PowerShell output returns more fields than selected

Question

I've been learning PowerShell over the past month and often times I've found through select statements or -properties iirc, that I get additional data that I don't ask for. Hoping someone can point out why it spits out extra data.

Example:

Get-WmiObject -query "Select name,vendor,version from win32_product where name = 'Service Name'" -property name,vendor,version

Result:

__GENUS          : 2
__CLASS          : Win32_Product
__SUPERCLASS     : 
__DYNASTY        : 
__RELPATH        : 
__PROPERTY_COUNT : 3
__DERIVATION     : {}
__SERVER         : 
__NAMESPACE      : 
__PATH           : 
Name             : <name stuff>
Vendor           : <vendor stuff>
Version          : <version number stuff>
PSComputerName   : 

Why is it giving me all these additional fields when I'm specifying only 3?

Answer 1

First the obligatory reminder:

• The CIM cmdlets (e.g., Get-CimInstance) superseded the WMI cmdlets (e.g., Get-WmiObject) in PowerShell v3 (released in September 2012). Therefore, the WMI cmdlets should be avoided, not least because PowerShell (Core) v6+, where all future effort will go, doesn't even have them anymore. Note that WMI still underlies the CIM cmdlets, however. For more information, see this answer.

• Therefore, I'm using Get-CimInstance below; substituting it for Get-WmiObject will typically work, but there are some basic differences - see the linked answer.

It is easier to let PowerShell extract the property values of interest after the fact, using Select-Object:

Get-CimInstance -Class Win32_Product -Filter  "Name = 'Service Name'" | 
  Select-Object -Property name, vendor, version

Using Select-Object ensures that the output object(s) have the specified properties only.

Note: I doubt that it is required for performance reasons, but you can limit property retrieval at the source as well:

$props = 'name', 'vendor', 'version'
Get-CimInstance -Class Win32_Product -Filter  "Name = 'Service Name'"  -Property $props | 
  Select-Object -Property $props

Note that a Select-Object call is still needed, because using -Property with Get-CimInstance still returns objects with all the usual properties^[1], and only limits what properties get filled in.

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^{[1] They are of type CimInstance, adapted by PowerShell in a class-specific manner.}