验证XML代码时出现意外异常

在.NET/C中,我想验证一些html代码。例如,我有以下HTML:

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
  <head><title></title></head>
  <body>
   CDATA section number 1?
  </body>
</html>

我有以下C代码:

string htmlCode = ... // for instance the html above
var settings = new XmlReaderSettings { ValidationType = ValidationType.DTD };
settings.ValidationEventHandler += delegate(object s, ValidationEventArgs e)
{
    throw new XmlException(e.Message);
};
using (var srdr = new StringReader(htmlCode))
using (var xrdr = new XmlTextReader(srdr))
using (var vrdr = XmlReader.Create(xrdr, settings))
{
    try
    {
        while (vrdr.Read()) { }
    }
    catch (XmlException ex)
    {
        // do some stuff
    }
}

运行此代码时出现以下异常:

位于System.Net.HttpWebRequest.GetResponse()