Union-Object

Most native PowerShell cmdlets that handle an array of objects (like e.g. Export-CSV) only look to the first object to define the concerned properties. This behavior can lead to unexpected results. Consider the following object:

$List = @(
    New-Object PSObject -Property @{Id = 2}
    New-Object PSObject -Property @{Id = 1}
    New-Object PSObject -Property @{Id = 3; Name = "Test"}
)

If you simply display this object array, only the Id property will be listed:

PS C:\> $List
 
Id
--
 2
 1
 3

You might quickly fix this by aligning the properties of the first object with the rest of the objects (Name = $Null):

$List = @(
    New-Object PSObject -Property @{Id = 2; Name = $Null}
    New-Object PSObject -Property @{Id = 1}
    New-Object PSObject -Property @{Id = 3; Name = "Test"}
)

This will work for most of the cases:

PS C:\> $List
 
Id Name
-- ----
 2
 1
 3 Test

But not in every case:

PS C:\> $List | Sort Id
 
Id
--
 1
 2
 3

In other words, it is required to align every object in de array to prevent that you lose any properties. The quickest way is to use the Select-Object cmdlet: Select-Object Id, Name, knowing that just selecting all objects: Select-Object *, doesn’t resolve the issue. This mean that it is still required to iterate through all the objects to define all the properties.

Cmdlet

Function Union-Object ([String[]]$Property = @()) {             # https://powersnippets.com/union-object/
    $Objects = $Input | ForEach {$_}                            # Version 02.00.01, by iRon
    If (!$Property) {ForEach ($Object in $Objects) {$Property += $Object.PSObject.Properties | Select -Expand Name}}
    $Objects | Select ([String[]]($Property | Select -Unique))
} Set-Alias Union Union-Object

Syntax

<Object[]> | Union-Object [[-Property] <String[]>]

Parameters

[-Property] <String[]> Redefines the properties, if omitted all objects will be scanned to define the common properties.

Example:

$list | Union-Object | Export-Csv

The Union-Object cmdlet was originally published at StackOverflow