How do I validate an XML document against a DTD in C#?

Instead of creating XmlValidatingReader class directly, you should construct an appropriate XmlReaderSettings object and pass it as an argument to the XmlReader.Create method:

var settings = new XmlReaderSettings { ValidationType = ValidationType.DTD };
settings.ValidationEventHandler += new ValidationEventHandler(OnValidationEvent);
var reader = XmlReader.Create("file.xml", settings);

The rest is unchanged.

P.S. OnValidationEvent is the name of the method you declare to handle validation events. Obviously, you can remove the line if you don't want to subscribe to validation events raised by the XmlReader.

var messages = new StringBuilder();
var settings = new XmlReaderSettings { ValidationType = ValidationType.DTD };
settings.ValidationEventHandler += (sender, args) => messages.AppendLine(args.Message);
var reader = XmlReader.Create("file.xml", settings);

if (messages.Length > 0)
{
    // Log Validation Errors
    // Throw Exception
    // Etc.
}

ValidationEventHandler

Lambda Expressions

Type Inference