Wednesday, December 13, 2006

Negating PowerShell switch parameters

The example below should tell it all. Just copy it into a file, execute it and see the output.


set-psdebug -trace 1
# f1 is a normal function with a switch parameter

function f1([switch] $showOutput) {
if ($showOutput.isPresent) {
"f1"
}
}

# f2 is a function with a switch parameter defaulted to present
function f2([switch] $showOutput=[switch]::present) {
if ($showOutput.isPresent) {
"f2"
}
}


# f3 is a function with a switch parameter defaulted to true, this gives the same result
# as using [switch]::present
function f3([switch] $showOutput=$true) {
if ($showOutput.isPresent) {
"f3"
}
}

# run f1, no output shown
f1
# run f1, show output
f1 -showOutput

# run f2, show output
f2

# run f2, show output
f2 -showOutput

# run f2, no output
f2 -showOutput:$false

# run f3, same behavior as f2
f3
f3 -showOutput
f3 -showOutput:$false

# Show the acceptable values for a switch parameter
f2 -showOutput:no
# f2 : Cannot convert value "System.String" to type "System.Management.Automation.SwitchParameter",
# parameters of this type only accept booleans or numbers, use $true, $false, 1 or 0 instead.
# At line:1 char:16
# + f2 -showOutput:n <<<< o


So what can this be used for? It is good for defaulting switches like WhatIf to true. This forces the user to enter -WhatIf:$false to get the destructive action executed

Updated: Fixed typo, thanks John!

1 comment:

John said...

There is a typo in the code I think:

f3 -showOutptu:$false

should be:

f3 -showOutput:$false

John.