为什么登录页面能看到链接，但是取不到按钮值呢

string url = "https://graph.qq.com/oauth2.0/show?which=Login&display=pc&response_type=code&client_id=101487368&redirect_uri=https%3A%2F%2Fpacaio.match.qq.com%2Fqq%2FloginBack%3Fsurl%3Dhttp%3A%2F%2Fcoral.qq.com%2F4343994945&state=5b481c68e379d";
webBrowser.Navigate(url );
var link =webBrowser.Document.GetElementById("switcher_plogin");link总是null，网页上能看到链接，帐号密码登录