PossibleIncorrectComparisonWithNull

Severity Level: Warning

Description

This rule detects comparisons where $null isn't on the left side of the comparison operator. To ensure that PowerShell performs comparisons correctly, the $null element should be on the left side of the operator.

There are multiple reasons why this placement matters:

• $null is a scalar value. When the value on the left side of an operator is a scalar, comparison operators return a Boolean value. When the value is a collection, the comparison operators return any matching values or an empty array if there aren't any matches in the collection.
• PowerShell performs type casting on the right-hand operand, resulting in incorrect comparisons when $null is cast to other scalar types.

The only way to reliably check if a value is $null is to place $null on the left side of the operator so that a scalar comparison is performed.

Comparison operator behavior

The comparison operator works by design in the following way.

# This example returns 'false' because the comparison doesn't return any objects from the array
if (@() -eq $null) { 'true' } else { 'false' }

# This example returns 'true' because the array is empty
if ($null -ne @()) { 'true' } else { 'false' }

This behavior can produce unexpected results, especially when you intend to perform a null check.

The following example demonstrates how comparison operators behave when the left-hand side is a collection. The operator compares each element in the collection to the right-hand side value and returns only the matching elements from the collection.

PS> 1,2,3,1,2 -eq $null
PS> 1,2,3,1,2 -eq 1
1
1
PS> (1,2,3,1,2 -eq $null).count
0
PS> (1,2,$null,3,$null,1,2 -eq $null).count
2

Example

Noncompliant

function Test-CompareWithNull
{
    if ($DebugPreference -eq $null)
    {
    }
}

Compliant

function Test-CompareWithNull
{
    if ($null -eq $DebugPreference)
    {
    }
}