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Validating an XML Document in the DOM

The 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 XmlReader to the Load method of the XmlDocument class, or validate a previously unvalidated XML document in the DOM using the Validate method of the XmlDocument class.

The XmlDocument class validates the XML data as it is loaded into the DOM when a validating XmlReader is passed to the Load method of the 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.

To create an XML schema-validating XmlReader, follow these steps.

  1. Construct a new XmlReaderSettings instance.

  2. Add an XML schema to the Schemas property of the XmlReaderSettings instance.

  3. Specify Schema as the ValidationType.

  4. Optionally specify ValidationFlags and a ValidationEventHandler to handle schema validation errors and warnings encountered during validation.

  5. Finally, pass the XmlReaderSettings object to the Create method of the XmlReader class along with the XML document, creating a schema-validating XmlReader.

In the code example that follows, a schema-validating 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.

using System;
using System.Xml;
using System.Xml.Schema;

class XmlDocumentValidationExample
{
    static void Main(string[] args)
    {
        try
        {
            // Create a schema validating XmlReader.
            XmlReaderSettings settings = new XmlReaderSettings();
            settings.Schemas.Add("http://www.contoso.com/books", "contosoBooks.xsd");
            settings.ValidationEventHandler += new ValidationEventHandler(ValidationEventHandler);
            settings.ValidationFlags = settings.ValidationFlags | XmlSchemaValidationFlags.ReportValidationWarnings;
            settings.ValidationType = ValidationType.Schema;

            XmlReader reader = XmlReader.Create("contosoBooks.xml", settings);

            // The XmlDocument validates the XML document contained 
            // in the XmlReader as it is loaded into the DOM.
            XmlDocument document = new XmlDocument();
            document.Load(reader);

            // Make an invalid change to the first and last  
            // price elements in the XML document, and write 
            // the XmlSchemaInfo values assigned to the price 
            // element during load validation to the console.
            XmlNamespaceManager manager = new XmlNamespaceManager(document.NameTable);
            manager.AddNamespace("bk", "http://www.contoso.com/books");

            XmlNode priceNode = document.SelectSingleNode(@"/bk:bookstore/bk:book/bk:price", manager);

            Console.WriteLine("SchemaInfo.IsDefault: {0}", priceNode.SchemaInfo.IsDefault);
            Console.WriteLine("SchemaInfo.IsNil: {0}", priceNode.SchemaInfo.IsNil);
            Console.WriteLine("SchemaInfo.SchemaElement: {0}", priceNode.SchemaInfo.SchemaElement);
            Console.WriteLine("SchemaInfo.SchemaType: {0}", priceNode.SchemaInfo.SchemaType);
            Console.WriteLine("SchemaInfo.Validity: {0}", priceNode.SchemaInfo.Validity);

            priceNode.InnerXml = "A";

            XmlNodeList priceNodes = document.SelectNodes(@"/bk:bookstore/bk:book/bk:price", manager);
            XmlNode lastprice = priceNodes[priceNodes.Count - 1];

            lastprice.InnerXml = "B";

            // Validate the XML document with the invalid changes. 
            // The invalid changes cause schema validation errors.
            document.Validate(ValidationEventHandler);

            // Correct the invalid change to the first price element.
            priceNode.InnerXml = "8.99";

            // Validate only the first book element. The last book 
            // element is invalid, but not included in validation.
            XmlNode bookNode = document.SelectSingleNode(@"/bk:bookstore/bk:book", manager);
            document.Validate(ValidationEventHandler, bookNode);
        }
        catch (XmlException ex)
        {
            Console.WriteLine("XmlDocumentValidationExample.XmlException: {0}", ex.Message);
        }
        catch(XmlSchemaValidationException ex)
        {
            Console.WriteLine("XmlDocumentValidationExample.XmlSchemaValidationException: {0}", ex.Message);
        }
        catch (Exception ex)
        {
            Console.WriteLine("XmlDocumentValidationExample.Exception: {0}", ex.Message);
        }
    }

    static void ValidationEventHandler(object sender, System.Xml.Schema.ValidationEventArgs args)
    {
        if (args.Severity == XmlSeverityType.Warning)
            Console.Write("\nWARNING: ");
        else if (args.Severity == XmlSeverityType.Error)
            Console.Write("\nERROR: ");

        Console.WriteLine(args.Message);
    }
}

The example takes the contosoBooks.xml file as input.

<?xml version="1.0" encoding="utf-8" ?>
<bookstore xmlns="http://www.contoso.com/books">
    <book genre="autobiography" publicationdate="1981-03-22" ISBN="1-861003-11-0">
        <title>The Autobiography of Benjamin Franklin</title>
        <author>
            <first-name>Benjamin</first-name>
            <last-name>Franklin</last-name>
        </author>
        <price>8.99</price>
    </book>
    <book genre="novel" publicationdate="1967-11-17" ISBN="0-201-63361-2">
        <title>The Confidence Man</title>
        <author>
            <first-name>Herman</first-name>
            <last-name>Melville</last-name>
        </author>
        <price>11.99</price>
    </book>
    <book genre="philosophy" publicationdate="1991-02-15" ISBN="1-861001-57-6">
        <title>The Gorgias</title>
        <author>
            <name>Plato</name>
        </author>
        <price>9.99</price>
    </book>
</bookstore>

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

<?xml version="1.0" encoding="utf-8"?>
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="http://www.contoso.com/books" xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xs:element name="bookstore">
        <xs:complexType>
            <xs:sequence>
                <xs:element maxOccurs="unbounded" name="book">
                    <xs:complexType>
                        <xs:sequence>
                            <xs:element name="title" type="xs:string" />
                            <xs:element name="author">
                                <xs:complexType>
                                    <xs:sequence>
                                        <xs:element minOccurs="0" name="name" type="xs:string" />
                                        <xs:element minOccurs="0" name="first-name" type="xs:string" />
                                        <xs:element minOccurs="0" name="last-name" type="xs:string" />
                                    </xs:sequence>
                                </xs:complexType>
                            </xs:element>
                            <xs:element name="price" type="xs:decimal" />
                        </xs:sequence>
                        <xs:attribute name="genre" type="xs:string" use="required" />
                        <xs:attribute name="publicationdate" type="xs:date" use="required" />
                        <xs:attribute name="ISBN" type="xs:string" use="required" />
                    </xs:complexType>
                </xs:element>
            </xs:sequence>
        </xs:complexType>
    </xs:element>
</xs:schema>

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

  • In the above example, the ValidationEventHandler is called whenever an invalid type is encountered. If a ValidationEventHandler is not set on the validating XmlReader,an XmlSchemaValidationException is thrown when Load 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 XmlDocument object with an associated schema that defines default values, the XmlDocument treats these defaults as if they appeared in the XML document. This means that the IsEmptyElement 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.

The Validate method of the XmlDocument class validates the XML data loaded in the DOM against the schemas in the XmlDocument object's Schemas 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 Validate method of the XmlDocument class. The Validate method validates the XML document against the XML schemas contained in the Schemas property of the 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.

using System;
using System.Xml;
using System.Xml.Schema;

class XmlDocumentValidationExample
{
    static void Main(string[] args)
    {
        try
        {
            // Create a new XmlDocument instance and load 
            // the XML document into the DOM.
            XmlDocument document = new XmlDocument();
            document.Load("contosoBooks.xml");

            // Add the XML schema for the XML document to the 
            // Schemas property of the XmlDocument.
            document.Schemas.Add("http://www.contoso.com/books", "contosoBooks.xsd");

            // Validate the XML document loaded into the DOM.
            document.Validate(ValidationEventHandler);

            // Make an invalid change to the first and last  
            // price elements in the XML document, and write 
            // the XmlSchemaInfo values assigned to the price 
            // element during validation to the console.
            XmlNamespaceManager manager = new XmlNamespaceManager(document.NameTable);
            manager.AddNamespace("bk", "http://www.contoso.com/books");

            XmlNode priceNode = document.SelectSingleNode(@"/bk:bookstore/bk:book/bk:price", manager);

            Console.WriteLine("SchemaInfo.IsDefault: {0}", priceNode.SchemaInfo.IsDefault);
            Console.WriteLine("SchemaInfo.IsNil: {0}", priceNode.SchemaInfo.IsNil);
            Console.WriteLine("SchemaInfo.SchemaElement: {0}", priceNode.SchemaInfo.SchemaElement);
            Console.WriteLine("SchemaInfo.SchemaType: {0}", priceNode.SchemaInfo.SchemaType);
            Console.WriteLine("SchemaInfo.Validity: {0}", priceNode.SchemaInfo.Validity);

            priceNode.InnerXml = "A";

            XmlNodeList priceNodes = document.SelectNodes(@"/bk:bookstore/bk:book/bk:price", manager);
            XmlNode lastprice = priceNodes[priceNodes.Count - 1];

            lastprice.InnerXml = "B";

            // Validate the XML document with the invalid changes. 
            // The invalid changes cause schema validation errors.
            document.Validate(ValidationEventHandler);

            // Correct the invalid change to the first price element.
            priceNode.InnerXml = "8.99";

            // Validate only the first book element. The last book 
            // element is invalid, but not included in validation.
            XmlNode bookNode = document.SelectSingleNode(@"/bk:bookstore/bk:book", manager);
            document.Validate(ValidationEventHandler, bookNode);
        }
        catch (XmlException ex)
        {
            Console.WriteLine("XmlDocumentValidationExample.XmlException: {0}", ex.Message);
        }
        catch(XmlSchemaValidationException ex)
        {
            Console.WriteLine("XmlDocumentValidationExample.XmlSchemaValidationException: {0}", ex.Message);
        }
        catch (Exception ex)
        {
            Console.WriteLine("XmlDocumentValidationExample.Exception: {0}", ex.Message);
        }
    }

    static void ValidationEventHandler(object sender, System.Xml.Schema.ValidationEventArgs args)
    {
        if (args.Severity == XmlSeverityType.Warning)
            Console.Write("\nWARNING: ");
        else if (args.Severity == XmlSeverityType.Error)
            Console.Write("\nERROR: ");

        Console.WriteLine(args.Message);
    }
}

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.

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 ValidationEventHandler. If a ValidationEventHandler was assigned to the XmlReaderSettings instance, or passed to the Validate method of the XmlDocument class, the ValidationEventHandler will handle schema validation errors; otherwise an XmlSchemaValidationException is raised when a schema validation error is encountered.

Note Note

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

Schema validation warnings are not reported unless the ReportValidationWarnings flag is specified to the XmlReaderSettings object.

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

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