What are the rules for automatic arrays wrapping/unwrapping? - powershell

Question

Consider this code:
$a = '[{"a":"b"},{"c":"d"}]'
"Test1"
$a | ConvertFrom-Json | ForEach-Object { $_.GetType() }
"Test2"
$b = $a | ConvertFrom-Json
$b | ForEach-Object { $_.GetType() }
This produces the following output:
Test1
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True Object[] System.Array
Test2
True False PSCustomObject System.Object
True False PSCustomObject System.Object
Evidently, if we use a temporary variable, whatever is passed down the pipeline is not the same thing that is passed if we do not use one.
I would like to know what the rules that powershell uses for automatic array wrapping / unwrapping are, and if the using a temp var the best course of action if we need to iterate through a json array.
Update 1
Logically ConvertFrom-Json should return an array with the input given and ForEach-Object should iterated on the said array. However in the first test this does not happen. Why?
Update 2
Is it possible that it's ConvertFrom-Json specific? Like bug/issue?

Answer 1

There is only one rule with regard to pipeline items' unwrapping: all arrays and collections written to the pipeline are always getting unwrapped to the items ("unwrapped one level down" or "unwrapped in a non-recursive fashion" would be more correct statement but for the sake of simplicity we are not going to consider nested arrays so far).
There is still a possibility to override this behavior by using unary comma operator:
$a = 1,2
$a | ForEach-Object{ $_.GetType() }
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True Int32 System.ValueType
True True Int32 System.ValueType
,$a | ForEach-Object{ $_.GetType() }
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True Object[] System.Array
In the second case Powershell pipeline engine unwrapped $a but afterwards the result got wrapped back to array by , operator.
As to ConvertFrom-Json case I personally believe that its observed behavior is more predictable as it allows you to capture JSON arrays as a whole by default.
If you are interested in the details, the function Get-WrappedArray in the code below imitates ConvertFrom-Json's behavior:
function Get-WrappedArray {
Begin { $result = #() }
Process { $result += $_ }
End { ,$result }
}
$a | Get-WrappedArray | ForEach-Object{ $_.GetType() }
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True Object[] System.Array
$b = $a | Get-WrappedArray
$b | ForEach-Object{ $_.GetType() }
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True Int32 System.ValueType
True True Int32 System.ValueType