下載-斷點續傳

     /// <summary>
        ///  斷點續傳
        /// </summary>
        /// <param name="savePath">保存路徑</param>
        /// <param name="loadPath">下載路徑</param>
        /// <param name="fileName">文件名稱</param>
        /// <param name="flag">備份區分，默認備份名，原文件+.back</param>
        /// <returns></returns>
        public static bool ResumeDownloadFile(string savePath, string loadPath, string fileName = null, bool flag = true)
        {
            // 自定義文件名爲空的情況
            if (string.IsNullOrEmpty(fileName))
            {
                // 取得下載文件名
                fileName = Path.GetFileName(loadPath);
            }
            savePath = savePath + fileName;
            // 判斷保存路徑是否存在，備份原文件
            JustFileAndDic(savePath, true);
            // 定義臨時下載路徑
            string strLocalPathTemp = savePath + PersistExp;
            // 文件下載結果.
            bool bDownload = true;

            //打開上次下載的文件或新建文件 
            long lStartPos = 0;
            FileStream fs;
            if (File.Exists(strLocalPathTemp))
            {
                fs = File.OpenWrite(strLocalPathTemp);
                lStartPos = fs.Length;
                fs.Seek(lStartPos, SeekOrigin.Current); //移動文件流中的當前指針 
            }
            else
            {
                fs = new FileStream(strLocalPathTemp, FileMode.Create);
                lStartPos = 0;
            }

            //打開網絡連接 
            try
            {
                // 定義驗證證書的回調函數.
                ServicePointManager.ServerCertificateValidationCallback = CheckValidationResult;
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(loadPath);
                if (lStartPos > 0)
                    request.AddRange((int)lStartPos); //設置Range值

                //向服務器請求，獲得服務器迴應數據流 
                Stream ns = request.GetResponse().GetResponseStream();

                byte[] nbytes = new byte[512];
                int nReadSize = 0;
                nReadSize = ns.Read(nbytes, 0, 512);
                while (nReadSize > 0)
                {
                    fs.Write(nbytes, 0, nReadSize);
                    nReadSize = ns.Read(nbytes, 0, 512);
                }
                fs.Close();
                ns.Close();
                // 將文件複製到指定文件夾
                File.Copy(strLocalPathTemp, savePath);
                // 刪除臨時文件
                File.Delete(strLocalPathTemp);
            }
            catch (Exception e)
            {
                fs.Close();
                // 設定下載結果爲false.
                bDownload = false;
            }

            // 返回下載結果.
            return bDownload;
        }