Check Login in ASP.Net C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;

public partial class media_admin : System.Web.UI.MasterPage {
    protected void Page_Load(object sender, EventArgs e) {
        if (!IsPostBack) {
            cek_login();
            lblAdmin.Text = "" + Session["namalengkap"] + "";
        }
    }
    void cek_login() {
        if (Session["username"] == null) {
            Response.Redirect("~/Login.aspx");
        }
    }
}