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

An entity reference consists of the entity name bracketed by an ampersand (&) and asemicolon (;). Not all parsers automatically expand entities upon document loading.When the XmlTextReader class encounters an entity reference, it returns an emptyinstance of the XmlEntityReference class in which the Value property is set to theempty string. By design, the XmlTextReader parser can't resolve entities, although itboasts a ResolveEntity method. Calling this method always throws an exception. Youmust use XmlValidatingReader to have entities properly expanded. (We'll covervalidating readers and validation schemas in Chapter 3.)Resolving External ReferencesIn the .NET Framework, external XML resources identified by a URI are resolvedthrough classes derived from the abstract class XmlResolver. Typical externalresources are entities and DTDs; however, the XmlResolver class can also successfullyprocess include and import elements for both XSD schemas and XSL style sheets.The .NET Framework provides only one concrete resolver class built atop XmlResolver:XmlUrlResolver. Programmers can design and implement custom resolvers, however,either by inheriting from the XmlUrlResolver class or completely from scratch byoverriding the methods and properties of XmlResolver. Let's take a look at the keyaspects, and the main tasks, of a resolver.The activity of an XML resolver revolves around two methods: GetEntity andResolveUri. The former takes the specified URI and returns the Stream object thatrepresents the desired contents. How the method actually manages to resolve the URIis implementation-specific. GetEntity, however, assumes to have at its disposal anabsolute URI. What if the URI read from the XML document is relative? Prior to callingGetEntity, you must be sure to call ResolveUri, passing both the relative URI and anybase URI. ResolveUri is responsible for combining these URIs into an absolute URI.Another problem a resolver must be ready to face arises when the resource referencedby the URI is protected and available only to authenticated users. In this case, theresolver must be passed valid credentials to carry out the task. Credentials arerepresented by an instance of the NetworkCredential class.The NetworkCredential class can be used to support a variety of authenticationschemes that make use of passwords. Among others, the list of authenticationschemes includes basic and digest authentication and Kerberos. The class does notsupport other types of authentication such as those based on a public key. You providethe credentials to the resolver through the XmlResolver.Credentials property, as shownhere:XmlUrlResolver resolver = new XmlUrlResolver();NetworkCredential cred = new NetworkCredential(user, pswd);resolver.Credentials = cred;reader.XmlResolver = resolver;You can also use the CredentialCache class to bind the resolver in a single shot to acollection of URI/credential pairs, as shown in the following code. The collection willthen be scanned, searching for a matching URI each time the resolver is called toaction.CredentialCache credCache = new CredentialCache();credCache.Add(new Uri(url1), "Basic", cred);credCache.Add(new Uri(url2), "Digest", cred);resolver.Credentials = credCache;37