HOW TO: Validate XML Fragments Against an XML Schema in Visual Basic .NET (317463)

SUMMARY

This step-by-step article describes how to use XmlValidatingReader and XMLSchemaCollection objects to validate an Extensible Markup Language (XML) fragment against an XML schema.

XmlValidatingReader implements the XmlReader class and provides support for XML data validation. The Schemas property of XmlValidatingReader connects the reader to the schema files cached in an XmlSchemaCollection. The ValidationType property of XmlValidatingReader specifies the type of validation the reader should perform. If you set the property to ValidationType.None, you create a nonvalidating reader.

You can only add XML Schema Definition Language (XSD) schemas and XML-Data Reduced (XDR) schemas to XmlSchemaCollection. Use the Add method with a namespace URI to load schemas. For XML schemas, the typical namespace URI is the targetNamespace property of the schema.

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Requirements

The following list outlines the recommended hardware, software, network infrastructure, and service packs that you will need:

• Microsoft Visual Studio .NET installed on a compatible Microsoft Windows operating system

This article assumes that you are familiar with the following topics:

• Visual Basic .NET
• Basic XML standards
• XSD schemas

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Create an XSD Schema

Paste the following code in a new text file named C:\Books.xsd:

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="urn:bookstore-schema" elementFormDefault="qualified" targetNamespace="urn:bookstore-schema">
	<xsd:element name="bookstore" type="bookstoreType" />
	<xsd:element name="comment" type="xsd:string" />
	<xsd:element name="author" type="authorName"/>
	<xsd:complexType name="authorName">
		<xsd:sequence>
			<xsd:element name="first-name" type="xsd:string" />
			<xsd:element name="last-name" type="xsd:string" />
		</xsd:sequence>
	</xsd:complexType>
	<xsd:complexType name="bookstoreType">
		<xsd:sequence maxOccurs="unbounded">
			<xsd:element name="book" type="bookType" />
			<xsd:element ref="comment" minOccurs="0" />
		</xsd:sequence>
	</xsd:complexType>
	<xsd:complexType name="bookType">
		<xsd:sequence>
			<xsd:element name="title" type="xsd:string" />
			<xsd:element ref="author" />
			<xsd:element name="price" type="xsd:decimal" />
		</xsd:sequence>
		<xsd:attribute name="genre" type="xsd:string" />
	</xsd:complexType>
	
</xsd:schema>
				

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Create a Visual Basic .NET Application

1. Create a new Visual Basic .NET Windows application.
2. Drag Button1 to Form1. Paste the following code to add a private member variable to Class Form1:

   Dim m_success As Boolean
   					

3. Paste the following sub procedure to create a ValidationEventHandler that raises validation errors in the XMLValidatingReader object:

   Public Sub ValidationEventHandle(ByVal sender As Object, ByVal args As ValidationEventArgs)
           m_success = False
           Console.WriteLine((ControlChars.CrLf & ControlChars.Tab & "Validation error: " & args.Message))
       End Sub 'ValidationEventHandle
   						

   NOTE: You must include an event handler to receive information about validation errors in the Data Type Definition (DTD), the XML-Data Reduced (XDR) schema, and the XML schema definition language (XSD) schema. The event handler receives an argument of type ValidationEventArgs that contains data related to this event.
   
   The callback handler can use the ValidationEventArgs.Severity property to guarantee that an XML instance document is validated against a schema. The Severity property enables you to distinguish between a validation error (Severity is equal to XmlSeverityType.Error) which indicates a fatal error, and a validation warning (Severity is equal to XmlSeverityType.Warning) which indicates that no schema information is available.
4. Paste the following code in the Button1_Click event procedure:

           Dim reader As XmlValidatingReader = Nothing
           Dim myschema As New XmlSchemaCollection()
   
           Try
               'Create the XML fragment to be parsed.
               Dim xmlFrag As String = "<author  xmlns='urn:bookstore-schema' xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'>" & _
                               "<first-name>Herman</first-name>" & _
                              "<last-name>Melville</last-name>" & _
                            "</author>"
   
               'Create the XmlParserContext.
               Dim context As New XmlParserContext(Nothing, Nothing, "", XmlSpace.None)
              
               'Implement the reader. 
               reader = New XmlValidatingReader(xmlFrag, XmlNodeType.Element, context)
               'Add the schema.
               myschema.Add("urn:bookstore-schema", "Books.xsd")
   
               'Set the schema type and add the schema to the reader.
               reader.ValidationType = ValidationType.Schema
               reader.Schemas.Add(myschema)
   
               'Add the handler to raise the validation event.
               AddHandler reader.ValidationEventHandler, AddressOf Me.ValidationEventHandle
   
               While reader.Read
   
               End While
               Console.WriteLine("Completed validating xmlfragment")
   
           Catch XmlExp As XmlException
               Console.WriteLine(XmlExp.Message)
           Catch XmlSchExp As XmlSchemaException
               Console.WriteLine(XmlSchExp.Message)
           Catch GenExp As Exception
               Console.WriteLine(GenExp.Message)
           End Try
       End Sub
   					

5. When the following message is displayed in the output window, the XML fragment is a valid element:

   Completed validating xmlfragment
   					

NOTE: The XMLValidatingReader object validates only the type declarations and the top level elements in the XML Schema. XML fragments, such as sub elements, are considered to be local. You cannot pass XML fragments to XmlValidatingReader for direct validation unless you declare the XML fragments as top-level elements and set the reference at the required level.

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