Applied XML Programming for Microsoft .NET.pdf - Csbdu.in

The most restrictive condition in qualifying for XML serialization is not having circularreferences. A lot of relatively complex .NET Framework classes can't be serialized toXML for this reason. Want an illustrious example? Consider the DataTable class.If you try to serialize an instance of the DataTable class, you get a fairly unclear errormessage. Try the following code:DataTable dt = new DataTable();XmlSerializer ser = new XmlSerializer(typeof(DataTable));ser.Serialize(writer, dt);The debugger stops on the constructor line and displays a message about a certainerror that occurred during reflection of the DataTable class. Like many other MicrosoftADO.NET and XML classes, the DataTable class has circular references. For example,DataTable contains the Rows property, which is a collection of DataRow objects. Inturn, each DataRow object has a Table property that points to the parent DataTableobject. This is clearly a circular reference, and, as such, is an appropriate justificationfor the run-time error.Why Is the DataSet Object XML-Serializable?The DataSet class (and the XmlNode and XmlElement classes) contains at least onecircular reference—specifically, the Tables collection, whose child DataTable objectsreference the parent DataSet object. Nevertheless, the DataSet object is serializablethrough the XmlSerializer class. Why is this so?The internal module that imports the XML schema for the type to serialize—the samemodule that does not handle circular references—specifically checks for the DataSettype. If the object turns out to be a DataSet object, the standard schema importationprocess aborts, and an alternative schema is applied. The schema importer uses themethods of the IXmlSerializable interface to serialize and deserialize a DataSet object.The MSDN documentation only touches on the IXmlSerializable interface, which isdefined in the System.Xml.Serialization namespace. This interface is not intended tobe used by applications—at least not yet. IXmlSerializable defines three methods:GetSchema, ReadXml, and WriteXml. Despite their names, these ReadXml andWriteXml methods have nothing to do with the methods we saw in Chapter 9 andChapter 10. Serialization methods are void, private, and accept only a single Xml-Reader argument.You can serialize XML classes with no circular references, the default constructor, andat least one public property. If the class implements the ICollection or IEnumerableinterface, other constraints apply. In addition to these classes, the XML serializersupports three more classes as an exception to the previous rules: DataSet, XmlNode,and XmlElement.The XmlSerializerNamespaces ClassA few of the Serialize overloads can take an extra parameter that denotes the XMLnamespaces and prefixes that the XmlSerializer uses to generate qualified names. TheXmlRootAttribute class we examined in the section "Configuring the Root Node," onpage 486, is useful for defining the default namespace but provides no way for you touse more namespaces and prefixes.The XmlSerializerNamespaces class can be used to cache multiple namespace URIsand prefixes that the target class will reference through attributes. You populate thenamespace container as follows:XmlSerializer ser = new XmlSerializer(typeof(Employee));XmlSerializerNamespaces ns = new XmlSerializerNamespaces();397