How Do I...Apply Validation When Reading XML?

This sample illustrates how to apply validation when reading and parsing XML data using the XmlReader class. Validation is the process of enforcing rules on the XML content either via a Document Type Definition (DTD) or a XML Schema Definition language (XSD) schema.

The XmlReader class can provide validation of a DTD or an XML Schema when parsing the document. XmlReader objects are created with the Create method. Settings on the XmlReaderSettings class determine what features are supported on the XmlReader object.

This sample validates several XML files using XML Schema validation. It shows validation successes and failures.

The following code sets a ValidationEventHandler to handle validation error results. If a handler is not provided, an XmlSchemaException is thrown when a validation error occurs.

XmlReaderSettings settings = new XmlReaderSettings(); settings.ValidationEventHandler += new ValidationEventHandler(this.ValidationEventHandler); Dim settings As New XmlReaderSettings() AddHandler settings.ValidationEventHandler, AddressOf Me.ValidationEventHandler XmlReaderSettings^ settings = new XmlReaderSettings(); settings->ValidationEventHandler += gcnew ValidationEventHandler (this, &ValidationReadingXMLSample::ValidationEventHandle); XmlReaderSettings settings = new XmlReaderSettings(); settings.add_ValidationEventHandler (new ValidationEventHandler(this.ValidationEventHandler));

The validation settings are specified with the ValidationType.DTD or ValidationType.Schema properties on the XmlReaderSettings class. The default setting for both properties is ValidationType.None so the XmlReader does not validate. The following code creates a DTD validating XmlReader object for the Validate subroutine.

settings.ValidationType = ValidationType.Schema; settings.Schemas.Add("schema.xsd", XmlReader.Create(document5)); using (XmlReader xmlValidatingReader = XmlReader.Create(document3, settings)) { while (xmlValidatingReader.Read()) { } } settings.ValidationType = ValidationType.Schema settings.Schemas.Add("schema.xsd", XmlReader.Create(document5)) Using XmlValidatingReader As XmlReader = XmlReader.Create(document3, settings) While XmlValidatingReader.Read() End While End Using settings->ValidationType = ValidationType::Schema; settings->Schemas->Add("schema.xsd", XmlReader::Create(document5)); xmlValidatingReader = XmlReader::Create(document3, settings); while (xmlValidatingReader->Read()) { } settings.set_ValidationType(ValidationType.Schema); settings.get_Schemas().Add("schema.xsd", XmlReader.Create(document5)); xmlValidatingReader = XmlReader.Create(document3, settings); while (xmlValidatingReader.Read()) {}

As the XmlReader parses the XML document, it calls the validation event handler when it encounters a validation error. The parser only stops if the parser finds data that is not well-formed. By not stopping for validation errors, you are able to find all the validation errors in one pass without having to repeatedly parse the XML document.

Summary

1. Validation can either be against a DTD or an XML Schema. The validation setting is specified with the DtdValidate or the XsdValidate properties on the XmlReaderSettings class.
2. Validation is performed during the read and parsing operations.
3. A ValidationEventHandler must be set to receive notification of validation errors.
4. Validation errors do not stop parsing. Parsing only stops if there is an error when the file is not well-formed XML. This enables all errors to be discovered in a single pass.