In what conditions does powershell unroll items in the pipeline?

by

Empirical Test Results

I’d rather have an analytical basis for these results, but I need an answer so I can move on. So, here are the results of my stab at an empirical test to discover which collections are unrolled by powershell’s pipeline, and which aren’t:

True in a column indicates there’s probably some unrolling occurring.

StartingType                          ChangedInCmdlet^  ChangedWhenEmitted**
------------                          ---------------   ------------------
System.String                                           
System.Collections.ArrayList          True              True
System.Collections.BitArray           True              True
System.Collections.Hashtable
System.Collections.Queue              True              True
System.Collections.SortedList
System.Collections.Stack              True              True
System.Collections.Generic.Dictionary                   
System.Collections.Generic.List       True              True

These are results for a line of powershell that looks like this:

$result = $starting | Cmdlet

^ The ChangedInCmdlet column indicates that the type of $starting is different when it appears inside Cmdlet.

** The ChangedWhenEmitted column indicates that the type of $result is different when it is assigned to $result from when it was emitted inside Cmdlet.

There’s probably some nuance in there for some types. That nuance can be analyzed by looking at the details of the output of the test script below. The whole test script is below.

Test Script

[System.Reflection.Assembly]::LoadWithPartialName('System.Collections') | Out-Null
[System.Reflection.Assembly]::LoadWithPartialName('System.Collections.Generic') | Out-Null

Function BackThroughPipeline{
    [CmdletBinding()]
    param([parameter(position=1)]$InputObject)
    process{$InputObject}
}

Function EmitTypeName{
    [CmdletBinding()]
    param([parameter(ValueFromPipeline=$true)]$InputObject)
    process{$InputObject.GetType().FullName}
}

$objects = (New-Object string 'TenTwentyThirty'),
           ([System.Collections.ArrayList]@(10,20,30)),
           (New-Object System.Collections.BitArray 16),
           ([System.Collections.Hashtable]@{ten=10;twenty=20;thirty=30}),
           ([System.Collections.Queue]@(10,20,30)),
           ([System.Collections.SortedList]@{ten=10;twenty=20;thirty=30}),
           ([System.Collections.Stack]@(10,20,30)),
           (& {
               $d = New-Object "System.Collections.Generic.Dictionary``2[System.String,int32]"
               ('ten',10),('twenty',20),('thirty',30) | % {$d.Add($_[0],$_[1])}
               $d
           }),
           (& {
               $l = New-Object "System.Collections.Generic.List``1[int32]"
               10,20,30 | % {$l.Add($_)}
               $l
           })

$objects | 
    % {
        New-Object PSObject -Property @{
                StartingType  = $_.GetType().FullName
                StartingCount = $_.Count
                StartingItems = $_
                InCmdletType  = $_ | EmitTypeName
                InCmdletCount = ($_ | EmitTypeName).Count
                AfterCmdletType   = (BackThroughPipeline $_).GetType().FullName
                AfterCmdletItems  = (BackThroughPipeline $_)
                AfterCmdletCount  = (BackThroughPipeline $_).Count
                ChangedInCmdlet    = if ($_.GetType().FullName -ne ($_ | EmitTypeName) ) {$true};
                ChangedWhenEmitted = if (($_ | EmitTypeName) -ne (BackThroughPipeline $_).GetType().Fullname ) {$true}
            }
    }

Out-Collection Cmdlet

This testing eventually led me to create a cmdlet that conditionally wraps collections in sacrificial arrays to (hopefully) reliably prevent loop unrolling. That cmdlet is called Out-Collection and is in this github repository.

More Related Contents:

  1. Why should I avoid using the increase assignment operator (+=) to create a collection
  2. Writing console output to a file – file is unexpectedly empty
  3. Are the cmdlets in a pipeline executing in parallel?
  4. How to run an EXE file in PowerShell with parameters with spaces and quotes
  5. Use PowerShell to run virus scan on multiple servers
  6. Unable to access UNC Paths in Powershell remote session
  7. PowerShell IE9 ComObject has all null properties after navigating to webpage
  8. PowerShell’s Invoke-RestMethod equivalent of curl -u (Basic Authentication)
  9. Remove files and foldes on Azure before a new deploy from VSTS
  10. PowerShell: Unable to find type when using PS 5 classes
  11. Prompt for user input in PowerShell
  12. Run a [Code] or PowerShell script in Inno Setup compiler
  13. Recreating a Dictionary from an IEnumerable
  14. Can the following Nested foreach loop be simplified in PowerShell?
  15. Add Write-Progress to Get-Job/Wait-Job
  16. Powershell with exchange– How do I append all verbose output to a file
  17. How to handle command-line arguments in PowerShell
  18. How to send email with PowerShell
  19. How to split string by string in Powershell
  20. Powershell: Colon in commandlet parameters
  21. Passing PowerShell variables into a script block
  22. Why is $false -eq “” true?
  23. Is there an alternative to GUID when using msiexec to uninstall an application?
  24. Can’t install nuget package because of “Failed to initialize the PowerShell host”
  25. What security setting is preventing Remote PowerShell 2.0 from accessing UNC paths
  26. PowerShell Capture Write-Host output
  27. start remote process within the context
  28. How can I elevate Powershell while keeping the current working directory AND maintain all parameters passed to the script?
  29. Are you able to use PtrToStringAuto to decrypt a secure string in Powershell 7 on macOS?
  30. Prevent trailing newline in PowerShell Out-File command

Leave a Comment