Pass Cmdlet as parameter to Function

Question

There are a couple good answers about passing functions

function pass_function([scriptblock] $func, [int] $a){
  func.invoke($a)
}

How would you pass Cmdlets that accept piped input? I have a poor solution

function pass_through([scriptblock]$command){
  $command.invoke()
}

1,2,3,4 | pass_through { $input | Where { $_ -gt 1} }

outputs 2, 3, 4.

Technically there are enough tools there, but they require implementation fiddling. I would prefer to pass Where and {$_ -gt 1} as separate parameters.

If there isn't good support for this, what is this language's style for solving similar problems?

Answer 1

So the problem is that different cmdlets are going to have different ways to invoke them. Such as, passing 1,2,3,4 to a Where clause is fine, it likes arrays of just about anything, but you can't pass that to Format-Table because it simply doesn't take an array of strings, it needs an array of objects.

For your Where purpose you could do something like:

Function Pass_Through {
Param(
    [string]$Cmd,
    [string]$Arguments
)
Process{[scriptblock]::Create("`$input|$cmd $arguments").Invoke()}
}

Then when we do:

1,2,3,4 | pass_through 'Where' '{ $_ -gt 1}'

It responds with 2, 3, 4 as expected. But what happens when you want to pass an object and not a string?

Get-ADUser $env:USERNAME | Pass_Through 'Format-Table' 'Name,DistinguishedName'

Not going to happen, because it tries to extrapolate each property and pass each string to the Format-Table cmdlet, and that just doesn't work.

Answer 2

Looks like you are looking for a way to change command in the middle of pipeline without writing another copy of pipeline. You can achieve that by invoking ScriptBlock as SteppablePipeline . Nice thing is that Invoke-Command cmdlet can do that for you, if all following conditions are meet:

• Invoke-Command expect pipeline input.
• ScriptBlock does not have direct reference to $input .
• ScriptBlock convertible to SteppablePipeline .

Here is Pass_Through function:

function Pass_Through {
    param(
        [ScriptBlock]$MiddleCommand
    )
    $input|
    Pre-Command|
    Invoke-Command $MiddleCommand|
    Post-Command
}

And you can invoke it like this:

1..10|Pass_Through {Where { $_ -gt 1}}