How to get a list of a particular field from a list of objects in powershell

Question

I am new to powershell scripting. Apologies if I am missing something simple.

Let's say I have an object called object that has a field called field. Now let there be a list of these objects.

How would I get the list of fields in the same order?

In python it would be:

list_of_objects = [o1, o2, o3]
list_of_fields = [i.field for i in list_of_object]

Answer 1

powershell is nice, and not so nice, because it unwraps collections for you, and sometimes this can hide that it is masking the elements' members. When you're using $parents.item, you're accessing the array's method, and trying to access its members (which there aren't any, so powershell is giving you $null):

Item           ParameterizedProperty System.Object IList.Item(int index) {get;set;}

You can overcome this by using the method I shared in the comments to iterate over each member and avoid this masking:

$list = $parents | ForEach-Object -MemberName item
$list.registration.parentCompoundNumber

Alternatively, a syntax more people are familiar with:

$list = $parents | Select-Object -ExpandProperty item

or unrolling it yourself:

# you could directly assign the outputs of a `foreach` loop to a variable by
# removing these comments (<##>)
<# $items = #> 
  foreach ($parent in $parents) {
    $parent.item.registration.parentCompoundNumber
  }

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To see when this masking is happening, consider this example which uses the unary array operator:

, @('a', 'b', 'c') | Get-Member

This will let you observe the wrapping array or collection's members.

Answer 2

To complement Maximilian Burszley's helpful answer:

The linked answer contains viable workarounds for the member-name collisions; let me add a PSv4+ alternative that is both more concise and faster than a pipeline-based approach:

$parent.ForEach('item').registration.parentCompoundNumber

Using the .ForEach() array method with a property name ('item') unambiguously targets the elements' members.

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To offer a slight reframing of the explanation for why a workaround is needed:

• PowerShell's member-access enumeration essentially treats $someCollection.someProp as if you had written foreach ($element in $someCollection) { $element.someProp }; that is, the elements of $someCollection are enumerated, and the the elements' .someProp property values are returned as an array.

  • Note: As in the pipeline, if the collection happens to have just one element, that element's property value is returned as-is, not as a single-element array.

• However, if the collection type itself happens to have a member named someProp, it is used, and no enumeration takes place; that is, collection-level members shadow (take precedence over) element-level members of the same name - and that is what happened with .Item in your case.

  • When in doubt, output $someCollection.someProp interactively / during debugging to see what it evaluates to.