How can I pass multiple values to a PowerShell function that accepts an array of input objects?

Question

How can I pass values to my functions in the same way the PowerShell cmdlet Write-Output can be passed multiple values and process them as expected?

'one', 'two' | Write-Output

which will product the following output

one
two

I thought this was a way to do that, but it isn't:

Function ProcessNames ([Parameter(ValueFromPipeline=$true)][string[]]$Names) {
  Foreach ( $name in $Names ) {
    # do something
  }
}
'bob', 'alice' | ProcessNames

In the above example, only the last element in the list gets processed - in this case 'alice'.

What am I doing wrong?

TheMadTechnician · Accepted Answer · 2023-08-23 16:01:12Z

3

You have built your function in the simplified style. Functions have a BEGIN, PROCESS, and END block, of which only the END block runs by default. You can fix it by declaring that you want to operate in the PROCESS scriptblock like this:

Function ProcessNames ([Parameter(ValueFromPipeline=$true)][string[]]$Names) {
  PROCESS{
      Foreach ( $name in $Names ) {
        $name
      }
  }
}
'bob', 'alice' | ProcessNames

answered Aug 23, 2023 at 16:01

TheMadTechnician

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mklement0 · Accepted Answer · 2023-10-11 22:42:26Z

To add to TheMadTechnician's helpful answer:

In addition to using a process block, it's better to also use a param(...) block for parameter declarations in advanced functions - which your function implicitly is, due to use of a [Parameter()] attribute.
- Only a param(...) block allows you to use the [CmdletBinding()] attribute, which modifies the behavior of advanced functions - see this answer.
If your intent is to support processing multiple input objects via the pipeline only, you needn't declare your pipeline-binding parameter as an array.
- In fact, the majority of cmdlets work this way - they do not meaningfully support processing an array of input objects passed to the pipeline-binding parameter directly, as an argument - see GitHub issue #4242 for a discussion.
- That said, Write-Output, Out-File and Set-Content are notable exceptions; e.g., the following commands are equivalent to providing 1, 2, 3 via the pipeline:
  Write-Output -InputObject 1, 2, 3
  Out-File out.txt -InputObject 1, 2, 3
  Set-Content out.txt -Value 1, 2, 3

Supporting multiple input objects via the pipeline only simplifies - and speeds up - things:

function ProcessNames {
  param(
    [Parameter(ValueFromPipeline)]
    [string] $Name # Defined as a *single* string
  ) 
  process {
    # Do something with $Name
    $Name + '!' # sample operation
  }
}

'bob', 'alice' | ProcessNames then yields the equivalent of: 'bob!', 'alice!'

If pipeline input is all you need to support - not also arguments - a simple alternative is to use a filter, which is a simplified version of a function that (a) has no parameters and (b) whose body implicitly runs inside a process block and receives pipeline input via automatic $_ variable (aka $PSItem):

filter ProcessNames {
  # Do something with $_
  $_ + '!' # sample operation
}

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