底层连接已关闭:连接被意外关闭

当我尝试获取html页面时，我得到这个错误:

The underlying connection was closed: The connection was closed unexpectedly

我想我正在访问的网站使用了一些基于ip的保护。

WebClient single_page_client = new WebClient();
single_page_client.Headers.Add("user-agent", "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705;)");
string cat_page_single = single_page_client.DownloadString(the_url);

我该怎么做呢?在Webclient中使用代理怎么样?

编辑

如果我使用这段代码，它可以工作。为什么?

HttpWebRequest webrequest = (HttpWebRequest)WebRequest.Create(current_url);
webrequest.KeepAlive = true;
webrequest.Method = "GET";
webrequest.ContentType = "text/html";
webrequest.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8";
//webrequest.Connection = "keep-alive";
webrequest.Host = "cat.sabresonicweb.com";
webrequest.Headers.Add("Accept-Language", "en-US,en;q=0.5");
webrequest.UserAgent = "Mozilla/5.0 (Windows NT 6.1; rv:18.0) Gecko/20100101 Firefox/18.0";
HttpWebResponse webresponse = (HttpWebResponse)webrequest.GetResponse();
Console.Write(webresponse.StatusCode);
Stream receiveStream = webresponse.GetResponseStream();

Encoding enc = System.Text.Encoding.GetEncoding(1252);//1252
StreamReader loResponseStream = new StreamReader(receiveStream, enc);
string current_page = loResponseStream.ReadToEnd();
loResponseStream.Close();
webresponse.Close();