System.Net.HttpWebRequest”不包含“Host”的定义,怎么解决？

try  
           {  
               req = req = (HttpWebRequest)HttpWebRequest.Create(url);  
               req.ContentType = "application/x-www-form-urlencoded";  
               req.Method = "Get";  
               req.Timeout = 1000 * 30;
               if (!string.IsNullOrEmpty(host))
               req.Host = host;  -----这个Host 出错了
               
               req.Referer = "http://www.wbto.cn/?c=login&m=index_login";  
               req.CookieContainer = cookie;  
               req.Headers.Add("Cookie:" + cookieStr);  
               rep = (HttpWebResponse)req.GetResponse();  
               newCookie = req.CookieContainer.GetCookieHeader(req.RequestUri);  
               st = rep.GetResponseStream();  
               StreamReader stre = new StreamReader(st, Encoding.GetEncoding("utf-8"));  
               json = stre.ReadToEnd();  
  
           }