用httpwebrequest方式采集某网页中的内容，为啥采集不到？
System.Net.HttpWebRequest request = (System.Net.HttpWebRequest)System.Net.WebRequest.Create(url);
request.UserAgent = "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; .NET CLR 2.0.50727; .NET CLR 3.0.04506.648; .NET CLR 3.5.21022)";
System.Net.WebResponse response = request.GetResponse();
System.IO.Stream resStream = response.GetResponseStream();
System.IO.StreamReader sr = new System.IO.StreamReader(resStream, Encoding.UTF8);
string html = (sr.ReadToEnd());
resStream.Close();
sr.Close();
return html
