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

The inspiration for designing such a read/write streaming parser is database servercursors. With database server cursors, you visit records one after the next and, ifneeded, can apply changes on the fly. Database changes are immediately effective,and actually the canvas on which your code operates is simply the database table. Thesame model can be arranged to work with XML documents.You will use a normal XML (validating) reader to visit the nodes in sequence. Whilereading, however, you are given the opportunity to change attribute values and nodecontents. Unlike the XML DOM, changes will have immediate effect. How can youobtain these results? The idea is to use an XML writer on top of the reader.You use the reader to read each node in the source document and an underlying writerto create a hidden copy of it. In the copy, you can add some new nodes and ignore oredit some others. When you have finished, you simply replace the old document withthe new one. You can decide to write the copy in memory or flush it in a temporarymedium. The latter approach makes better use of the system's memory and saves youfrom possible troubles with the application's security level and zones. (For example,partially trusted Windows Forms applications and default running ASP.NET applicationscan't create or edit disk files.)Built-In Support for Read/Write OperationsWhen I first began thinking about this lightweight XML DOM component, one of keypoints I identified was an efficient way to copy (in bulk) blocks of nodes from the readonlystream to the write stream. Luckily enough, two somewhat underappreciatedXmlTextWriter methods just happen to cover this tricky but boring aspect of two-waystreaming: WriteAttributes and WriteNode.The WriteAttributes method reads all the attributes available on the currently selectednode in the specified reader. It then copies them as a single string to the current outputstream. Likewise, the WriteNode method does the same for any other type of node.Note that WriteNode does nothing if the node type is XmlNodeType.Attribute.The following code shows how to use these methods to create a copy of the originalXML file, modified to skip some nodes. The XML tree is visited in the usual node-firstapproach using an XML reader. Each node is then processed and written out to theassociated XML writer according to the index. This code scans a document and writesout every other node.XmlTextReader reader = new XmlTextReader(inputFile);XmlTextWriter writer = new XmlTextWriter(outputFile);// Configure reader and writerwriter.Formatting = Formatting.Indented;reader.MoveToContent();// Write the rootwriter.WriteStartElement(reader.LocalName);// Read and output every other nodeint i=0;while(reader.Read()){149