Validating XML in .Net

There might be a case where you are importing XML file from somewhere and need to validate it before performing some operations on the XML file. Though, there can be many ways to do so, probably the best is using XMLValidatingReader or XMLReaderSettings. It provides you mechanism to provide a XML file, add a schema and validate the XML file supplied against the schema. It also provides an event (ValidationEventHandler) which is fired as soon as an exception occurs in validating the XMLDocument. Please note that this class has been marked obsolete in .Net framework 2.0 which recommends using XMLReader.Create() with proper XMLReaderSettings for validating the document. We will be looking at both the cases i.e. using XMLValidatingReader as well as XMLReader.Create().

Lets assume that the XML file that we wish to validate has details about orders. Each of the Order contains the Order ID, details about the Customer, Discount percent given and the details about Products. Following graphic(click for larger image) shows the XML file that we are using:




Orders.xml

To validate the XML shown above, we have the schema. The schema for the XML can be found here.

Validating XML using XmlValidatingReader:

To validate the XML file against a schema, we can use XmlValidatingReader. The steps involved are:

1. Create instance of XmlValidatingReader using the XML file to validate.
2. Create an object of XmlSchema using the schema file to validate against.
3. Hookup event for XmlValidatingReader.ValidationEventHandler.
4. Read the Xml to the end using XmlValidatingReader.

Please note that in absence of event mentioned in point (3), an exception will be thrown on the first error encountered in XML file where as in case of having the ValidationEventHandler hooked up, all the errors in the XML file can be displayed. Following code shows the steps mentioned above:

using (XmlValidatingReader xmlValidatingReader = new XmlValidatingReader(new XmlTextReader("Orders.xml")))
            {
                // Create the schema object to validate XML files
                XmlSchema xmlSchema = XmlSchema.Read(new XmlTextReader("Orders.xsd"), new ValidationEventHandler(Schema_ValidationError));
                // Add to the collection of schemas for XmlValidatingReader
                xmlValidatingReader.Schemas.Add(xmlSchema);
                // Attach an event which will be filed on validating error
                xmlValidatingReader.ValidationEventHandler += new ValidationEventHandler(xmlValidatingReader_ValidationEventHandler);
                // Read the XML to the end
                while (xmlValidatingReader.Read()) ;
                Console.WriteLine("\nFinished validating XML file....");
            }

Validating XML using XmlReaderSettings:

Validating XML using XmlReaderSettings follows almost the same path as that for XmlValidatingReader, with a difference that here we will be using XmlReaderSettings to pass in the schema details. Following are the steps to using XmlReaderSettings for validating an XML file:

1. Create a XmlSchema object using the schema file.
2. Create an object for XmlReaderSettings.
3. Set the ValidationType of XmlReaderSettings as "Schema".
4. Add the XmlSchema to the collection of schemas of XmlReaderSettings.
5. Attach the event for XmlReaderSettings.ValidationEventHandler.
6. Create an instance of XmlReader using XmlReader.Create() and passing in XML file to validate.
7. Read the XmlFile to the end.

In this case also, absence of ValidationEventHandler will cause exception on the first error encountered in reading the XML file. Following code shows the above mentioned steps:

            // Create the schema object
            XmlSchema xmlSchema = XmlSchema.Read(new XmlTextReader("Orders.xsd"), new ValidationEventHandler(Schema_ValidationError));
            // Create reader settings
            XmlReaderSettings xmlReaderSettings = new XmlReaderSettings();
            // Set validation type to schema
            xmlReaderSettings.ValidationType = ValidationType.Schema;
            // Add to the collection of schemas in readerSettings
            xmlReaderSettings.Schemas.Add(xmlSchema);
            // Attach event handler whic will be fired when validation error occurs
            xmlReaderSettings.ValidationEventHandler += new ValidationEventHandler(xmlReaderSettings_ValidationEventHandler);
            // Create object of XmlReader using XmlReaderSettings
            using (XmlReader xmlReader = XmlReader.Create(new XmlTextReader("Orders.xml"), xmlReaderSettings))
            {
                // Read XML to the end
                while (xmlReader.Read()) ;
                Console.WriteLine("\nFinished validating XML file....");
            }

The code behaves the same in both the cases. However, there is a small difference in how the exception message is displayed. Using theXmlValidatingReader tells about the element/attribute with incorrect value as per the data type, but using XmlReaderSetting provides a detailed error showing the incorrect value along with the expected datatype.

I hope I was able to provide most of you with a good insight on validating XML files using schemas. If you have been using some other method, please do share it here.