C#利用HttpClient实现反向代理分块下载

最近遇到的一个项目，附件在A服务器，对外是B服务器，A与B服务器可以内网通信，但是A服务器不对外，就需要用到B服务器的对外访问地址浏览附件与文件，参考了部分网上的代码，附上函数

    /// 

    /// 异步下载
    /// 
    /// 
    /// 
    private async void FileDownloadAsync(string url)
    {
        //开启一个异步线程
        await System.Threading.Tasks.Task.Run(async () =>
        {
            long downloadSize = 0;//已经下载大小
            long downloadSpeed = 0;//下载速度

            using (HttpClient http = new HttpClient())
            {
                var httpResponseMessage = await http.GetAsync(url, HttpCompletionOption.ResponseHeadersRead);//发送请求
                var contentLength = httpResponseMessage.Content.Headers.ContentLength;   //文件大小            
                string contentType = httpResponseMessage.Content.Headers.ContentType.MediaType??MimeMapping.GetMimeMapping(url);
                string fileName = httpResponseMessage.Content.Headers.ContentDisposition!=null?httpResponseMessage.Content.Headers.ContentDisposition.FileName:Path.GetFileName(url);
                //兼容各种URL文件
                if (fileName.IsNull())
                    fileName = Path.GetFileName(url);

                using (var stream = await httpResponseMessage.Content.ReadAsStreamAsync())
                {
                    var readLength = 102400;//100K
                    byte[] bytes = new byte[readLength];
                    int writeLength;
                    var beginSecond = DateTime.Now.Second;//当前时间秒

                    Response.Clear();
                    Response.ContentType = "application/octet-stream"; //二进制流
                    Response.AddHeader("Content-Disposition", "attachment; filename=" + HttpUtility.UrlEncode(fileName, System.Text.Encoding.UTF8));//下载保存的文件名

                    Response.AddHeader("Content-Length", contentLength.HasValue?contentLength.Value.ToString():"0");//文件总大小
                    while ((writeLength = stream.Read(bytes, 0, readLength)) > 0)
                    {

                        if (Response.IsClientConnected)
                        {
                            Response.OutputStream.Write(bytes, 0, writeLength);
                            Response.Flush();//如果客户端 暂停下载时，这里会阻塞。
                        }

                        downloadSize += writeLength;
                        downloadSpeed += writeLength;
                    }
                    Response.Close();
                }
            }
        });
    }