Accessing and Manipulating XML Data in Microsoft .NET Framework

The XML Schema Object Model

The structure of XML documents is based on rules that are specified in an XML Schema Definition (XSD) file also known as an XML Schema. An XSD file consists of the definitions of elements, attributes, and data types. XML Schema is used to create, define and validate the structure of XML documents. The structure of an XML document can be specified by the names of elements that are used in the XML documents, and the structure and types of elements that must be valid for a specific schema.

An XML schema is an XML file that has an .xsd file name extension and uses valid XML objects to describe the contents of XML document. The structure of the XML document can be created by using simpleType and complexType elements. A simpleType element is used to define built-in data types or existing simple types and contains no element or attribute. A complexType element consists of elements and attributes.

The XML Schema Object Model (SOM) derived from the System.Xml.Schema namespace consists of a set of classes that read the schema definition from a file and also create the schema definition files programmatically. When a schema is created, it needs to be validated and compiled before writing it to a file. The XML SOM can load and save valid XSD schemas from and to files. In-memory schemas can be created by using strongly typed classes. The XmlSchemaCollection class is used to cache and retrieve schemas, and the XmlValidatingReader class is used to validate XML instance documents.

After creating a schema definition file, the XML Schema Object Model is used to edit files, which is similar to the way that the XML DOM uses for editing files. For validating an XSD file, the Compile method of the XmlSchema class is used to verify whether the schema is semantically correct. During the validation process, a validation callback is used if the parser gives an error or a warning, and simultaneously the ValidationEventHandler event is raised for the semantic validation checking of XML Schema.

Additionally, the XmlSchema.Read method is used to load schemas from a file. The XML SOM is also used to read and write XSD language schemas from files or other sources using the XmlTextReader, XmlTextWriter, and XmlSchema classes. The following code example in Visual Basic .NET shows how to read XML Schema from a file and then to write the schema to a new file.

After creating, reading and writing XML data of an XML document, and creating XML schemas, now comes the use of the XmlValidatingReader class to validate an XML document. The following sub-heading is a description of an XML validation.