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

abstraction and explicitly indicate the XML elements to create—nodes, attributes,comments, or processing instructions. The writer works on a stream, dumping contentincrementally, one node after the next, without the random access capabilities of theXMLDOM but also without its memory footprint.To grasp the importance of XML writers, consider that, in general, the only alternativeyou have for writing XML contents to any storage media consists of preparing the entireoutput as a string and then writing it off. In this case, the markup nature of XML is morehindrance than real help, because you must yourself take care of the intricacies ofquotation marks, attributes, indentation, and end tags.In the .NET Framework, XML writers come to the rescue and let you write XMLdocuments programmatically in much the same way you write them through texteditors. For example, you can specify whether you want a namespace prefix, thepadding character and the size of the indentation, the quotation mark and the newlinecharacter, and even how you want white spaces to be treated. To create nodes, yousimply use ad hoc methods to write comments, attributes, and element nodes. Theoverall method of working is simple and extremely effective.The .NET Framework provides several types of writers that use heterogeneous outputdevices—strings, HTTP response, and HTML documents. You could also use an XMLtext writer to dump contents to a stream object or a new text file. In the latter two cases,you could also specify character encoding. If the encoding argument is null, theUnicode 8-bits-per-character schema (UTF-8) will be used.XML writers, and in particular the XmlTextWriter class, are used throughout the .NETFramework for creating any sort of XML output. We'll look at XML writers in detail inChapter 4.The XML Document Object API in .NETAs mentioned, along with XML readers and writers, the .NET Framework also providesclasses that load and edit XML documents according to the W3C DOM Level 1 andLevel 2 Core. The key XMLDOM class in the .NET Framework is XmlDocument—notmuch different from the DOMDocument class, which you might recognize from workingwith MSXML.The XMLDOM supplies an in-memory tree-based representation of XML documentsand supports both navigation and editing of the document. In addition, the XMLDOMclasses can handle both XPath queries and XSLT.Tightly coupled with the XmlDocument class is the XmlDataDocument class. It extendsXmlDocument and focuses on XML storage and retrieval of structured tabular data. Inparticular, XmlDataDocument can import data from an ADO.NET DataSet object andexport regular XML contents to the DataSet relational format. Regular XML content is aset of nodes with exactly one level of subnodes, with each node having the samenumber of children. The ultimate goal of this requirement is enabling the XML contentsto fit into a relational table.The XMLDOM representation of an XML document is fully editable. Attributes and textcan be randomly accessed, and nodes can be added and removed. You performupdates on a loaded XMLDOM document by first creating a node object (the XmlNodeclass) and then binding it to the existing tree. All in all, the underlying writing pattern isclose to that of XML writers—you write nodes to the stream in one case, and you addnodes to the tree in the other. Of course, if you are using the XMLDOM, bear in mindthat all changes occur in memory and must be flushed to the storage medium prior toreturn. (The XMLDOM API is described in detail in Chapter 5.)17