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

Limitations of the XML DOM Eventing ModelAlthough you receive notifications before and after an action takes place, you can't alterthe predefined flow of operations. In other words, you can perform any action whilehandling the event, but you can't cancel the ongoing operation. This also means thatyou can't just skip some nodes based on run-time conditions. In fact, the event handlerfunction is void, and all the arguments passed with the event data structure are readonly.Programmers have no way to pass information back to the reader and skip thecurrent node. There is only one way in which the event handler can affect the behaviorof the reader. If the event handler throws an exception, the reader will stop working. Inthis case, however, the XML DOM will not be built.Selecting Nodes by QueryAs mentioned, the XML DOM provides a few ways to traverse the document forest tolocate a particular node. The ChildNodes property returns a linked list formed by thechild nodes placed at the same level. You move back and forth in this list using theNextSibling and PreviousSibling methods.You can also enumerate the contents of the ChildNodes list using a foreach-styleenumerator. This enumerator is built into the XmlDocument class and returned ondemand by the GetEnumerator method, as shown here:foreach(XmlNode n in node.ChildNodes){// Do something}Direct Access to ElementsTheGetElementById method returns the first child node below the current node that hasan ID attribute with the specified value. Note that ID is a particular XML type and notsimply an attribute with that name. An attribute can be declared as an ID only in anXML Schema Definition (XSD) or a DTD schema. The following XML fragment definesan employeeid attribute of type ID. The attribute belongs to the Employee node.A corresponding XML node might look like this:As you can see, the source XML is apparently unaffected by the use of an ID attribute.An ID attribute can be seen as an XML primary key, and the GetElementById method—part of the W3C DOM specification—represents the search method that applicationsuse to locate nodes. The following code retrieves the node element in the documentwhose ID attribute (employeeid) matches the specified value:employeeNode = node.GetElementById("1");If you call GetElementById on a node whose children have no ID attributes or matchingvalues, the method returns null. The search for a matching node stops when the firstmatch is found.Another query method at your disposal is GetElementsByTagName. As the namesuggests, this method returns a list of nodes with the specified name.GetElementsByTagName looks similar to ChildNodes but differs in one aspect.Whereas ChildNodes returns all the child nodes found, including all elements andleaves, GetElementsByTagName returns only the element nodes with a particularname. The name specified can be expressed as a local as well as a namespacequalifiedname.188