RenderControl

       protected void Page_Load(object sender, EventArgs e)
        {
             Response.ContentType = "text/plain";
             Response.Write(this.Output());
             Response.End();
            // HttpContext.Current.ApplicationInstance.CompleteRequest();

        }

        public string Output()
        {

            DataTable myTable = new DataTable();
            myTable.Columns.Add("A1");
            myTable.Rows.Add("a");
            myTable.Rows.Add("a");
            myTable.Rows.Add("a");


            this.RepList.DataSource = myTable;
            this.RepList.DataBind();

            StringBuilder strBuilder = new StringBuilder();
            HtmlTextWriter htmlWriter = new HtmlTextWriter(new StringWriter(strBuilder));
            this.RepList.RenderControl(htmlWriter);
            return strBuilder.ToString();
        }