How can I list possible XPath queries for an XML object in PowerShell?

Question

How can I get the list of possible XPath queries for an xml object in PowerShell?

Answer 1

As the comments note, it is impossible to list all possible XPath queries for a given XML document, given the complexity of this open-ended query language, with different ways to target the same nodes, ...

However, it is possible and may be useful to output XPath path expressions to the leaf elements of a document, so as to get a sense of the document structure, and to be able to formulate XPath queries based on them.

Assuming that helper function Get-XmlElementPath is defined (source code below), you can do something the following:

# Sample XML doc.
$xmlDocText = @'
<?xml version="1.0"?>
<doc>
    <catalog>
        <book id="bk101">
            <title>De Profundis</title>
        </book>
        <book id="bk102">
            <title>Pygmalion</title>
        </book>
    </catalog>
    <foo>
        <bar>one</bar>
        <bar>two</bar>
    </foo>
</doc>
'@

Get-XmlElementPath $xmlDocText

This outputs the following strings, representing the XPath path expressions that select the document's leaf elements:

/doc/catalog/book[@id="bk101"]/title
/doc/catalog/book[@id="bk102"]/title
/doc/foo/bar[1]
/doc/foo/bar[2]

Note:

• Caveat: The function does not (fully) support namespaces - while elements with explicit namespace prefixes are reported as such, those implicitly in a namespace are reported by their name only; if the input document uses namespaces and you want to query it based on the path expressions returned, you'll need to:

  • Create a namespace manager with self-chosen prefixes to refer to the namespace URIs, including the default one.
  • Use these prefixes in the XPath path expression, even for elements that are in the default namespace.
  • The following answers demonstrate these techniques:
    • In the context of the .SelectNodes() and .SelectSingleNode() .NET API methods: see this answer.
    • In the context of the Select-Xml cmdlet: see this answer.

• Only element nodes are considered, and only leaf elements, i.e. those elements that themselves do not have any element children.

• If a given child element has an "id" or "name" attribute, its path is represented with an XPath conditional ([@id="..."] or [@name="..."]; "id" takes precedence), under the assumption that these values are unique (at least among the sibling elements).

• Multiple child elements with the same name that do not have "id" or "name" attributes are each represented by their 1-based positional index (e.g, [1]).

---

Get-XmlElementPath source code; run Get-XmlElementPath -? for help:

function Get-XmlElementPath {
  <#
  .SYNOPSIS
  Outputs XPath paths for all leaf elements of a given XML document.
  
  .DESCRIPTION
  Leaf elements are those XML elements that have no element children.
  
  If a given child element has an "id" or "name" attribute, its path is 
  represented with an XPath conditional ([@id="..."] or [@name="..."])

  Multiple child elements with the same name that do not have "id" or "name" 
  attributes are each represented by their 1-based positional index.
  
  Note: Namespaces are NOT (fully) supported: while elements with
        explicit namespace prefixes are reported as such, those
        that are implicitly in a namespace are reported by name only.
  
  .EXAMPLE
  Get-XmlElementPath '<catalog><book id="bk101">De Profundis</book><book id="bk102">Pygmalion</book></catalog>'
  
  /catalog/book[@id="bk101"]
  /catalog/book[@id="bk102"]
  #>

  param(
    [Parameter(Mandatory)] $Xml,            # string, [xml] instance, or [XmlElement] instance
    [Parameter(DontShow)] [string] $Prefix, # used internally
    [Parameter(DontShow)] [string] $Index   # used internally
  )

  if ($Xml -is [string]) {
    $Xml = [xml] $Xml
  }
  if ($Xml -is [xml]) { $Xml = $Xml.DocumentElement}

  # Construct this element's path.
  $Prefix += '/' + $Xml.psbase.Name # !! .psbase.Name must be used to guard againts a "name" *attribute* preempting the type-native property.
  if ($Index) { $Prefix += '[{0}]' -f $Index }

  $childElems = $Xml.ChildNodes.Where({ $_ -is [System.Xml.XmlElement]})
  if ($childElems) {
    # Create a hashtable that maps child element names to how often they occur.
    $htNames = [hashtable]::new() # Note: case-*sensitive*, because XML is.
    foreach ($name in $childElems.get_Name()) { $htNames[$name]++ }
    # Create a hashtable that maintains the per-name count so far in the iteration.
    $htIndices = [hashtable]::new() 
    # Iterate over all child elements and recurse.
    foreach ($child in $childElems) {
      $Index = ''
      if ($htNames[$child.psbase.Name] -gt 1) { $Index = ++$htIndices[$child.psbase.Name] }
      # If an 'id' attribute is present, use it instead of a positional index.
      if ($id = $child.GetAttribute('id')) { $Index = '@id="{0}"' -f $id }
      elseif ($id = $child.GetAttribute('name')) { $Index = '@name="{0}"' -f $id }
      # Recurse
      Get-XmlElementPath $child $Prefix $Index
    }
  } else { # leaf element reached
    $Prefix # output the path
  }

}