string imgurlhard = doc.DocumentNode.
Element("html").
Element("body").
Elements("div").Single(el => el.Attributes["id"].Value == "main").
Elements("div").Single(el => el.Attributes["id"].Value == "onlineIntegrator").
Elements("div").Single(el => el.Attributes["id"].Value == "results").
Element("img").Attributes["src"].Value;
MessageBox.Show(imgurlhard);
这是要检索的网页的 URL:
http://integrals.wolfram.com/index.jsp?expr=sin(x)&random=false
谁能告诉我我做错了什么,因为我抓住了一个NullReferenceException
?