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

While the secondary thread works, the user can freely use the application's userinterface and the huge size of the XML file is no longer a serious issue—at least as itpertains to loading.Extracting XML DOM SubtreesYou normally build the XML DOM by loading the entire XML document into memory.However, the XmlDocument class also provides the means to extract only a portion ofthe document and return it as an XML DOM subtree. The key method to achieve thisresult is ReadNode, shown here:public virtual XmlNode ReadNode(XmlReader reader);The ReadNode method begins to read from the current position of the given reader anddoesn't stop until the end tag of the current node is reached. The reader is then leftimmediately after the end tag. For the method to work, the reader must be positionedon an element or an attribute node.ReadNode returns an XmlNode object that contains the subtree representingeverything that has been read, including attributes. ReadNode is different fromChildNodes in that it recursively processes children at any level and does not stop atthe first level of siblings.Visiting an XML DOM SubtreeSo far, we've examined ways to get XML DOM objects out of an XML reader. Is itpossible to call an XML reader to work on an XML DOM document and have the readervisit the whole subtree, one node after the next?Chapter 2 introduced the XmlNodeReader class, with the promise to return to it later.Let's do that now. The XmlNodeReader class is an XML reader that enables you toread nodes out of a given XML DOM subtree.Just as XmlTextReader visits all the nodes of the specified XML file, XmlNodeReadervisits all the nodes that form an XML DOM subtree. Note that the node reader is reallycapable of traversing all the nodes in the subtree no matter the level of depth. Let'sreview a situation in which you might want to take advantage of XmlNodeReader.The XmlNodeReader ClassSuppose you have selected a node about which you need more information. To scan allthe nodes that form the subtree using XML DOM, your only option is to use a recursivealgorithm like the one discussed with the LoopThroughChildren method in the section"Loading XML Documents," on page 219. The XmlNodeReader class gives you aneffective, and ready-to-use, alternative, shown here:// Select the root of the subtree to processXmlNode n = root.SelectSingleNode("Employee[@id=2]");if (n != null){// Instantiate a node reader objectXmlNodeReader nodeReader = new XmlNodeReader(n);// Visit the subtreewhile (nodeReader.Read()){// Do something with the node...186