导出Excel（npoi）

public static void ExportExcel(DataTable dt, string fileName = "")        {            //生成Excel            IWorkbook book = BuildWorkbook(dt);            //web 下载            if (fileName == "")                fileName = string.Format("{0:yyyyMMddHHmmssffff}", DateTime.Now);            fileName = fileName.Trim();            string ext = Path.GetExtension(fileName);            if (ext.ToLower() == ".xls" || ext.ToLower() == ".xlsx")                fileName = fileName.Replace(ext, string.Empty);            HttpResponse httpResponse = HttpContext.Current.Response;            httpResponse.Clear();            httpResponse.Buffer = true;            httpResponse.Charset = Encoding.UTF8.BodyName;            httpResponse.AppendHeader("Content-Disposition", "attachment;filename=" + fileName + ".xls");            httpResponse.ContentEncoding = Encoding.UTF8;            httpResponse.ContentType = "application/vnd.ms-excel; charset=UTF-8";            book.Write(httpResponse.OutputStream);            httpResponse.End();        }