//客戶端儲存的檔名
 string fileName = "222.xls";
 //被下載檔案的伺服器絕對路徑
 string filePath = Server.MapPath("file/" + doc.FileName);

 Response.Clear(); //清空快取

 //以字元流的形式下載檔案
 FileStream fs = new FileStream(filePath, FileMode.Open);

 byte[] bytes = new byte[(int)fs.Length]; //獲取檔案位元組長度
 fs.Read(bytes, 0, bytes.Length);
 fs.Close();
 Response.ContentType = "application/octet-stream";
 //通知瀏覽器下載檔案而不是開啟
 Response.AddHeader("Content-Disposition", "attachment; filename=" + HttpUtility.UrlEncode(fileName, System.Text.Encoding.UTF8));
 Response.BinaryWrite(bytes);
 Response.Flush();
 //下載完成後刪除檔案
 File.Delete(Server.MapPath("file/" + doc.FileName)); 
 Response.End();