How do I implement a filter in PowerShell?

Question

I'm trying to figure out a way to incorporate a filter into a custom function and I haven't been able to get it working:

Function Test-Me{
    Param
    (
      $Filter = "Number -like ""Three"""
    )

    $Obj = New-Object PSObject -Properties &{
                                             Number = "One","Two","Three"
                                            }

    If($Filter){
     $Obj | Where-Object $Filter
    }else{
     $Obj
    }
}

I've tried various means, but they all fail:

$Filter = 'Where-Object{$_' + $Filter + '}'
$Obj | & $Filter

.

$Filter = "Number -like ""One"""
$Obj | Where & $Filter

How do you incorporate Filter support into custom functions?

Community · Accepted Answer · 2017-05-23 11:51:23Z

3

Set the parameter type to ScriptBlock:

Function Test-Me {
  Param(
    [ScriptBlock]$Filter = {$_.Number -like 'Three'}
  )

  $Obj = 'One','Two','Three' | % {[PSCustomObject]@{Number = $_}}
  $Obj | Where-Object $Filter
}

and define the filters as actual scriptblocks:

PS C:\> Function Test-Me {
>>   Param([ScriptBlock]$Filter = {$_.Number -like 'Three'})
>>   $Obj = 'One','Two','Three' | % {[PSCustomObject]@{Number = $_}}
>>   $Obj | Where-Object $Filter
>> }
>>
PS C:\> Test-Me

Number
------
Three

PS C:\> $sb = { $_.Number -like 't*' }
PS C:\> Test-Me $sb

Number
------
Two
Three

PS C:\> Test-Me {$_.Number -like '*e'}

Number
------
One
Three

If your function must accept string input for some reason you could create scriptblocks from the strings, as @Richard suggested:

Function Test-Me {
  Param(
    [string]$Filter = '$_.Number -like "Three"'
  )

  $fltr = [ScriptBlock]::Create($Filter)

  $Obj = 'One','Two','Three' | % {[PSCustomObject]@{Number = $_}}
  $Obj | Where-Object $fltr
}

edited May 23, 2017 at 11:51

CommunityBot

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answered Aug 4, 2015 at 15:44

Ansgar Wiechers

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2 Comments

Colyn1337 Over a year ago

You don't actually have to include the $_ with your filter. But this is the closest to a real answer so I'll accept yours.

Ansgar Wiechers Over a year ago

@Colyn1337 The simplified syntax only works for simple expressions and only in PowerShell v3 and newer. If for instance you want to combine 2 or more conditions with logical operators ($_.Number -like 't*' -or $_.Number -like '*e') you must use $_.

Peter Mortensen · Accepted Answer · 2018-12-18 19:31:22Z

1

You cannot just pass a string and expect it to be expanded as the cmdlet's parameters: PowerShell's parsing is more sophisticated than that.

But you should be able to convert a string into a script block (for example, see here). Then pass the script block to Where-Object's -FilterScript parameter.

edited Dec 18, 2018 at 19:31

Peter Mortensen

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answered Aug 4, 2015 at 14:58

Richard

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