HttpWebRequest Headers Location

做Post 提交时 对方的服务器返回给我的信息中 并没有Location 这个属性
// 设置参数
request = WebRequest.Create(posturl) as HttpWebRequest;

request.AllowAutoRedirect = false;
request.Method = "POST";
request.ContentType = "application/x-www-form-urlencoded";

request.Headers.Set("Accept-Encoding", "gzip, deflate");
request.Headers.Set("Accept-Language", "zh-cn,zh;q=0.8,en-us;q=0.5,en;q=0.3");
request.UserAgent = "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:22.0) Gecko/20100101 Firefox/22.0";
request.Referer = posturl;
request.ContentLength = data.Length;
outstream = request.GetRequestStream();
outstream.Write(data, 0, data.Length);
outstream.Close();
//发送请求并获取相应回应数据
response = request.GetResponse() as HttpWebResponse;
string location = response.Headers["Location"];
//直到request.GetResponse()程序才开始向目标网页发送Post请求
instream = response.GetResponseStream();
sr = new StreamReader(instream, encoding);

string location 这个变量的值 总是得到null

请高手指教 不胜感激。