Why do PowerShell comparison operators not enumerate collections of size 1?

tl;dr

In PowerShell conditionals / implicit Boolean contexts:

• Single-element arrays are treated like scalars: that is, their one and only element itself is interpreted as a Boolean.^[1]

• 2+-element arrays are always $true, irrespective of their content.

With an array as the LHS, array-aware operators such as -eq invariably also output an array.

Since your array elements are all $null and you compare to $null, your comparison is an effective no-op - e.g., @( $null ) -eq $null results in @( $null ) - and your conditionals are equivalent to:

[bool] @( $null, $null ) # -> $true - array with 2+ elements is always $True
[bool] @( $null )        # -> $false(!) - treated like: [bool] $null

Perhaps surprisingly, the implicit Boolean logic applies pipeline logic to an array:

That is, a single-element array is (conceptually) unwrapped and its element is interpreted as a Boolean.

Therefore, [bool] @( $null ) is treated the same as [bool] $null, which is $false.

Generally, @( <one-and-only-element> ) (or , <one-and-only-element>) is treated the same as <one-and-only-element> in a Boolean context.

By contrast, if an array has 2 or more elements, it is always $true in a Boolean context, even if all its elements would individually be considered $false.

Workaround for testing whether an arbitrary array is empty:

Base your conditional on the .Count property:

if ( (<array>).Count ) { $true } else { $false }

You could append -gt 0, but that's not strictly necessary, because any nonzero value is implicitly $true.

Applied to your example:

PS> if ( ( @($null) -eq $null ).Count ) { $true } else { $false }
True

Testing an arbitrary value for being a (scalar) $null:

if ($null -eq <value>) { $true } else { $false }

Note how $null must be used as the LHS in order to prevent the array-filtering logic from taking effect, should <value> be an array.

That's also the reason why Visual Studio Code with the PowerShell extension advises "$null should be on the left side of comparisons" if you write something like $var -eq $null.

[1] To-Boolean conversion summary:

• Among scalars:

  • The following are implicitly $false:

    • ''/"" (empty string)

    • 0 (of any numeric type).

    • $null

      • Pitfall: Comparing $null to a Boolean explicitly with -eq is always $false, even with $null as the RHS (despite the RHS normally getting coerced to the type of the LHS):

        $false -eq $null # !! $false - unlike `$false -eq [bool] $null`
        

  • Pitfall: Any non-empty string evaluates to $true

    • e.g., [bool] 'False' is $true

    • Note that this differs from explicit string parsing: [bool]::Parse('false') does return$false (and $true for 'true', but recognizes nothing else).

  • Instances of any other (non-collection) type are implicitly $true, including of type [pscustomobject] and [hashtable] (which PowerShell treats as a single object, not as a collection of entries).

• Among collections such as arrays (more accurately, collection-like types that implement the IList interface - see the source code):

  • Empty collections are always $false, as is the special "null collection" value indicating the absence of output from a command, [System.Management.Automation.Internal.AutomationNull]::Value.

  • Pitfall: Single-element collections evaluate to:

    • If the one and only element is a scalar: its Boolean value
    • If that element is itself a collection: $true if it has at least 1 element (irrespective of what that element is).

  • 2+-element collections are always $true.

The following items evaluate to $false:

@()
0
$null
$false
''

In your first example:

@($null, $null) -eq $null

This evaluates to $null, $null which is a non-zero collection, so it is $true. You can observe this with the following:

[bool]($null, $null)

In your second example, what you're observing is filtering of an array like the first case, but returning a scalar (instead of an array) since only one item of the array matched the filter:

@($null) -eq $null

This evaluates to @($null) but powershell is evaluating it as a scalar in a boolean context, so it returns $false, observed by:

[bool]@($null)

Footnote: in powershell v2, there was a bug with $null filtering which spawned the left-hand $null comparison. This bug caused if/else blocks to be skipped entirely.