Setting a default namespace for a namespace manager in PowerShell

Question

I have the following code which correctly fetches the AD Property links from the MSDN documentation:

$uri = 'https://msdn.microsoft.com/en-us/library/ms675090(v=vs.85).aspx' #lists all AD attributes
$results = [xml](Invoke-RestMethod -Method Get -Uri $uri -UseBasicParsing -UseDefaultCredentials)
[System.Xml.XmlNamespaceManager] $nsMgr = New-Object -TypeName System.Xml.XmlNamespaceManager($results.NameTable)
$nsMgr.AddNamespace('ns','http://www.w3.org/1999/xhtml')
$results.SelectNodes("/ns:html/ns:body/ns:div[@id = 'page']/ns:div[@id = 'body']/ns:div[@id = 'content']/ns:div[@class = 'topic']/ns:div[@id = 'mainSection']/ns:dl/ns:dd/ns:a/@href",$nsMgr)

Originally I'd hoped to avoid adding the namespace prefix (ns:), which the documentation implies can be done by adding a namespace with the prefix string.Empty. This seems to work in terms of setting the default namespace; but SelectNodes does not make use of this default.

$uri = 'https://msdn.microsoft.com/en-us/library/ms675090(v=vs.85).aspx' #lists all AD attributes
$results = [xml](Invoke-RestMethod -Method Get -Uri $uri -UseBasicParsing -UseDefaultCredentials)
[System.Xml.XmlNamespaceManager] $nsMgr = New-Object -TypeName System.Xml.XmlNamespaceManager($results.NameTable)
#$nsMgr.AddNamespace('ns','http://www.w3.org/1999/xhtml') #tried with and without this line
$nsMgr.AddNamespace([string]::Empty,'http://www.w3.org/1999/xhtml')
$nsMgr.DefaultNamespace #returns http://www.w3.org/1999/xhtml as hoped
$results.SelectNodes("/html",$nsMgr).Name #should return `html` but doesn't (though works if we register the prefix and use /ns:html)

Question:

Is there any way to have PowerShell use SelectNodes without requiring a namespace prefix / via setting a default namespace?

In case it's useful to anyone, this is the resultant script for fetching info on the various AD attributes: gist.github.com/JohnLBevan/09021f504247454f0e603587f82bcf58 — JohnLBevan
– JohnLBevan, Commented May 25, 2018 at 14:20

mklement0 · Accepted Answer · 2018-05-26 20:42:23Z

From what I can tell, the concept of a default namespace in the sense of not having to prefix node names with it:

applies to XML documents
does not apply to XPath expressions.

In other words:

If a document / element subtree declares a default namespaces along the lines of <foo xmlns='http://example.org'>, that element and all descendants not using a namespace prefix are implicitly in that default namespace.
By contrast, referring to such nodes in the context of an XPath expression requires that you:
- choose a prefix to map the document's default namespace URI to (ns in your example)
- explicitly use that prefix to match nodes in the default namespace (e.g., ns:div)

The above is supported by the following excerpt from the documentation (emphasis added):

If the XPath expression does not include a prefix, it is assumed that the namespace Uniform Resource Identifier (URI) is the empty namespace. If your XML includes a default namespace, you must still add a prefix and namespace URI to the XmlNamespaceManager; otherwise, no nodes will be selected.

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