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

What the XmlWriter Class Can't DoAlthough powerful and considerably feature-rich, an XML writer is not perfect; it stillleaves some margin for errors. To be more precise, the XmlWriter class certainlygenerates 100-percent well-formed code, but only if you pass on correct information. Inparticular, an XML writer does not check for invalid characters in element and attributenames. It also does not guarantee that any Unicode characters you use fit into thecurrent encoding schema. As a consequence, any characters outside the encodingschema are not escaped into character entities and might lead to incorrect output.An XML writer also does not verify duplicate attributes; it simply dumps the text outwhen you call the appropriate method. Nor does an XML writer validate any identifiers(for example, the SYSTEM identifier) you specify when you create a DOCTYPE node.In addition, the XmlWriter class does not validate against any schema or document typedefinition (DTD). Creating a validating writer is not difficult, however; I'll give you sometips on how to build one in the section "XML Validating Writers," on page 168. By theway, an XmlValidatingWriter class is just one of the extensions to the System.Xmlnamespace slated for the next version of the .NET Framework.Properties of the XmlWriter ClassTable 4-1 lists the properties that belong to the XmlWriter class.Table 4-1: Properties of the XmlWriter ClassPropertyWriteStateXmlLangXmlSpaceDescriptionRead-only property that gets the state of the writer. The statecan be any value taken from the WriteState enumeration anddescribes the element being written.Read-only property that returns the current xml:lang scope.You set the language of the document by writing an xml:langattribute to the output stream.Read-only property that indicates the current xml:space scopethrough a value taken from the XmlSpace enumeration(Default, None, or Preserve).All of these properties are read-only and abstract—that is, they must be overridden inany derived class. The behavior described in Table 4-1 simply indicates what theproperties have been designed for and does not necessarily reflect the actual behaviorof these properties in a custom implementation.In general, the XmlWriter class properties serve to track the state in which anothercomponent might have left the writer. Note that these properties belong to the currentinstance of the writer object. If you are using the same writer to generate moredocuments on the same stream, these properties are not automatically reset when youstart a new document.XML Writer StatesTable 4-2 summarizes the allowable states for an XML writer. Values come from theWriteState enumeration type. Any XML writer is expected to properly and promptlyupdate its WriteState property as various internal operations take place.Table 4-2: States of an XML WriterStateAttributeDescriptionThe writer enters this state when an attribute is beingwritten.114