No error when selecting non-existing property

Question

I want PowerShell to throw an error when trying to select non-existing properties, but instead I get empty column as output. Example:

$ErrorActionPreference=         


        

Answer 1

PowerShell expanding non-existing properties to $null behaves as designed. AFAICS the only thing you could do is to explicitly check if all properties exist:

$props = 'Id', 'ProcessName', 'xxx'

$p = Get-Process *ex*
$missing = $p | Get-Member -Type *property |
           Select-Object -Expand Name |
           Compare-Object -Reference $props |
           Where-Object { $_.SideIndicator -eq '<=' } |
           Select-Object -Expand InputObject

if ($missing) {
  throw "missing property $missing."
} else {
  $p | Select-Object $props
}

Of course you could wrap this in a custom function:

function Select-ObjectStrict {
  [CmdletBinding()]
  Param(
    [Parameter(
      Position=0,
      Mandatory=$true,
      ValueFromPipeline=$true,
      ValueFromPipelineByPropertyName=$true
    )]$InputObject,

    [Parameter(
      Position=1,
      Mandatory=$true
    )][string[]]$Property
  )

  Process {
    $missing = $InputObject | Get-Member -Type *property |
               Select-Object -Expand Name |
               Compare-Object -Reference $Property |
               Where-Object { $_.SideIndicator -eq '<=' } |
               Select-Object -Expand InputObject

    if ($missing) {
      throw "missing property $missing."
    } else {
      $InputObject | Select-Object $Property
    }
  }
}

so it could be used like this:

Get-Process *ex* | Select-ObjectStrict -Property 'Id', 'ProcessName', 'xxx'