public static void Main()
@"<INPUT class=box id='ss' value=John maxLength=16 size=16 name=user_name>
box1' value=Tony maxLength=16 size=16 name=user_name>
var htmlDoc = new HtmlDocument();
var htmlNodes = htmlDoc.DocumentNode.SelectNodes("//input[@class='box1']");
Console.WriteLine(htmlNodes == null);
htmlNodes = htmlDoc.DocumentNode.SelectNodes("//input[@class='box']");
Console.WriteLine(htmlNodes == null);