PowerShell advanced function output PipelineVariable doesn't work - function

Question

I created an advanced function to get the mac address from a VM running on VMware ESXi.
function Get-MacFromVm {
[CmdletBinding(SupportsShouldProcess=$true)]
Param(
# The name of the VM of which we want to obtain the mac address.
[Parameter(Mandatory=$true,
ValueFromPipeline=$true,
ValueFromPipelineByPropertyName=$true)]
[string[]]
$Name
)
Begin {}
Process {
foreach ($item in $Name) {
if ($PSCmdlet.ShouldProcess($item, "Getting the mac address")) {
Get-VM $item -PipelineVariable vm |
Get-NetworkAdapter |
Select-Object #{n="Name"; e={$vm.Name}},
#{n="ClientId"; e={$_.MacAddress -replace ":","-"}}
}
}
}
End {}
}
So far everything works perfect.
I can use it in any of the following ways and get results back.
It accepts either a single or array of string via the named parameter or as pipeline input.
Get-MacFromVm -Name "playground"
Get-MacFromVm -Name "playground", "DC01"
"playground", "DC01" | Get-MacFromVm
The output is a [PSCustomObject] with 2 properties, a Name and the ClientId.
Now the problem starts when I want to chain the result to multiple other cmdlets by using the -PipelineVariable parameter.
Normally I should be able to use it like this:
Get-MacFromVm -Name "playground" -PipelineVariable pv | % {$pv}
But it doesn't show me any results back. If i substitute the $pv with $_ it does show the correct result, but I cannot use that automatic variable 2 or 3 cmdlets farther in the pipeline chain.
Although I can solve this by using the -OutVariable and/or split it into multiple lines.
I want to know why this doesn't work, I want to know what I'm missing here.

Answer 1

I have no experience with -PipelineVariable parameter. So, I took this as an opportunity to learn about the -PipelineVariable parameter:
Because I do not have easy access to VMs either, I simulated the function as follows:
Function Get-MacFromVm {
[CmdletBinding(SupportsShouldProcess=$true)]
Param(
[Parameter(Mandatory=$true,
ValueFromPipeline=$true,
ValueFromPipelineByPropertyName=$true)]
[string[]]
$Name
)
Function MacAddress {(1..4 | ForEach {'{0:x2}' -f (Get-Random -Min 0 -Max 255)}) -Join ":"}
Function Get-VM {[cmdletbinding()] Param ($Name) [pscustomobject]#{Name = $Name; MacAddress = (MacAddress)}}
ForEach ($Item in $Name) {
If ($PSCmdlet.ShouldProcess($Item, "Getting the mac address")) {
Get-VM $item -PipelineVariable vm |
Select-Object #{n="Name"; e={$vm.Name}}, #{n="ClientId"; e={$_.MacAddress -replace ":","-"}}
}
}
}
But I am not able to reproduce the issue:
PS C:\> Get-MacFromVm -Name "playground", "DC01" -PipelineVariable pv | % {$pv}
Name ClientId
---- --------
playground 3c-23-55-c4
DC01 4f-38-42-a7
So I wonder if this simulated function also works for you.
If yes, maybe you can find the differences with a real VM object.
Knowing PowerShell, I would question whether there is nothing outputted or nothing displayed. So what happens if you just output a single property:
Get-MacFromVm -Name "playground" -PipelineVariable pv | % {$pv.Name}
Or:
Get-MacFromVm -Name "playground" -PipelineVariable pv | % {$pv.GetType()}
Does this return a "You cannot call a method on a null-valued expression." error?
Update 30 October 2019
As per mklement0 comment:
The reason you're not seeing the bug is that your function, due to not using begin / process / end blocks, implicitly runs in the end block, and the very first invocation of any of these script blocks is captured in the pipeline variable, but not the output from subsequent script blocks calls, which would happen with a function that has a process block.
Meaning the correct simulation should be:
Function Get-MacFromVm {
[CmdletBinding(SupportsShouldProcess=$true)]
Param(
[Parameter(Mandatory=$true,
ValueFromPipeline=$true,
ValueFromPipelineByPropertyName=$true)]
[string[]]
$Name
)
Begin {
Function MacAddress {(1..4 | ForEach {'{0:x2}' -f (Get-Random -Min 0 -Max 255)}) -Join ":"}
Function Get-VM {[cmdletbinding()] Param ($Name) [pscustomobject]#{Name = $Name; MacAddress = (MacAddress)}}
}
Process {
ForEach ($Item in $Name) {
If ($PSCmdlet.ShouldProcess($Item, "Getting the mac address")) {
Get-VM $item -PipelineVariable vm |
Select-Object #{n="Name"; e={$vm.Name}}, #{n="ClientId"; e={$_.MacAddress -replace ":","-"}}
}
}
}
}
Which indeed doesn't give any result for the PipelineVariable:
PS C:\> Get-MacFromVm -Name "playground", "DC01" -PipelineVariable pv | % {$pv}
PS C:\>
While the current item ($_) does:
PS C:\> Get-MacFromVm -Name "playground", "DC01" -PipelineVariable pv | % {$_}
Name ClientId
---- --------
playground e7-b5-a0-31
DC01 0f-ed-94-cc