Without www to URL with www. (301 Redirect)

   protected void Application_BeginRequest(object sender, EventArgs e)
    {
        string Domainloc = Request.Url.Authority;
        Domainloc = Domainloc.Substring(0, 9);
        if (Domainloc != "localhost")
        {
            if (!Request.Url.Authority.StartsWith("www"))
            {
                string MainUrl = Request.Url.Authority + Request.Url.PathAndQuery;
                string url = string.Format("{0}://www.{1}{2}",
                        Request.Url.Scheme,
                        Request.Url.Authority,
                        Request.Url.PathAndQuery);
                Response.Status = "301 Moved Permanently";
                if (MainUrl.ToUpper().Trim() == "YOUR_DOMAIN_NAME.COM/DEFAULT.ASPX")
                {
                    string urlMain = string.Format("{0}://www.{1}",
                                Request.Url.Scheme,
                                Request.Url.Authority);
                    Response.AddHeader("Location", urlMain);
                }
                else
                {
                    Response.AddHeader("Location", url);
                }
            }
        }
   }