Why do process blocks correctly return a hashtable, instead of an array of hashtables? - powershell

Question

I am trying to sort some REST API responses and extract data from them, and found this helpful answer on how to end up with a hashtable at the end.
$res= (Invoke-RestMethod #commonParams -uri "foo" -Method GET -FollowRelLink) | ForEach-Object { $_ } | Select-Object -Property id, filename | ForEach-Object -begin {$h=#{}} -process {$h[$_.filename] = $_.id} -end {$h}
Great, so I thought I'd shorten it a bit by getting rid of the begin/process/end blocks like so:
$res = (Invoke-RestMethod #commonParams -uri "foo" -Method GET -FollowRelLink) | ForEach-Object { $_ } | Select-Object -Property id, filename | ForEach-Object { $h=#{}; $h[$_.filename] = $_.id; $h }
All good, I should now be able to reference each item by it's key value using square brackets, but that doesn't work, only referencing by dot notation.
So I went searching and found this answer, which turned out to be correct:
$res.GetType()
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True Object[] System.Array
But, even changing from the generic hashtable constructor to using the .Add() method didn't help:
$res = (Invoke-RestMethod #commonParams -uri "foo" -Method GET -FollowRelLink) | ForEach-Object { $_ } | Select-Object -Property id, filename | ForEach-Object { $h=#{}; $h.add($_.filename, $_.id); $h }
$res.GetType()
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True Object[] System.Array
I add the process blocks back in:
$res = (Invoke-RestMethod #commonParams -uri "$foo" -Method GET -FollowRelLink) | ForEach-Object { $_ } | Select-Object -Property id, filename | ForEach-Object -begin {$h=#{}} -process {$h[$_.filename] = $_.id} -end {$h}
$res.GetType()
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True Hashtable System.Object
And it's all working as expected.
So I'd like to know, if possible, what exactly is happening when I use the process blocks as opposed to just making 'fake' multi-line functions in the pipeline?
And if anyone cares, I actually ended up avoiding the above issue completely by using Group-Object from here.

Answer 1

ForEach-Object emulates how script blocks work in the pipeline. The reason why the first and last snippets work as expected, returning a hashtable is because -Begin and -End not because of -Process which executes by default and is a mandatory parameter. If you don't use -Begin you would simply be creating and outputting a new hashtable per input object.
Using ForEach-Object
0..10 | ForEach-Object -Begin {
"executes only once, before the first input object is processed"
} -Process {
"processes each input object: $_"
} -End {
"executes only once, after the last input object is processed"
}
Using a script block
0..10 | & {
begin {
"executes only once, before the first input object is processed"
}
process {
"processes each input object: $_"
}
end {
"executes only once, after the last input object is processed"
}
}
about_Functions_Advanced_Methods explain how these blocks work.
Also your code could be simplified to this:
$h = #{}
Invoke-RestMethod #commonParams -uri "foo" -Method GET -FollowRelLink |
Write-Output |
ForEach-Object { $h[$_.filename] = $_.id }
Or using Group-Object -AsHashTable as you have already found:
$h = Invoke-RestMethod #commonParams -uri "foo" -Method GET -FollowRelLink |
Write-Output |
Group-Object FileName -AsHashTable
Write-Output in this case may not be needed if the output from Invoke-RestMethod is already enumerated.