how to display message box in c# asp.net inside of codebehind

              mes = chpwd.updat(txtrpwd.Text, txtuname.Text);
                        if (mes)
                        {
                           
Page.RegisterClientScriptBlock("mes", "<script
language='javascript'>alert('New Password is
Created')</script>");

                            string f = Session["flag"].ToString();
 &nbs ...