Conditional on empty SelectNodes set - powershell

Question

Given XML like this
<?xml version="1.0"?>
<Definitions>
<Products>
<Product_Group id="Revit">
<Product id="RVT2017">
<RSAccelerator>RSACCELERATOR2017</RSAccelerator>
<ShortcutPath os="6.0 6.1">C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Autodesk\Revit 2017</ShortcutPath>
<ShortcutPath os="6.2 6.3 10.0">C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Revit 2017</ShortcutPath>
</Product>
</Product_Group>
</Products>
</Definitions>
I want to test for the presence of the OS attribute so I can handle getting the value differently when there are two nodes differentiated by that attribute value vs no attribute at all.
I would have thought this would work, with appropriate values for the two variables.
if ($script:pxResources.SelectNodes("//Product[#id='$product']/$resource[#os]")) {
However, this is returning true even when no nodes are selected. I can use
if ($script:pxResources.SelectNodes("//Product[#id='$product']/$resource[#os]").count -gt 0) {
but that seems clumsy. Is there a better way to handle this, or is testing for an empty set the only option?

Answer 1

AFAIK you will always have to test as SelectNodes will return a System.Xml.XPathNodeList object, which PowerShell will consider to be true even if it is empty.
Agreed adding some code to test is not pretty but AFAIK it's necessary.
My preferred method is IsNullOrEmpty:
[String]::IsNullOrEmpty(<thing>)
# example
$exp = $script:pxResources.SelectNodes("//Product[#id='$product']/$resource[#os]")
if (-not [String]::IsNullOrEmpty($exp)) {# do something}

Answer 2

There's a;so the approach where you don't use XPath:
[xml]$xml = #"
<?xml version="1.0"?>
<Definitions>
<Products>
<Product_Group id="Revit">
<Product id="RVT2017">
<RSAccelerator>RSACCELERATOR2017</RSAccelerator>
<ShortcutPath os="6.0 6.1">C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Autodesk\Revit 2017</ShortcutPath>
<ShortcutPath os="6.2 6.3 10.0">C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Revit 2017</ShortcutPath>
<ShortcutPath os="">C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Anything\Revit 2017</ShortcutPath>
<ShortcutPath>C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Something\Revit 2017</ShortcutPath>
</Product>
</Product_Group>
</Products>
</Definitions>
"#
$product = "RVT2017"
$resource = "ShortcutPath"
($xml.Definitions.Products.Product_Group.Product | Where-Object { $_.id -eq $product }).$resource | ForEach-Object {
if ($null -eq $_.os) {
Write-Host "'os' attribute missing on $_"
}
elseif ([string]::IsNullOrWhiteSpace($_.os)) {
Write-Host "'os' attribute empty on element $($_.outerXml)"
}
else {
Write-Host "'os' = $($_.os) on element $($_.outerXml)"
}
}
This will output
'os' = 6.0 6.1 on element <ShortcutPath os="6.0 6.1">C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Autodesk\Revit 2017</ShortcutPath>
'os' = 6.2 6.3 10.0 on element <ShortcutPath os="6.2 6.3 10.0">C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Revit 2017</ShortcutPath>
'os' attribute empty on element <ShortcutPath os="">C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Anything\Revit 2017</ShortcutPath>
'os' attribute missing on C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Something\Revit 2017