricula.DataSource = lstMatri;
            GridMatricula.DataKeyNames = new string[] { "Codigo" };
            GridMatricula.DataBind();
        }