运行时出现错误。次错误的地址为0x53af7e06,在线程0x1fc4上。错误代码为0xc0000005

HtmlAgilityPack.HtmlDocument listhtmldoc = new HtmlAgilityPack.HtmlDocument();
HttpClient httpclient = new HttpClient();
httpclient.Url = “http://wap.ganji.com/zz/fang1/”;
string source = httpclient.GetString();
listhtmldoc.LoadHtml(source);
程序报错，