public static void HttpDownloadEx(string url,
            string path,
            bool overwrite,
            Action<string, HttpWebResponse> doneCallback = null,
            Action<string, string, long, long> downloadingCallback = null) 
        {            // 设置参数
            HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
            //发送请求并获取相应回应数据
            HttpWebResponse response = request.GetResponse() as HttpWebResponse;            string contentType = response.Headers["Content-Type"];
            //断点续传
            FileStream fStream = null;
            long sPosition = 0;
            Stream responseStream = null;            try
            {
                long totalLength = response.ContentLength;
                if (System.IO.File.Exists(path))
                {
                    fStream = System.IO.File.OpenWrite(path);
                    sPosition = fStream.Length;
                    if (sPosition == totalLength)
                    {
                        doneCallback?.Invoke(path, response); //文件是完整的，直接结束下载任务
                        return;
                    }
                    fStream.Seek(sPosition, SeekOrigin.Current);
                }
                else
                {
                    fStream = new FileStream(path, FileMode.Create);
                    sPosition = 0;
                }                HttpWebRequest myRequest = (HttpWebRequest)HttpWebRequest.Create(url);
                if (sPosition > 0)
                {
                    myRequest.AddRange(sPosition);             //设置Range值
                }                //向服务器请求,获得服务器的回应数据流
                responseStream = myRequest.GetResponse().GetResponseStream();                //定义一个字节数据
                byte[] btContent = new byte[512];
                int intSize = 0;
                intSize = responseStream.Read(btContent, 0, 512);
                while (intSize > 0)
                {
                    fStream.Write(btContent, 0, intSize);
                    intSize = responseStream.Read(btContent, 0, 512);
                    downloadingCallback?.Invoke(path, contentType, totalLength, fStream.Length);
                }
            }
            catch
            {//(Exception ex)
                throw;
            }
            finally
            {
                //关闭流
                if (responseStream != null)
                {
                    responseStream.Close();
                    responseStream.Dispose();
                }
                if (fStream != null)
                {
                    fStream.Close();
                    fStream.Dispose();
                }
            }
        }