Alert in the codebehind

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;
using System.Data.SqlClient;
public partial class _Default : System.Web.UI.Page
{
       protected void Page_Load(object sender, EventArgs e)
    {
        Page.RegisterStartupScript("startup", "<script>alert('Nothing selected to perform selected operation.');</script>");
        if (!IsPostBack)
        {
         }
      }