How to display all properties with Format-Table cmdlet

For that, you can use below function to merge all properties into the first object:

function Complete-ObjectHeaders {
    # function to add properties to the first item in a collection of PSObjects
    # when this object is missing properties from items further down the array.
    # you may need this if you have such a collection and want to export it
    # to Csv, since Export-Csv (and also Format-Table) only looks at the FIRST
    # item to create the csv column headers.
    [CmdletBinding()]
    param (
        [Parameter(Mandatory = $true, Position = 0)]
        [PSObject[]]$Collection,

        [int]$MaxItemsToTest = -1,  # < 0 --> test all items in the collection
        [switch]$SortHeaders
    ) 

    # Try and find all headers by looping over the items in the collection.
    # The headers will be captured in the order in which they are found. 
    if ($MaxItemsToTest -gt 0) {
        $MaxItemsToTest = [math]::Min($MaxItemsToTest, $Collection.Count)
        $headers = for($i = 0; $i -lt $MaxItemsToTest; $i++) {
            ($Collection[$i].PSObject.Properties).Name
        }
        $headers = $headers | Select-Object -Unique
    }
    else {
        $headers = $Collection | ForEach-Object {($_.PSObject.Properties).Name} | Select-Object -Unique
    }

    if ($SortHeaders) { $headers = $headers | Sort-Object }

    # update the first object in the collection to contain all headers
    $Collection[0] = $Collection[0] | Select-Object $headers

    ,$Collection
}

Use like this:

$1 = [pscustomobject]@{ A='a1'; B='b1' }
$2 = [pscustomobject]@{ A='a2'; C='c2' }

# just output to console
Complete-ObjectHeaders -Collection $1,$2 | Format-Table -AutoSize

# or capture the merged array of objects in a new variable you can save as CSV file for instance
$merged = Complete-ObjectHeaders -Collection $1,$2
$merged | Export-Csv -Path 'D:\Test\Merged.csv' -NoTypeInformation

Output:

A  B  C 
-  -  - 
a1 b1   
a2    c2

Thanks @theo for the answer.

I used it to write my own version of a function that supports pipelining.

function Expand-Properties {
  [Cmdletbinding()]
  param(
    [Parameter(ValueFromPipeline)]
    $InputObject,
    [Parameter()]
    [Alias('All')]
    [switch]
    $ExpandAll,
    [Parameter()]
    [switch]
    $SortHeaders
  )

  begin {
    $collection = [System.Collections.ArrayList]::new()
    $properties = [System.Collections.ArrayList]::new()
  }

  process {
    [void]$collection.Add($InputObject)
    $properties.AddRange((($InputObject.PSObject.Properties).Name))
  }

  end {
    if ($SortHeaders) {
      $properties = $properties | Sort-Object -Unique
    } else {
      $properties = $properties | Select-Object -Unique
    }
    if ($ExpandAll) {
      for ($i = 0; $i -lt $collection.Count; ++$i) {
        $collection[$i] = $collection[$i] | Select-Object -Property $properties
      }
    } else {
      $collection[0] = $collection[0] | Select-Object -Property $properties
    }
    $collection
  }
}

EXAMPLE:

PS> $1 = [pscustomobject]@{ A='a1'; B='b1' }
PS> $2 = [pscustomobject]@{ A='a2'; C='c2' }
PS> $1, $2 | Expand-Properties

A  B  C 
-  -  - 
a1 b1   
a2    c2