Response.Rewrite not working

dt = (DataTable)ViewState["Data"];
           if (dt.Rows.Count != 0)
           {
               string file = new ExcelHelper().ExportToExcel(dt);
               string rootPath = HttpContext.Current.Server.MapPath("~\\ExcelFiles\\").ToString();
               string localCopy = Guid.NewGuid().ToString() + ".xlsx";
               File.Copy(file, rootPath + localCopy);
               string str = rootPath + localCopy;
               Response.Write(str);  // not writing anything
               Response.Redirect(str); // not going to path..
           }