下载文件

  #### 只适合处理小文件，文件太大会撑爆内存
  public FileResult DownLoadFile(string key = "", string fileName = "")
        {
            string filePath = GetObjectFromCache<string>(key) as string;
            if (!string.IsNullOrEmpty(filePath))
            {
               
                FileStream stream = new FileInfo(filePath).OpenRead();
                Byte[] buffer = new Byte[stream.Length];
                ////下载后将Excel立即删除
                //从流中读取字节块并将该数据写入给定缓冲区buffer中
                stream.Read(buffer, 0, Convert.ToInt32(stream.Length));
                stream.Close();
                if (System.IO.File.Exists(filePath))
                {
                   System.IO.File.Delete(filePath);
                }
                MemoryStream m = new MemoryStream(buffer);
                if (m!=null)
                {
                    return File(m, "application/octet-stream", fileName);
                }
            }
            return null;
        }