Null in PowerShell

• null is an automatic variable in PowerShell used to represent NULL
• PowerShell treats $null as an object with a value of NULL
• Anytime you try to use a variable that you have not initialized, the value is $null (this is one of the most common ways that $null sneaks into code)
• If you mistype a variable name, then PowerShell sees it as a different variable and the value is $null
• Value coming from a command that don’t give any results will be $null
• If you have a collection but try to access an element that is not in the collection, you get a $null result
• If you try to access a property or sub property of an object that doesn’t have the specified property, you get a $null (It doesn’t matter if the variable is $null or an actual object)
• Calling a method on a $null object throws a RuntimeException
• $null should be on the left side of equality comparisons (use PSScriptAnalyzer to check: Invoke-ScriptAnalyzer .\program.ps1)
• $null.Count => 0
• foreach loop doesn’t enumerate over a $null collection
• More: https://learn.microsoft.com/en-us/powershell/scripting/learn/deep-dives/everything-about-null

Nullable variable

The concept “nullable type for value types” in C# is different than how null is handled in PowerShell.

Only reference types can be $null, but PowerShell allows for variables to be any type. If you decide to strongly type a value type, it cannot be $null, but still if you assign $null then PowerShell converts that $null to a default value for many types. For example:

# Altough only reference types can be $null, PowerShell allows $null to be assigned to a value type variable
# The trick here is PowerShell converts that $null to a default value for the corresponding value type (not all value types)
# There are some types that do not have a valid conversion from $null. These types generate a "Cannot convert null to type" error.

PS> [int] $number = $null
PS> $number
0

PS> [bool] $isDone = $null
PS> $isDone
False