Testing `type = @( $data )` and `[type]::new( $data )`

Using Powershell New or Assignment.md

Explicitly calling ::new()

using namespace System.Collections.Generic
$example = 9, 4, 3

[List[int]]::new( [int[]]$example ) 

• I had to type it as an [int[]]

Using Coercion

$stuff = [list[int]] $example 

• $stuff.GetType() is [List[Int]]
• $stuff points to / was assigned to a value that was coerced into that type.
• But you can change the type that $stuff points to with a different type (see examples below)

Warning

Later you can change the datatype that $stuff points to ( example below )

As a type constraint

# Implicitly calling the constructor
# I didn't have to type it in this case
# this is type constraint on 'nums' when non null
[List[int]] $nums  = @( $example ) 

• $nums.GetType() is [List[Int]
• You can't replace it with an incompatible type

$stuff = 314
$nums = 314

• $stuff.GetType() is now a [int]. It is not an array.
• $nums.GetType() is still a [List[int]], even though it was set to a single value

Note

$nums.GetType() is still a List, even though It's a single value

$nums = @()

Note

$nums.GetType() a [List[int]] with 0 elements

$nums = 'a'
    # Error: MetadataError: Cannot convert value "a" to type "System.Int32".  Error: 
    # "The input string 'a' was not in a correct format."

Caution

Cannot convert value "a" to type "System.Int32" The input string 'a' was not in a correct format.

Note

nums.GetType() is still the type [List[int]] with 0 elements

You might be able to inspect the constraint if you drill down into

Get-variable 'stuff' -Verbose | Find-Member

Depending on the type, $null might not remove a constraint

$nums = $null

$null -eq $nums
   # true
   
$nums.gettype()
   # Error: InvalidOperation: You cannot call a method on a null-valued expression.

$nums = @( 'a' )
   # MetadataError: Cannot convert value "a" to type "System.Int32". Error: 
   # "The input string 'a' was not in a correct format."

It shows up as being a true null value, but still has some constraints attached

# both don't give a metadata error
$nums = @() 
$nums = 100 

Note

$nums.GetType() was null

now it's a [List[Int]]

Testing if $nums is null verses undefined

$null -eq $nums # True

$null -eq $nums -and 
   ( (get-variable 'nums' -ea Ignore).count -eq 0 )  # False

Other related options

$nums = $null
$null -eq (Get-variable 'nums' -ValueOnly -ea ignore)
$null -eq (Get-variable 'stuff' -ValueOnly -ea ignore)
    # out: True, False
    
$null -eq (Get-variable 'nums' -ea ignore).count -gt 0
$null -eq (Get-variable 'stuff' -ea ignore).count -gt 0
    # out: False, False
    
# or
$null -eq $nums
$null -eq $ExecutionContext.SessionState.PSVariable.Get('nums') 
    # out: True, False

Note

If it both exists and is null, -ValueOnly doesn't work as a test for existing

Other Links

• Why $null -eq $x vs $x -eq $null ? See linter rule PossibleIncorrectComparisonWithNull
• The type [List[Type]] runs on Win Powershell 5 and Pwsh 7. See Generics in Dotnet

Where does $ExecutionContext.SessionState.PSVariable take place? Maybe in SessionStateScope.cs