如何获得你要下载文件的大小？

//获得下载文件的大小
  private static long GetFileSize(string url, out bool progressKnown)
  {
   HttpWebResponse response = null;
   long size = -1;

   try
   {
    response = (HttpWebResponse) GetRequest(url).GetResponse();

    size = response.ContentLength;    
   }
   finally
   {
    if(response != null)
     response.Close();
   }

   return size;
  }

  //下载请求
  private static HttpWebRequest GetRequest(string url)
  {
   HttpWebRequest request = (HttpWebRequest) WebRequest.Create(url);
   request.Credentials = CredentialCache.DefaultCredentials;
   return request;
  }