Create an XmlNamespaceManager pre-populated with an XML document's namespace prefixes to simplify XPath queries

New-XmlNamespaceManager.ps1

<#
.DESCRIPTION
Create an XmlNamespaceManager pre-populated with an XML document's namespace prefixes to simplify XPath queries

.PARAMETER XmlDocument
An instance of [xml] with custom namespaces to register in the XmlNamespaceManager.

.PARAMETER DefaultNamespacePrefix
Optional prefix to use for the default namespace in XPath queries.

.EXAMPLE
$NsMgr = New-XmlNamespaceManager -XmlDocument $XmlDoc -DefaultNamespacePrefix def
This then enables nodes in the default namespace to be queries using the 'def' prefix:
    $XmlDoc.SelectNodes('//def:SomeElement', $NsMgr)

.EXAMPLE
$NsMgr = New-XmlNamespaceManager -XmlDocument $XmlDoc
Assuming the document's root element declares namespaces with prefixes like this:
    <Root xmlns:foo="http://namespaces/foo">

Then nodes in the 'http://namespaces/foo' namespace can be queried by the prefix:
    $XmlDoc.SelectNodes('//foo:AFooElement', $NsMgr)


#>
[CmdletBinding()]
param (
    [Parameter(Mandatory=$true, Position=0)]
    [xml]
    $XmlDocument,
    [string]
    $DefaultNamespacePrefix
)

$NsMgr = New-Object -TypeName System.Xml.XmlNamespaceManager -ArgumentList $XmlDocument.NameTable

$DefaultNamespace = $XmlDocument.DocumentElement.GetAttribute('xmlns')
if ($DefaultNamespace -and $DefaultNamespacePrefix) {
    $NsMgr.AddNamespace($DefaultNamespacePrefix, $DefaultNamespace)
}

$XmlDocument.DocumentElement.Attributes | 
    Where-Object { $_.Prefix -eq 'xmlns' } |
    ForEach-Object {
        $NsMgr.AddNamespace($_.LocalName, $_.Value)
    }

return ,$NsMgr # unary comma wraps $NsMgr so it isn't unrolled

Many thanks Jason. I was googling for this since three hours at least.

Works great. 🥇

A Huge thank you Jason! You made may day!