Powershell Array parameter from pipline - powershell

Question

I'm trying to duplicate a typical powershell -Computername parameter that's available from the pipeline and as a normal parameter using CmdletBinding and ValueFromPipeline. My challenge is that I'm getting different results from specifying the parameter versus piping in values.
My code looks like this:
[CmdletBinding()]
param(
[parameter(Mandatory=$true, ValueFromPipeline=$true)] [string[]]$ComputerName
)
BEGIN { "Begin script`n-----" }
PROCESS {
" `$ComputerName '$ComputerName'"
" `$_ '$_'"
" +++ Count: " + $ComputerName.Count
foreach($computer in $ComputerName) {
" `$computer '$computer'"
}
" -----"
}
END { "Complete" }
When I run this using a pipeline, I get this:
PS> (1, 2, 3) | .\BPEParamTest.ps1
Begin script
-----
$ComputerName '1'
$_ '1'
+++ Count: 1
$computer '1'
-----
$ComputerName '2'
$_ '2'
+++ Count: 1
$computer '2'
-----
$ComputerName '3'
$_ '3'
+++ Count: 1
$computer '3'
-----
Complete
However, when run with a parameter, I get different results:
PS> .\BPEParamTest.ps1 -ComputerName (1, 2, 3)
Begin script
-----
$ComputerName '1 2 3'
$_ ''
+++ Count: 3
$computer '1'
$computer '2'
$computer '3'
-----
Complete

Answer 1

I always use the following construction. This works for arrays in a parameter as well as from the pipeline:
[CmdletBinding()]
param(
[parameter(Mandatory=$true, ValueFromPipeline=$true)] [string[]]$ComputerName
)
process {
foreach($computer in $computername){
#do the stuff
}
Full explanation: The process block is run once for each item in the pipeline, so that's how it handles lists on the pipeline (i.e. $computername is set to each item in turn). If you pass the values as a parameter, the $computername is set to the list which is why there's a loop.

Answer 2

.\BPEParamTest.ps1 -ComputerName (1, 2, 3) does not use the pipeline but rather a single input (a 3-element array). In contrast, (1, 2, 3) | .\BPEParamTest.ps1 uses the pipeline (3 separate inputs).
To work around this, you can check bound parameters and determine if there is pipeline input. Here is a short example:
function out-item {
[CmdletBinding()]
param(
[Parameter(ValueFromPipeline=$TRUE)]
$item
)
begin {
$PipelineInput = -not $PSBoundParameters.ContainsKey("item")
write-host "Pipeline input? $PipelineInput"
$n = 0
}
process {
if ($PipelineInput) {
$_
$n++
}
else {
$item | foreach-object {
$_
$n++
}
}
}
end {
write-host "Output $n item(s)"
}
}
With this function, both 1,2,3 | out-item and out-item 1,2,3 produce the same output (except for the "Pipeline input?" part).