[HttpGet]

        public IHttpActionResult ExportData()

        {

            ...

            var dt = ExcelHelper.ListToDataTable(list);

            var fileName = SysConfig.WebTempPath + Guid.NewGuid().ToString() + ".xls";

            ExcelHelper.ExportToFile(dt,fileName);

            

　　　　　　　　//http://www.cnblogs.com/enternal/p/5417519.html

            HttpResponseMessage httpResponseMessage = new HttpResponseMessage(HttpStatusCode.OK);

            FileStream fileStream = File.OpenRead(fileName);

            httpResponseMessage.Content = new StreamContent(fileStream);

            httpResponseMessage.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");

            httpResponseMessage.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")

            {

                FileName =HttpUtility.UrlEncode(Path.GetFileName(fileName))

            };

            return ResponseMessage(httpResponseMessage);

        }