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

Figure 1-1: Streams can be read and written using made-to-measure reader and writerclasses.The base classes are TextReader, TextWriter, BinaryReader, BinaryWriter, andStream. With the exception of the binary classes, all of these classes are marked asabstract (MustInherit, if you speak Visual Basic) and cannot be directly instantiated incode. You can use abstract classes to reference living instances of derived classes,however.In the .NET Framework, base reader and writer classes find a number of concreteimplementations, including StreamReader and StringReader and their writingcounterparts. By design, reader and writer classes work on top of .NET streams andprovide programmers with a customized user interface able to handle a particular typeof underlying data or file format. Although each specific reader or writer class is tailormadefor the content of a given type of stream, they share a common set of methodsand properties that defines the official .NET interface for reading and writing data.The Cursor-Like ApproachA reader works in much the same way as a client-side database cursor. The underlyingstream is seen as a logical sequence of units of information whose size and layoutdepend on the particular reader. Like a cursor, the reader moves through the data in aread-only, forward-only way. Normally, a reader is not expected to cache anyinformation, but this is only common practice, rather than a strict requirement for allstandard .NET readers.ADO.NET data reader classes (for example, SqlDataReader) are simply .NET readersthat move from one record to the next and expose the contents of the current recordthrough a tailor-made interface. The unit of information read at every step is thedatabase row. Similarly, a reader working on a disk file stream would consider as itsown atomic unit of information the single byte, whereas a text reader would perhapsspecialize in extracting one row of text at a time.XML readers are simply another, very peculiar, type of .NET reader. The class parsesthe contents of an XML file, moving from one node to the next. In this case, the finergrain of the information processed is represented by the XML node—be it an element,an attribute, a comment, or a processing instruction.XML ReadersAn XML reader makes externally available a programming interface through whichcallers can connect and pull out all the data they need. This is in no way different fromwhat happens when you connect to a database and fetch data. The database serverreturns a reference to an internal object—the cursor—which manages all the queryresults and makes them available on demand. This statement applies regardless of thefact that the database world might provide several flavors of cursors—client, scrollable,server-side, and so on.With XML readers, client applications are returned a reference to an instance of thereader class, which abstracts the underlying data stream. Methods on the reader classallow you to scroll forward through the contents, moving from node to node rather thanfrom byte to byte or from record to record. When viewed from the perspective ofreaders, an XML document ceases to be a tagged text file and becomes a serializedcollection of nodes. Such a cursor model is specific to the .NET platform, and to date,you will not find a similar programming API available for other platforms, includingMicrosoft Win32.15