C#文件下载（实现断点续传）

    public class WebDown
     {
         /// 下载文件方法
         /// 文件保存路径和文件名
         /// 返回服务器文件名
         public static bool DeownloadFile(string sourceFile, string desFile)
         {
             bool flag = false;
             long SPosition = ;
             FileStream FStream = null;
             Stream myStream = null;
             string fileName = sourceFile.Substring(sourceFile.LastIndexOf(@"/") + );
             if (desFile.EndsWith("\\"))
             {
                 desFile = desFile + fileName;
             }
             else
             {
                 desFile = desFile + "\\" + fileName;
             }
             try
             {
                 //判断要下载的文件夹是否存在
                 if (File.Exists(desFile))
                 {
                     //打开上次下载的文件
                     FStream = File.OpenWrite(desFile);
                     //获取已经下载的长度
                     SPosition = FStream.Length;
                     long serverFileLength = GetHttpLength(sourceFile);
                     if (SPosition == serverFileLength)
                     {//文件是完整的，直接结束下载任务
                         return true;
                     }
                     FStream.Seek(SPosition, SeekOrigin.Current);
                 }
                 else
                 {
                     //文件不保存创建一个文件
                     FStream = new FileStream(desFile, FileMode.Create);
                     SPosition = ;
                 }
                 //打开网络连接
                 HttpWebRequest myRequest = (HttpWebRequest)HttpWebRequest.Create(sourceFile);
                 if (SPosition > )
                 {
                     myRequest.AddRange(SPosition);             //设置Range值
                 }
                 //向服务器请求,获得服务器的回应数据流
                 myStream = myRequest.GetResponse().GetResponseStream();
                 //定义一个字节数据
                 byte[] btContent = new byte[];
                 int intSize = ;
                 intSize = myStream.Read(btContent, , );
                 while (intSize > )
                 {
                     FStream.Write(btContent, , intSize);
                     intSize = myStream.Read(btContent, , );
                 }
                 flag = true;        //返回true下载成功
             }
             catch (Exception ex)
             {
                 SysLog.WriteTxtLog("下载文件时异常：" + ex.Message);
             }
             finally
             {
                 //关闭流
                 if (myStream != null)
                 {
                     myStream.Close();
                     myStream.Dispose();
                 }
                 if (FStream != null)
                 {
                     FStream.Close();
                     FStream.Dispose();
                 }
             }
             return flag;
         }
         static long GetHttpLength(string url)
         {
             long length = ;
             try
             {
                 var req = (HttpWebRequest)WebRequest.CreateDefault(new Uri(url));
                 req.Method = "HEAD";
                 req.Timeout = ;
                 var res = (HttpWebResponse)req.GetResponse();
                 if (res.StatusCode == HttpStatusCode.OK)
                 {
                     length = res.ContentLength;
                 }
                 res.Close();
                 return length;
             }
             catch (WebException wex)
             {
                 return ;
             }
         }
     }