Why is an empty PowerShell pipeline not the same as null?

Question

I am trying to understand the behavior of the @() array constructor, and I came across this very strange test.

It seems that the value of an empty pipeline is \"not

Answer 1

The reason you're experiencing this behaviour is becuase $null is a value. It's a "nothing value", but it's still a value.

PS P:\> $y = 1,2,3,4 | ? { $_ -ge 5 }

PS P:\> Get-Variable y | fl *

#No value survived the where-test, so y was never saved as a variable, just as a "reference"

Name        : y
Description : 
Value       : 
Visibility  : Public
Module      : 
ModuleName  : 
Options     : None
Attributes  : {}


PS P:\> $z = $null


PS P:\> Get-Variable z | fl *

#Our $null variable is saved as a variable, with a $null value.

PSPath        : Microsoft.PowerShell.Core\Variable::z
PSDrive       : Variable
PSProvider    : Microsoft.PowerShell.Core\Variable
PSIsContainer : False
Name          : z
Description   : 
Value         : 
Visibility    : Public
Module        : 
ModuleName    : 
Options       : None
Attributes    : {}

The way @() works, is that it guarantees that the result is delievered inside a wrapper(an array). This means that as long as you have one or more objects, it will wrap it inside an array(if it's not already in an array like multiple objects would be).

$y is nothing, it's a reference, but no variable data was stored. So there is nothing to create an array with. $z however, IS a stored variable, with nothing(null-object) as the value. Since this object exists, the array constructor can create an array with that one item.