Refresh or click

using System;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;

public partial class _Default : System.Web.UI.Page
{

    protected override void OnPreRender(EventArgs e)
    {
        ViewState["update"] = Session["update"];
    }
    void page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack) // If page loads for first time
        {
            // Assign the Session["update"] with unique value
            Session["update"] = Server.UrlEncode(System.DateTime.Now.ToString());
            //=============== Page load code =========================
            //============== End of Page load code ===================
        }
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        if (Session["update"].ToString() == ViewState["update"].ToString())
        {
            //=============== On click event code =========================

            lblDisplayAddedName.Text = txtName.Text;

            //=============== End of On click event code ==================

            // After the event/ method, again update the session

            Session["update"] = Server.UrlEncode(System.DateTime.Now.ToString());
        }
        else // If Page Refreshed
        {
            // Do nothing
        }
    }
}