Today I had a problem with Tridion Content Porter. This is an application that can be used to move data in and out of the Tridion web content management system. It has an "unattended" mode of operation which allows you to run it from the command line, passing a reference to a configuration file as a parameter. I discovered that sometimes (and if anyone can tell me exactly when that is, I'd be grateful),  if you try to run it this way, instead of "blocking" it returns asynchronously and continues to run in the background, writing data to disk that you'd really rather wait for before you embark on further processing.

I called this in to customer support, but I can't wait for them to fix it, so in the meantime, I cracked my knuckles and settled down to figuring out a workaround for my batch script. This post isn't about Tridion Content Porter; this is a generic technique that you can use in batch scripts for any process that you'd like to give a chance to finish.

This (with most of the irrelevant stuff elided) is what I came up with:

REM Dump out the data using Content Porter
.... ContentPorter.exe .... config.xml .... /unattended

:TEST_FOR_CONTENT_PORTER
for /f "usebackq" %%i in (`tasklist ^| find /C "ContentPorter.exe"`) DO if /I %%i GTR 0 GOTO TEST_FOR_CONTENT_PORTER

REM do something with the exported data

At long last this blog is back on the air. A few weeks ago, I started hearing a deathwatch-beetle-like ticking noise coming from under the television. It turned out to be coming from the hard drive of the server which used to support this web site. To my shame, I didn't have any proper backups. In any case, there's only one right thing to do with a ticking hard drive; it had to go.

In the intervening period, my free time has been taken up with the following tasks, among others:

• Go out and buy a second hand computer to become my new server
• Install Gentoo Linux on it.
• Install and configure Apache, Zope, Plone and EasyBlog

Needless to say, it wasn't quite as simple as that, and I've had various technical challenges to overcome along the way. Still, with this web site, or rather cluster of web sites (the same server hosts web sites for the whole family) that's what it says on the tin. One of the primary goals of this activity has always been to erm.. "boldly go".

As has been said before, I choose to use technologies that I wouldn't usually encounter in my working life, even though I'm a computer programmer who works mainly with web sites and the software used to manage them. This sometimes means that the home projects are a little amateur, but hopefully not amateurish.

Now all I have to do is put some new content on the web sites, and pretty them up. For the content, I'll be trying to recover some of the old data, but don't hold your breath. For the look and feel, well there again, I'll be trying to do things a different way to try and learn something along the way, so I hope it doesn't take too long. In the meantime things are going to look distincly plone-ish around here.

Today I was struggling with trying to get my .NET application to pick up it's config file. I'd tried creating a .config in the same directory as the application dll, but it didn't work. This was because the application in question is activated via COM+ interop using the /codebase setting. (In other words, the COM+ settings in the registry tell the COM+ loader to use the CLR, and additionally specify the location of the assembly DLL for the benefit of the .NET loader.) This means that as far as .NET is concerned, the directory isn't the "base" of the application.

This was a tough problem, and I might have ended up doing one of two ugly things to solve it:

• Creating a %windir%/system32/dllhost.exe.config file
• Adding my configuration settings to machine.config

Either of these would have polluted a much wider area than I'd have liked.

Then I came across a blog entry by Rinat Shagisultanov which gave me sufficient of a clue to find a much better solution. The basic idea is this:

• Configure the application in COM+ (if necessary adding sufficient System.EnterpriseServices goo to make this work)
• Create a directory and point to it from the ApplicationRootDirectory setting of the COM+ application
• Create a .manifest file

Now the CLR will regard that directory as the base of your application, and look there for the config file.

I thought I was pretty much an old hand at XML by now, and that the standard gotchas wouldn't catch me any more. Not so!  The standardest gotcha of 'em all jumped up out of the slime today like something undead and there I was, gotcha'd again.

I suppose I was led astray by putting too much trust in the notion that the methods surfaced in an API would always be relevant. Stupid really, but perhaps if you follow along with the story you'll forgive me.  The API in question was that of the .NET framework; specifically the XmlNamespaceManager class.

Let's say you have some XML like this:

<?xml version='1.0'?>
<a:one xmlns:a='aaa'>
	<two xmlns='bbb'/>
</a:one>

If you have this in an XmlDocument and you want to XPath to the < two/> element, the first thing you'd usually do is create an XmlNamespaceManager and add the namespaces that you want to use in your XPath expression. This allows you to create a mapping between the namepace prefixes in your expression and the namespaces they represent. In the document itself, the same thing is achieved by the namespace declarations you can see in the sample above, but these don't exist in the XPath expression; it needs its own namespace context, or it won't be able to address anything that's in a namespace. But more of this just now...

So there I was today trying to demonstrate some techniques to a colleague, and I wrote some code something like this:

            NameTable nt = new NameTable();
            XmlNamespaceManager nm = new XmlNamespaceManager(nt);
            nm.AddNamespace("a", "aaa");
            nm.AddNamespace(String.Empty, "bbb");

            if (null == doc.SelectSingleNode("a:one/two", nm))
                Console.WriteLine ("Couldn't match default namespace");
            else
                Console.WriteLine ("Matched default namespace");

... and to my horror the match failed. After a few quick changes I had something like this:

            nm.AddNamespace("b", "bbb");
            if (null == doc.SelectSingleNode("a:one/b:two", nm))
                Console.WriteLine("Couldn't match b: namespace");
            else
                Console.WriteLine("Matched b: namespace");

... and on this occasion the match succeeded.

What was going on?  Could I possiibly be the first human to set foot on a previously undiscovered bug in the framework? Suffice it to say that my moment of glory will have to wait. A quick hunt around on Microsoft's web site showed that half a dozen other people had reported this as a bug, and that Microsoft's response was "Won't fix."

What was going on?

Five or so years ago Martin Gudgin and the other luminaries teaching Developmentor's "Guerilla XML" course, had gone to extraordinary lengths to teach me and my fellow victims the difference between a node that is in a namespace, and one that isn't. Sorry Martin, I failed you.

It turns out that in the XPath standard it says the following

A QName in the node test is expanded into an expanded-name using the namespace
declarations from the expression context.  This is the same way
expansion is done for element type names in start and end-tags except
that the default namespace declared with xmlns is not used: if the QName does not have
a prefix, then the namespace URI is null (this is the same way attribute names are 
expanded).  It is an error if the QName has a prefix for which there is
no namespace declaration in the expression context.

So - in XPath, no prefix means not in any namespace at all, just like for attributes. Microsoft's implementation is correct. Otherwise, if you had XML like this:

<?xml version='1.0'?>
<one>
	<two xmlns='bbb'>
		<c:three xmlns:c="po"/>
	</two>
<one>

... you'd be unable to write an XPath from the root element down to <c:three/>.

Now for the part where you forgive me for my stupidity:

If you look at the documentation for XmlNamespaceManager, it states very clearly that you can use the empty string to set the Default namespace. (To be fair, the documentation for selectSingleNode has a note which attempts to clarify matters.)

If you look at the rest of the API of XmlNamespaceManager, the reason for the confusion becomes clear. XmlNamespaceManager also supports methods like PushScope, GetNamespacesInScope, etc. which plainly aren't intended for use with XPath at all. It looks rather as though you could use an XmlNamespaceManager for managing your namespaces as you navigate through an XmlDocument, perhaps with a streaming library. In that context, setting the default namespace makes perfect sense. If you're using it with XPath though, it's completely irrelevant.