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

for defining the structure of XML data. Second, it is the newest schema technology, andas such, has been built specifically to fix bugs and flaws in the other schemas (mostlyproblems with DTDs). And remember, more than an alternative schema technology,XDR is Microsoft's implementation of an early working draft of the XML Schemaspecification.Although no developer can seriously think of denying the significance of XML, manyperceive XML as a sort of extraneous entity that lies outside the main body of the codeand that must be integrated through distinct objects. XML parsers process strings madeof text and markup and come up with binary representations of that content. When youtry to integrate this with the rest of the caller program, you must effectively transformtext content into more specific data types.The same issue arises in the other direction. To export your binary objects to XML, youperform a kind of text serialization that looks more like a normalization of involved typeswith subsequent loss of type information. You shouldn't be surprised by this informationloss, because XML doesn't have a type system.DTD is a format designed to describe the structure and the contents of a documentrather than to endow XML with an effective type system. XDR, on the other hand,introduces the concept of typed attributes. XSD thinks a little bigger. Not only does itreinforce the importance of typed attributes, but it also distinguishes between simpleand complex types, simplifies type inheritance, and exposes a full-blown and officialXML type system.The .NET Framework has been designed around XML standards, including XSD.Although the .NET Framework type system is a separate entity from the XML typesystem, a conversion API does exist that greatly simplifies software interaction throughintegration technologies such as SOAP and Web services.What Is a Schema, Anyway?A schema is an XML file (with typical extension .xsd) that describes the syntax andsemantics of XML documents using a standard XML syntax. An XML schema specifiesthe content constraints and the vocabulary that compliant documents mustaccommodate. For example, compliant documents must fulfill any dependenciesbetween nodes, assign attributes the correct type, and give child nodes the exactcardinality.The XML Schema specification is articulated into two distinct parts. Part I contains thedefinition of a grammar for complex types—that is, composite XML elements. Part IIdescribes a set of primitive types—the XML type system—plus a grammar for creatingnew primitive types, said to be simple types. New types are defined in terms of existingtypes.An XML schema also supports rather advanced and object-oriented concepts such astype inheritance. In the .NET Framework, the SOM provides a suite of classes held inthe System.Xml.Schema namespace to read a schema from an XSD file. Theseclasses also enable you to programmatically create a schema that can be eithercompiled in memory or written to a disk file.Simple and Complex TypesXML simple types consist of plain text and don't contain any other elements. Examplesof simple types are string, date, and various flavors of numbers (long, double, andinteger). XML complex types can include child elements and attributes. In practice, acomplex type is always rendered as an XML subtree. A complex type can beassociated only with an XML element node, whereas a simple type applies to bothelements and attributes.The diagram in Figure 3-7 illustrates the structure of the XSD type system.86