public void GetWetheaerByCity(string cityName) {
  string city = cityName;
  string mycity = HttpUtility.UrlEncode(city, System.Text.Encoding.GetEncoding("GB2312"));
  System.Net.HttpWebRequest request = (System.Net.HttpWebRequest)System.Net.WebRequest.Create("http://www.sina.com/index.php" + cityName);
  //HttpWebRequest 对象 获取GetResponse 转换成HttpWebResponse
  System.Net.HttpWebResponse response = request.GetResponse() as System.Net.HttpWebResponse;
  //通过HttpWebResponse response.GetResponseStream()获取输出流
  Stream str = response.GetResponseStream();
  StreamReader reader = new StreamReader(str, System.Text.Encoding.GetEncoding("GB2312"));
  string weathhtml = reader.ReadToEnd();
  str.Close();
  reader.Close();
}