PipelineVariable not available when using Foreach-Object -Parallel

Steps to reproduce

$MyDict = [System.Collections.Concurrent.ConcurrentDictionary[string,object]]::new()
Get-Process -PipelineVariable Processinfo | ForEach-Object -Parallel {
    $dict = $using:MyDict
    $dict.TryAdd($Processinfo.id, $Processinfo) | Out-Null
}

$MyDict.Keys.Count
1

$MyDict.Keys

Expected behavior

$MyDict.Keys.Count
200
$MyDict.Keys
0
2
3
9
1480
...

Actual behavior

$MyDict.Keys.Count
1

$MyDict.Keys

Environment data

$PSVersionTable

Name                           Value
----                           -----
PSVersion                      7.1.3
PSEdition                      Core
GitCommitId                    7.1.3
OS                             Microsoft Windows 10.0.17763
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

The variable defined as the pipeline variable is not available in the foreach-object script when the option -Parallel is used, However then -Parallel is not used, the variable can be used correctly

[PaulHigin]

Pipeline variables are not supported for Foreach-Object -Parallel. The RFC and blog specify that variables can be passed in only via pipeline input $_ and with the $using: keyword.

...
The only variable that automatically appears in the parallel script block is the piped in object. Other variables are passed in using the $using: keyword.
...

But we may need to update the documentation to make this more clear. This may be something that can be considered for future releases.

[mklement0]

Thanks for clarifying, @PaulHigin; documenting this will certainly help, but providing an appropriate error message is important too.

If we don’t want to introduce a specific error message for this scenario, the existing, catch-all message should be amended, along the lines of:

A Using variable can be used only with Start-Job, Start-ThreadJob, 
ForEach-Object -Parallel, except with pipeline variables,
and Invoke-Command when targeting remote computers.

Note that I’ve cut out the workflow reference, as it doesn’t apply to PS Core anymore, and that I’ve also added Start-ThreadJob.