如何获得iFrame id的价值

Question

我有一个html页面，但我不能得到iframe src的值。 这是我的代码

                string requestUrl = URL.Text;
                WebRequest request = WebRequest.Create(requestUrl);
                WebResponse response = request.GetResponse();
                StreamReader reader = new StreamReader(response.GetResponseStream());
                string responseText = reader.ReadToEnd();

                doc.LoadHtml(responseText);

                var content = doc.DocumentNode.SelectNodes("//iframe[@class='ifram']//iframe");
                var src= content.Select(x => x.Attributes["src"]);