In what conditions does powershell unroll items in the pipeline?

Question

Consider the following:

function OutputArray{
    $l = @(,(10,20))
    $l
}

(OutputArray) -is [collections.ienumerable]
# C:\ PS> True
(OutputArray).Count
# C:\ PS> 2

$l is "unrolled" when it enters the pipeline. This answer states that powershell unrolls all collections. A hashtable is a collection. However, a hashtable is of course unaffected by the pipeline:

function OutputHashtable{
    $h = @{nested=@{prop1=10;prop2=20}}
    $h
}

(OutputHashtable) -is [collections.ienumerable]
# C:\ PS> True
(OutputHashtable).Count
# C:\ PS> 1

This comment suggests that it is all IEnumerable that are converted to object arrays. However, both array and hashtable are ienumerable:

@(,(10,20)) -is [collections.ienumerable]
#True
@{nested=@{prop1=10;prop2=20}} -is [collections.ienumerable]
#True

What, exactly, are the conditions where powershell "unrolls" objects into the pipeline?

Answer 1

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.