.NET:如何在没有DOCTYPE声明的情况下使用DTD验证XML文件

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE rootElement [
    <!ELEMENT rootElement (childElement*)>
    <!ELEMENT childElement (#PCDATA)>
    <!ATTLIST childElement attribute CDATA #REQUIRED>
]>
<rootElement>
   <childElement attribute="value">Text</childElement>
</rootElement>

using System;
using System.Xml;

class Program
{
    static void Main(string[] args)
    {
        string xml = @"<?xml version=""1.0"" encoding=""UTF-8""?>
<!DOCTYPE rootElement [
    <!ELEMENT rootElement (childElement*)>
    <!ELEMENT childElement (#PCDATA)>
    <!ATTLIST childElement attribute CDATA #REQUIRED>
]>
<rootElement>
   <childElement attribute=""value"">Text</childElement>
</rootElement>";

        XmlReaderSettings settings = new XmlReaderSettings();
        settings.ValidationType = ValidationType.DTD;
        settings.ValidationEventHandler += new ValidationEventHandler(ValidationEventHandler);

        XmlReader reader = XmlReader.Create(new System.IO.StringReader(xml), settings);

        while (reader.Read()) { }
    }

    static void ValidationEventHandler(object sender, ValidationEventArgs e)
    {
        Console.WriteLine("Validation error: " + e.Message);
    }
}