Mark Wragg's helpful answer works well, but you may wonder why the Add-Member cmdlet cannot be piped to directly, as opposed to requiring an enclosing ForEach-Object call:
Arguably, the following should work, but in Windows PowerShell doesn't:
# !! Does NOT work as expected in Windows PowerShell.
# OK in PowerShell (Core) 7+
$JSON | ConvertFrom-Json |
Add-Member -MemberType NoteProperty -Name 'Source' -Value 'CPU' -PassThru
The idea is that Add-Member uses pipeline input directly, and, after modifying each input object, outputs it, thanks to -PassThru (by default Add-Member produces no output).
The reason that it doesn't work is that when Windows PowerShell's ConvertFrom-Json outputs an array, it outputs it as a single object rather than sending its elements one by one through the pipeline, as one would expect.
- In PowerShell [Core] 7.0, the behavior was changed to align with the usual enumeration-of-elements behavior, and a
-NoEnumerate switch was added as an opt-in to the old behavior. For the discussion that led to this change, see GitHub issue #3424.
Workarounds:
- Use
(...), which forces enumeration of the array:
# Enclosing the ConvertFrom-Json command in (...) forces enumeration.
($JSON | ConvertFrom-Json) |
Add-Member -MemberType NoteProperty -Name 'Source' -Value 'CPU' -PassThru
Note that, generally, using (...) to force collection of a command's entire output in-memory in an array is convenient, but can be problematic with large output sets. As PetSerAl points out, however, in this case it is fine, because ConvertFrom-Json itself constructs the entire output array in memory up front anyway.
- Alternative: A pass-through call to
Write-Output -NoEnumerate (Windows PowerShell) /
just Write-Output (PowerShell Core), whose sole purpose is to force enumeration of the array elements:
# Inserting Write-Output [-NoEnumerate] between ConvertFrom-Json and Add-Member
# forces enumeration of the array elements.
# *Windows PowerShell*, as of v5.1:
$JSON | ConvertFrom-Json | Write-Output -NoEnumerate |
Add-Member -MemberType NoteProperty -Name 'Source' -Value 'CPU' -PassThru
# PowerShell *Core*:
$JSON | ConvertFrom-Json | Write-Output |
Add-Member -MemberType NoteProperty -Name 'Source' -Value 'CPU' -PassThru
Optional reading: the quirks of Write-Output:
Windows PowerShell as of v5.1:
Due to a bug, Write-Output invariably enumerates single objects that are collections themselves, even when you add -NoEnumerate.
Paradoxically, -NoEnumerate is actually needed in this case - even though we do want to enumerate! - so as to prevent Write-Output from applying enumeration twice: once (invariably) to the input array, and again to the individual array elements (thanks again, PetSerAl); e.g.:
# !! Unexpectedly returns 4(!): enumerates the outer 2-element array
# !! *and* its elements.
# (In PowerShell *Core*, this yields 2, as expected.)
Write-Output -InputObject (1, 2), (3, 4) | Measure-Object
# BETTER: yields 2, because only the outer 2-element array is enumerated
# (In PowerShell *Core*, this yields 1, as expected.)
Write-Output -NoEnumerate -InputObject (1, 2), (3, 4) | Measure-Object
PowerShell (Core) v6.2.3+:
The above problem has been fixed - see GitHub issue #5955 - which means that - sensibly - you mustn't use -NoEnumerate if you do want Write-Output to enumerate pipeline objects that are themselves collections (and enumeration no longer recurses 1 level).
