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();
}