XmlPreprocess is an elegant hack - or - "The hack is in the eye of the beholder"
Hack? Brilliance? Sloppy? Simple? Here's when I know something is good:
- It's easily explained.
- It's not ambiguous.
- There aren't many files.
- Self-containment.
- It's simple
Seeing a pattern? Loren has come up with an interesting and elegant hack called XmlPreprocess to deal with the deployment of config files to multiple environments without the need for multiple copies. It would be EASILY integrated into a Continuous Integration environment like ours.
The idea is bone simple:
<configuration>
<system.web>
<!-- ifdef ${production} -->
<!-- <compilation defaultLanguage="c#" debug="false"/> -->
<!-- else -->
<compilation defaultLanguage="c#" debug="true"/>
<!-- endif -->
</system.web>
</configuration>
gets loaded into "XmlPreprocess.exe /d production" and you get:
<configuration>
<system.web>
<compilation defaultLanguage="c#" debug="false"/>
</system.web>
</configuration>
Cool, eh? Now, some may thing preprocessor/if statements "tunneled" in XML Comments are gross, but to the nay-sayers I say show me one man's extensibility hack and I'll show you xs:annotation. Would you feel better if the "language" was hidden in other XML elements? Or another namespace? Sure, it's possible, but the goal isn't a new language (if statement = new language) it's #ifdef for XML for a specific purpose.
Reasons it's cool:
- The files are well-formed and legit XML before and after the process. No processing is needed to use them as found in Source Control.
- The files are self-describing. Just look at them, it's clear what's up.
- As Loren says, Get and Go.
Charlie says in comments on Jon Galloway's blog that it's a hack that this is "definitely not the way most natural way of dealing with XML" and he'd prefer XSLT. He does point out that XSLT isn't for newbies. However, for this small task I say, Wow, no way.
Using XSLT to change a config file in such a small way seems like hammering a screw. It's possible, but overkill that WILL get messy. Look at the previous requirements and apply XSLT. XSLT is great for transforming Infosets into other totally different Infosets, and while I'm deep into XSLT, I'm not a fan of using XSLT for small "pruning" changes.
I think XmlPreprocess is clever thing that's good for exactly what it's good for. Check it out!
About Scott
Scott Hanselman is a former professor, former Chief Architect in finance, now speaker, consultant, father, diabetic, and Microsoft employee. He is a failed stand-up comic, a cornrower, and a book author.
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