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Learn more about: Validating an XML Document in the DOM
Validating an XML Document in the DOM
03/30/2017
csharp
vb
cpp
2c61c920-d0f8-4c72-bfcc-6524570f3060

Validating an XML Document in the DOM

Thexref:System.Xml.XmlDocument class does not validate the XML in the Document Object Model (DOM) against an XML Schema definition language (XSD) schema or document type definition (DTD) by default; the XML is only verified to be well-formed.

To validate the XML in the DOM, you can validate the XML as it is loaded into the DOM by passing a schema-validating xref:System.Xml.XmlReader to the xref:System.Xml.XmlDocument.Load%2A method of the xref:System.Xml.XmlDocument class, or validate a previously unvalidated XML document in the DOM using the xref:System.Xml.XmlDocument.Validate%2A method of the xref:System.Xml.XmlDocument class.

Validating an XML Document As It Is Loaded into the DOM

The xref:System.Xml.XmlDocument class validates the XML data as it is loaded into the DOM when a validating xref:System.Xml.XmlReader is passed to the xref:System.Xml.XmlDocument.Load%2A method of the xref:System.Xml.XmlDocument class.

After successful validation, schema defaults are applied, text values are converted to atomic values as necessary, and type information is associated with validated information items. As a result, typed XML data replaces previously untyped XML data.

Creating an XML Schema-Validating XmlReader

To create an XML schema-validating xref:System.Xml.XmlReader, follow these steps.

  1. Construct a new xref:System.Xml.XmlReaderSettings instance.

  2. Add an XML schema to the xref:System.Xml.XmlReaderSettings.Schemas%2A property of the xref:System.Xml.XmlReaderSettings instance.

  3. Specify Schema as the xref:System.Xml.XmlReaderSettings.ValidationType%2A.

  4. Optionally specify xref:System.Xml.XmlReaderSettings.ValidationFlags%2A and a xref:System.Xml.XmlReaderSettings.ValidationEventHandler to handle schema validation errors and warnings encountered during validation.

  5. Finally, pass the xref:System.Xml.XmlReaderSettings object to the xref:System.Xml.XmlReader.Create%2A method of the xref:System.Xml.XmlReader class along with the XML document, creating a schema-validating xref:System.Xml.XmlReader.

Example

In the code example that follows, a schema-validating xref:System.Xml.XmlReader validates the XML data loaded into the DOM. Invalid modifications are made to the XML document and the document is then revalidated, causing schema validation errors. Finally, one of the errors is corrected, and then part of the XML document is partially validated.

[!code-cppXmlDocumentValidation.Load#1] [!code-csharpXmlDocumentValidation.Load#1] [!code-vbXmlDocumentValidation.Load#1]

The example takes the contosoBooks.xml file as input.

[!code-xmlXPathXMLExamples#2]

The example also takes the contosoBooks.xsd file as input.

[!code-xmlXPathXMLExamples#3]

Consider the following when validating XML data as it is loaded into the DOM.

  • In the above example, the xref:System.Xml.XmlReaderSettings.ValidationEventHandler is called whenever an invalid type is encountered. If a xref:System.Xml.XmlReaderSettings.ValidationEventHandler is not set on the validating xref:System.Xml.XmlReader,an xref:System.Xml.Schema.XmlSchemaValidationException is thrown when xref:System.Xml.XmlDocument.Load%2A is called if any attribute or element type does not match the corresponding type specified in the validating schema.

  • When an XML document is loaded into an xref:System.Xml.XmlDocument object with an associated schema that defines default values, the xref:System.Xml.XmlDocument treats these defaults as if they appeared in the XML document. This means that the xref:System.Xml.XmlReader.IsEmptyElement%2A property always returns false for an element that was defaulted from the schema. Even if in the XML document, it was written as an empty element.

Validating an XML Document in the DOM

The xref:System.Xml.XmlDocument.Validate%2A method of the xref:System.Xml.XmlDocument class validates the XML data loaded in the DOM against the schemas in the xref:System.Xml.XmlDocument object's xref:System.Xml.XmlDocument.Schemas%2A property. After successful validation, schema defaults are applied, text values are converted to atomic values as necessary, and type information is associated with validated information items. As a result, typed XML data replaces previously untyped XML data.

The example below is similar to the example in "Validating an XML Document As It Is Loaded into the DOM" above. In this example, the XML document is not validated as it is loaded into the DOM, but rather is validated after it has been loaded into the DOM using the xref:System.Xml.XmlDocument.Validate%2A method of the xref:System.Xml.XmlDocument class. The xref:System.Xml.XmlDocument.Validate%2A method validates the XML document against the XML schemas contained in the xref:System.Xml.XmlDocument.Schemas%2A property of the xref:System.Xml.XmlDocument. Invalid modifications are then made to the XML document, and the document is then revalidated, causing schema validation errors. Finally, one of the errors is corrected, and then part of the XML document is partially validated.

[!code-csharpXmlDocumentValidation.Validate#1] [!code-vbXmlDocumentValidation.Validate#1]

The example takes as input the contosoBooks.xml and contosoBooks.xsd files referred to in "Validating an XML Document as it is Loaded into the DOM" above.

Handling Validation Errors and Warnings

XML schema validation errors are reported when validating XML data loaded in the DOM. You are notified about all schema validation errors found while validating the XML data as it is being loaded, or when validating a previously unvalidated XML document.

Validation errors are handled by the xref:System.Xml.Schema.ValidationEventHandler. If a xref:System.Xml.Schema.ValidationEventHandler was assigned to the xref:System.Xml.XmlReaderSettings instance, or passed to the xref:System.Xml.XmlDocument.Validate%2A method of the xref:System.Xml.XmlDocument class, the xref:System.Xml.Schema.ValidationEventHandler will handle schema validation errors; otherwise an xref:System.Xml.Schema.XmlSchemaValidationException is raised when a schema validation error is encountered.

Note

The XML data is loaded into the DOM despite schema validation errors unless your xref:System.Xml.Schema.ValidationEventHandler raises an exception to stop the process.

Schema validation warnings are not reported unless the xref:System.Xml.Schema.XmlSchemaValidationFlags.ReportValidationWarnings flag is specified to the xref:System.Xml.XmlReaderSettings object.

For examples illustrating the xref:System.Xml.Schema.ValidationEventHandler, see "Validating an XML Document As It Is Loaded into the DOM" and "Validating an XML Document in the DOM" above.

See also