XmlReader and the Factory Design Pattern


Daniel Cazzulino writes in response to Don Demsak’s post on Waking Up From A DOM Induced Coma



So, in this regard, I believe SUN is doing a good job at concentrating on pluggable and standard interfaces and specifications, and letting whoever wants to take the time to implement custom stuff.
I don’t want to “new XmlTextReader”. I want some app/system-wide factory take care of creating the appropriate parser implementation for me based on declarative configuration, and I want my to code to work against a single unified interface/base class always.
Changing the parser shouldn’t mean I have to change my working app code. If MS provides the appropriate abstractions, it wouldn’t even be necessary to rely on some implementation-specific feature such as XmlTextReader.GetRemainder that is not part of the abstract contract defined by XmlReader.


I both agree and disagree with Daniel. We do have a single unified interface for processing XML which developers can program against, it is called the XmlReader. Unfortunately, we subclassed this class into the XmlTextReader and XmlValidatingReader which are actually what most developers program against including our devs internally. In the next version of the .NET Framework we are moving away from the XmlTextReader and XmlValidating reader. Instead we will emphasize programming directly to the XmlReader and will provide an implementation of the factory design patterns which returns different XmlReader instances based on which features the user is interested. More importantly users will be able to layer different XmlReader implementations on those created by our factory which was always our intention since v1.0 of the .NET Framework. For example, one could layer XSD Validation on top the XIncludingReader from XInclude.NET to combine third party XInclude support with Microsoft’s W3C XML Schema validation technologies.


As for whether the Sun’s approach of just providing interfaces instead of concrete for XML parsing was such a great thing in Java I’d claim that it’s been hit and miss. Most XML developers from the Java world despise the DOM for the reasons described in Chapter 33 of Elliotte Rusty Harold’s Effective XML. This is the reason for the existence of extensions and alternatives to the DOM API which extend it such as Oracle’s XDK, dom4J, JDOM, Xerces and XOM. Heck, you can’t even get the XML as a string out of node or save an XML document object to a file without using extensions since these aren’t in the base DOM API. As for SAX, the API just gives you access to regular parsing events nothing fancy.  There isn’t much difference functionally from programming against the base SAX APIs and programming against XmlReader


The one point of interest is that Daniel claims that the Java way of not shipping with any XML APIs but just interfaces is somehow better than the .NET way.  In Java one can programa against interfaces and loads the XML parser by passing the class name to a factory method. One could put this name in a config file and change it at runtime. The question is whether anyone in the .NET world actually thinks being able to change your XML parser implementation at runtime is anything more than a geek feature. I consider it as geeky as asking why you can’t change the implementation of the System.String class to a user defined class that uses less memory at runtime without having to recompile. An interesting idea but one primarily of interest to the ultimate of power users.


The funny thing is that even if we shipped functionality where we looked in the registry or in some config file before figuring out what XML parser to load it’s not as if there are an abundance of third party XML parsers targetting the .NET Framework in the first place. There is definitely no intention to ship any functionality like this in future versions of the .NET Framework.


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