PowerShell Switch type never $null

Tales from the code review…

How do you test for a switch type in a PowerShell script?

How do you test for the *absence* of a switch in a PowerShell Script?

Came across this recently, and decided to dig into it further.

Script:

Function Test-Switch ( [switch] $Test ) {
# Correct use of a switch Test (True case)
if ( $test ) {
"Do Something"
}
# Bad use of a switch test (False case)
if ( $test -eq $null ) {
"Never going to do it!"
}
# Better use of a switch test (False case)
if ( -not $test ) {
"Don't do it"
}
}
Test-Switch
Test-Switch Test:$False
Test-Switch Test
Test-Switch Test:$True

view raw
test-switches.ps1
hosted with ❤ by GitHub

IN the example above, We have a function with a single switch argument. We then test against that argument, displaying “do something” if it’s set, and “Don’t do it” if not set.

Example Output:

PS C:\Users\Keith> C:\Users\Keith\Source\Example\test-switches.ps1
Don't do it
Don't do it
Do Something
Do Something

Cool!  Um… where did the “Never going to do it!” go? Well turns out that even when we don’t specify -test as an argument to the function, it’s still a switch defined as IsPresent = $false. So testing to see if it’s equal to $null will always fail, because it’s never $null.

 

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