Description

To ensure that PowerShell performs comparisons correctly, the $null element should be on the left side of the operator.

There are a number of reasons why this should occur:

$null is a scalar. When the input (left side) to an operator is a scalar value, comparison operators return a Boolean value. When the input is a collection of values, the comparison operators return any matching values, or an empty array if there are no matches in the collection. 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.
PowerShell will perform type casting left to right, resulting in incorrect comparisons when $null is cast to other scalar types.

How

Move $null to the left side of the comparison.

Example of incorrect code:

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

Example of correct code:

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