winform=> post+COOKIE登录网站取出新的ＣＯＯＫＩＥ

方法： public static string GetCookie(string URL, string postData, string cookie, out string newCookie)
        {
            byte[] byteRequest = Encoding.Default.GetBytes(postData);
            long contentLength;
            HttpWebRequest httpWebRequest;
            HttpWebResponse webResponse;
            Stream getStream;
            httpWebRequest = (HttpWebRequest)HttpWebRequest.Create(URL);
            CookieContainer co = new CookieContainer();
            co.SetCookies(new Uri(URL), cookie);
            httpWebRequest.CookieContainer = co;
            httpWebRequest.ContentType = "application/x-www-form-urlencoded";
            httpWebRequest.Referer = "http://test.com";
            httpWebRequest.UserAgent =
                "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; Maxthon; .NET CLR 1.1.4322)";
            httpWebRequest.Method = "Post";
            httpWebRequest.ContentLength = byteRequest.Length;
            httpWebRequest.AllowAutoRedirect = false;
            Stream stream;
            stream = httpWebRequest.GetRequestStream();
            stream.Write(byteRequest, 0, byteRequest.Length);
            webResponse = (HttpWebResponse)httpWebRequest.GetResponse();
            CookieContainer cookies = httpWebRequest.CookieContainer;
            newCookie = httpWebRequest.Headers.Get("Set-Cookie"); 
            stream.Close();
            return newCookie;          
                  }
//调用
string newCookie="";
newCookie=GetCookie("http://test.com/登录验测.aspx",post提交过来的数据,验证码cookie,输出新的cookie);
取出newCookie怎么为空呢？我同时试了：
 httpWebRequest.CookieContainer.GetCookieHeader(new Uri("http://test.com/logindo.aspx")).ToString();
这个能取得到，但是发现和使用的抓包工具http://test.com/logindo.aspx的cookie：Set-Cookie: .ASPXFORMSAUTHXXForTest=7CCB87024AB3A9DA3D90F46FFCDA3F0D4F3BD3657D62997F8AA2141B943D243BE69E54B34E855F2012BDFF8B24F0BCFAA91D3CEDA4593943F6658B09E35D156699E1F24DB045A10DB272C1922EC3019842C2603BF988908F4E36CC9B000D71D8; domain=.test.com; path=/; HttpOnly
Set-Cookie: forever.test.com=UserID=PBjdvkHaNTQ=&Time=zULFFSqYKXe668wECCO0nO5Mvo9jV6qI&Status=KAyeeDyZo6Y=; domain=www.test.com; expires=Tue, 05-Jun-2018 12:32:22 GMT; path=/
 httpWebRequest.CookieContainer.GetCookieHeader这方法取到的是：ASP.NET_SessionId=pfxkjfmnnx5aut3wq0af0f45; .ASPXFORMSAUTHXXForTest=C8EAA98C6B26D3442D1E7B2E7FFD1218B9526A69A3075E559A90BEE4927E2BE0A2BF77DD576FD7BBDFA14FEC4864076B3943C7837FE872C31571D908E3A3B1FA2B939226311350A8451205B51414731AF309902868E54B9B9083A961F84C6C5B
帮帮忙呀，高手们