"Property" output is similar as if "Passthru" is specified

[gwpelletier]

Not sure if this is a bug or expected behavior.

When I use Property to handle comparison, the output is similar as if "Passthru" was specified, but just shows the properties I'm comparing against

Source object:

PS C:\WINDOWS\system32> $MonitorSetup_Current


BitsPerPixel : 32
Bounds       : {X=0,Y=0,Width=3440,Height=1440}
DeviceName   : \\.\DISPLAY1
Primary      : True
WorkingArea  : {X=0,Y=0,Width=0,Height=0}

BitsPerPixel : 32
Bounds       : {X=3440,Y=-307,Width=1200,Height=1920}
DeviceName   : \\.\DISPLAY2
Primary      : False
WorkingArea  : {X=0,Y=0,Width=0,Height=0}

Comparison object:

PS C:\WINDOWS\system32> $MonitorSetup_New


BitsPerPixel : 32
Bounds       : {X=0,Y=0,Width=3440,Height=1440}
DeviceName   : \\.\DISPLAY1
Primary      : True
WorkingArea  : {X=0,Y=0,Width=3440,Height=1400}

Property output (comparing DeviceName,Bounds,Primary)

PS C:\WINDOWS\system32> Compare-Object -ReferenceObject $MonitorSetup_Current -DifferenceObject $MonitorSetup_New -Property DeviceName,Bounds,Primary

DeviceName   Bounds                                 Primary SideIndicator
----------   ------                                 ------- -------------
\\.\DISPLAY2 {X=3440,Y=-307,Width=1200,Height=1920}   False <=           

Full output, no parameters:

PS C:\WINDOWS\system32> Compare-Object -ReferenceObject $MonitorSetup_Current -DifferenceObject $MonitorSetup_New

InputObject                                                                                                                       SideIndicator
-----------                                                                                                                       -------------
Screen[Bounds={X=0,Y=0,Width=3440,Height=1440} WorkingArea={X=0,Y=0,Width=3440,Height=1400} Primary=True DeviceName=\\.\DISPLAY1  =>           
Screen[Bounds={X=0,Y=0,Width=3440,Height=1440} WorkingArea={X=0,Y=0,Width=0,Height=0} Primary=True DeviceName=\\.\DISPLAY1        <=           
Screen[Bounds={X=3440,Y=-307,Width=1200,Height=1920} WorkingArea={X=0,Y=0,Width=0,Height=0} Primary=False DeviceName=\\.\DISPLAY2 <=           

PSVersionTable output:

PS C:\WINDOWS\system32> $PSVersionTable

Name                           Value                                                                                                                            
----                           -----                                                                                                                            
PSVersion                      5.1.17134.858                                                                                                                    
PSEdition                      Desktop                                                                                                                          
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}                                                                                                          
BuildVersion                   10.0.17134.858                                                                                                                   
CLRVersion                     4.0.30319.42000                                                                                                                  
WSManStackVersion              3.0                                                                                                                              
PSRemotingProtocolVersion      2.3                                                                                                                              
SerializationVersion           1.1.0.1                                                                                                                          

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Document Details

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• ID: a7615306-98e3-0b79-ac87-58bb191b4349
• Version Independent ID: 85f1e969-8216-bed4-2204-409cbd9befd4
• Content: Compare-Object (Microsoft.PowerShell.Utility)
• Content Source: reference/6/Microsoft.PowerShell.Utility/Compare-Object.md
• Product: powershell
• Technology: powershell-cmdlets
• GitHub Login: @JamesWTruher
• Microsoft Alias: jimtru

[JamesWTruher]

When Compare-Object runs, it has 2 paths to emit an object.

• If you use -PassThru, the objects which are emitted, have a NoteProperty attached to them called SideIndicator which shows which bucket the object is in (reference/difference/both - depending on the other parameters), but they don't lose their original object type which means the usual formatting takes place.
• Otherwise, a new PSCustomObject is created which has two properties:
  • InputObject which is the comparison object
  • SideIndicator which is the bucket of ref/diff/both
    This new object now looks like an object which is disconnected from the original (it's not _really- since that object is in the InputObject property)

The following examples might make it more clear:

PS> $a = get-date
PS> compare-object -IncludeEqual $a $a 

InputObject            SideIndicator
-----------            -------------
10/18/2019 10:57:07 AM ==

PS> compare-object -IncludeEqual $a $a -pass

Friday, October 18, 2019 10:57:07 AM

PS> (compare-object -IncludeEqual $a $a ).SideIndicator     
==
PS> (compare-object -IncludeEqual $a $a -pass).SideIndicator
==

PS> (compare-object -IncludeEqual $a $a -pass).psobject.typenames [0]                
System.DateTime

PS> (compare-object -IncludeEqual $a $a ).psobject.typenames[0]     
System.Management.Automation.PSCustomObject

The difference in the last 2 examples is pretty clear. When using -PassThru, the original object is what is emitted (with a NoteProperty SideIndicator). Without -PassThru, a new object is created, which has 2 properties; InputObject and SideIndicator which means different formatting will come into play, since they're different object types.