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

managed application, you must first create a .NET wrapper class that exposes all thepublic methods and properties found in the component's type library. Microsoft VisualStudio .NET, for example, creates such a class on the fly, immediately after adding theproper library reference to the current project.During the process, the involved types are converted from COM types and adapted tofit into the .NET Framework type system. After the importation is complete, the originalCOM object is ready for use in the .NET Framework, and more importantly, it haspreserved the original interface while adding some .NET Framework-specific memberssuch as ToString and GetType. In the end, for a Microsoft Visual Basic 6.0 programmerwho happens to use Visual Basic .NET, the code to be written is nearly identical.NoteTo generate a .NET wrapper class for a COM object, you can alsouse the tlbimp.exe utility from the command line. This utility givesyou full control over the entire process, and by using command-lineswitches, you can intervene in many useful areas, including the(strong) name of the assembly and the wrapping namespace.Although importing MSXML functionality into a .NET application is straightforward, youmust have a good reason for doing so. Jumping continuously in and out of the .NETcommon language runtime (CLR) can result in a performance hit—not to mention thefact that you end up using a programming model that, although perfectly functional, isnot the best suited for the surrounding environment.The .NET Framework XML APIThe essence of XML in the .NET Framework is found in two abstract classes—XmlReader and XmlWriter. These classes are at the core of all other .NET FrameworkXML classes, including the XMLDOM classes, and are used extensively by varioussubsystems to parse or generate XML text. For example, ADO.NET data adaptersretrieve the data to store in a DataSet object using a database reader, and the DataSetobject serializes its contents to the DiffGram format using an XmlTextWriter object,which derives from XmlWriter.XML readers and writers constitute the primitive I/O functions for XML documents andare used to build more sophisticated functionalities. So overall, you have two possibleapproaches when it comes to processing XML data. You can use any of the specializedclasses built on top of XmlReader and XmlWriter as well as document classes thatexpose the contents through the well-known and classic XMLDOM.The direct use of readers represents a stream-based, but fast and stateless, approachto XML parsing. The use of XMLDOM classes (for example, XmlDocument) representsthe traditional XMLDOM parsing model. Readers are representative of a pull model, asopposed to the SAX parser's typical push model. You can certainly build a push modelatop a pull model-based API. Unfortunately, the reverse is never true, and that's whythere is no SAX support in the .NET Framework. (In Chapter 2, you'll learn the basics ofimplementing a SAX parser using .NET Framework XML readers.)The XML API for the .NET Framework comprises the following set of functionalities:• XML readers• XML writers• XML document classesAll of these functionalities must overcome the rather subtle problem of type mapping.The .NET Framework XML type system has several things in common with the XSDSchema type system, and ad hoc conversion classes in the .NET Framework providefor applicable transformations.13