在 winfrom中 不显示删除回复显示所有回复显示星级回复显示得分回复 在用DownloadFile下载图片前，怎么用程序判断远端图片是否存在

private bool UrlIsExist(String url){  System.Uri u = null;  try  {    u = new Uri(url);  }  catch { return false; }  bool isExist = false;  System.Net.HttpWebRequest r = System.Net.HttpWebRequest.Create(u) as System.Net.HttpWebRequest;  r.Method = "HEAD";  try  {    System.Net.HttpWebResponse s = r.GetResponse() as System.Net.HttpWebResponse;    if (s.StatusCode == System.Net.HttpStatusCode.OK)    {      isExist = true;    }  }  catch (System.Net.WebException x)  {    try    {      isExist = ((x.Response as System.Net.HttpWebResponse).StatusCode != System.Net.HttpStatusCode.NotFound);    }    catch { isExist = (x.Status == System.Net.WebExceptionStatus.Success); }  }  return isExist;}