用HTTPWebrequest的CharacterSet属性时，结果总是空字符

private void WebRequestButton_Click(object sender, System.EventArgs e)
{
PageUrl = UrlText.Text;
WebRequest  request = WebRequest.Create(PageUrl);
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
Stream resStream = response.GetResponseStream(); 
StreamReader sr;
string encoding = "";
string contenttype = response.Headers["Content-Type"];
 Response.Write ("contenttype = " + contenttype);
 if(contenttype.ToLower().IndexOf("charset") == -1)
 {
  contenttype = "";
 }
 
if( contenttype != "")
{
 string[] headers = System.Text.RegularExpressions.Regex.Split(contenttype,";");
 for(int i = 0;i < headers.Length; i++)
 {
  if(headers[i].ToLower().IndexOf("charset") > -1)
  {
   encoding =  System.Text.RegularExpressions.Regex.Split(headers[i],"=")[1];
    break;
  }
 }
}

if(encoding == "")
 sr= new StreamReader(resStream, System.Text.Encoding.Default);
else
{
 sr= new StreamReader(resStream, System.Text.Encoding.GetEncoding(encoding));
}
ContentHtml.Text = sr.ReadToEnd();
resStream.Close();
sr.Close();
}