public IActionResult ExportCod3es(string codes)
        {
            try
            {
                byte[] bs = { };
                if (string.IsNullOrEmpty(codes))
                    return File(bs, "text/xml", "匯出出錯.txt");

                var filename = DateTime.Now.ToString("yyyyMMddHHmmss
 
") + $"PC退貨.txt";

                using (MemoryStream mem = new MemoryStream())
                {
                    using (StreamWriter sw = new StreamWriter(mem))
                    {
                        sw.BaseStream.Seek(0, SeekOrigin.End);
                        var list = codes.Split(',
 
').ToList();
                        //開始寫入
                        foreach (var item in list)
                        {
                            sw.WriteLine(item);
                        }
                        //清空緩衝區
                        sw.Flush();
                        //關閉流
                        sw.Close();
                    }
                    
 
return File(mem.ToArray(), "text/xml", filename);
                }
            }
            catch (Exception ex)
            {
                byte[] bs = { };
                return File(bs, "text/xml", "匯出出錯.txt");
            }
        }

參考：https://www.cnblogs.com/ShenJA/p/12580622.html