public const int MaxLength = 104857600;         //100M
        private const string DefaultUserAgent = "Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.87 Safari/537.36";
        public string Url { get; set; }                 //下载链接
        public string SavePath { get; set; }            //保存的完整路径
        public long ContentLength { get; private set; }                  //本次下载的总长度，并非文件总长度
        private int unitDownloadLength = MaxLength;
        bool isStop;
        private int Offset { get; set; }
        private WebResponse Response;
HttpWebRequest req = (HttpWebRequest)WebRequest.Create(Url);
if (Url.StartsWith("https", StringComparison.OrdinalIgnoreCase))
{
     ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(CertificateValidationCallback);
     req.ProtocolVersion = HttpVersion.Version11;
     req.UserAgent = DefaultUserAgent;
}
 if(RecievedSize > 0)
     req.AddRange(RecievedSize);
Response = (HttpWebResponse)req.GetResponse();
using (Stream stream = Response.GetResponseStream())
{
     if (State == DownloadState.Stoping) return;                                        //判断停止
     using (FileStream fs = new FileStream(SavePath, FileMode.Create, FileAccess.Write))
     {
          using (BinaryReader br = new BinaryReader(stream))
          {
               byte[] b = new byte[MaxLength];
               while (!isStop && (Offset = br.Read(b, 0, (unitDownloadLength = unitDownloadLength > ContentLength - Offset ? (int)(ContentLength - Offset) : unitDownloadLength))) > 0)  //offset等于0时完成下载
               {
                    fs.Write(b, 0, Offset);
                    b = new byte[MaxLength];
                    RecievedSize += Offset;
                    ProgressChanged?.Invoke(null, new DownloadEventArgs(this));
                    Thread.Sleep(50);
                }
           }
     }
}