Validation of XML with XSD

Introduction

This article explains about how to validate an XML document with XSD schema. The validation is performed by checking whether the XML document is a well-formed one by programmatically using .NET classes in C#.

XML document

An XML document contains elements, attributes, and values of primitive data types. For example, consider the following XML document:

<address xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:noNamespaceSchemaLocation="address.xsd"> 
<name>John Smith</name>
<street>109 Abbey Close</street>
<city>Hayes</city>
<country> UK</country>
</address>

XSD schema

XSD schema defines elements, attributes, and the relationship between them. It conforms to the W3C XML schema standards and recommendations. XSD schema for the above XML document, address.xsd, can be given as follows:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="address">
<xs:complexType>
<xs:sequence>
<xs:element name="name" type="xs:string"/>
<xs:element name="street" type="xs:string"/>
<xs:element name="city"type="xs:string"/>
<xs:element name="country"type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>

Validation event handler

The ValidationEventHandler event is used to define an event handler for receiving the notification about XSD schema validation errors. The validation errors and warnings are reported through the ValidationEventHandler call-back function. If any validation error occurs, an exception is thrown, and the XmlValidatingReader cannot be restarted.

XML validation with .NET classes

Microsoft .NET framework classes support the W3C XML schema recommendation. The classes used to validate the XML document are XmlTextReader, XmlSchemaCollection, and XmlValidatingReader. The sequence of steps to validate an XML document is given as follows:

1. The ValidationHandler event handler method is defined.
2. The XSD schema is being parsed using XmlTextReader class.
3. The parsed schema is added to the schema collection using XmlSchemaCollection class.
4. The schema collection is associated with XmlValidatingReader class.
5. The event handler method is associated with XmlValidatingReader class.
6. XmlValidatingReader class validates the XML document using the namespace URI specified when the schema was added to the collection.

Sample Code

The following example shows how to validate XML document against XSD schema by using Microsoft .NET framework classes.

using System;
using System.Collections;
using System.Data;
using System.IO;
using System.Xml;
using System.Xml.Schema;
using System.Text;

public class XMLValidator
{
    // Validation Error Count
    static int ErrorsCount = 0;

     // Validation Error Message
    static string ErrorMessage = "";

    public static void ValidationHandler(object sender,
                                         ValidationEventArgs args)
    {
        ErrorMessage = ErrorMessage + args.Message + "\r\n";
        ErrorsCount ++;
    }

    public void Validate(string strXMLDoc)
    {
        try
        {
            // Declare local objects
            XmlTextReader         tr   = null;
            XmlSchemaCollection   xsc  = null;
            XmlValidatingReader   vr   = null;

            // Text reader object
            tr  = new XmlTextReader(urlpath);
            xsc = new XmlSchemaCollection();
            xsc.Add(null, tr);

            // XML validator object

            vr = new XmlValidatingReader( strXMLDoc,
                         XmlNodeType.Document, null);

            vr.Schemas.Add(xsc);

            // Add validation event handler

            vr.ValidationType = ValidationType.Schema;
            vr.ValidationEventHandler +=
                     new ValidationEventHandler(ValidationHandler);

            // Validate XML data

            while(vr.Read());

            vr.Close();

            // Raise exception, if XML validation fails
            if (ErrorsCount > 0)
            {
                throw new Exception(ErrorMessage);
            }

            // XML Validation succeeded
            Console.WriteLine("XML validation succeeded.\r\n");
        }
        catch(Exception error)
        {
            // XML Validation failed
            Console.WriteLine("XML validation failed." + "\r\n" +
            "Error Message: " + error.Message);
        }
    }
}

Conclusion

This article explained about the XML document, XSD schema, and how to validate XML document against XSD schema using Microsoft .NET framework classes.

Author:
Syed Hameed
Wipro Technologies
Bangalore