Powershell: Colon in commandlet parameters

What's the deal with Powershell commandlet switch parameters that require a colon?

Consider Exchange 2010 management shell cmdlet Move-ActiveMailboxDatabase. The Confirm switch is a System.Management.Automation.SwitchParameter and must be used like so,

Move-ActiveMailboxDatabase -Confirm:$false

Without the colon the command fails to recognize the don't confirm switch like so,

Move-ActiveMailboxDatabase -Confirm $false

Why is that? What's the difference the colon makes there? Why Exchange2010 seems to be about the only thing I've noticed this behavior?

I've browsed through Powershell in Action and Powershell 2.0, but didn't find anything about this syntax. Scope resolution and .Net object access uses are documented on those books though.

My Google-fu found an article which claims that it explicitly forwards switch parameter values, but fails to explain what that is about.

Solution 1:

When you do:

Move-ActiveMailboxDatabase -Confirm $false

you are not saying Confirm parameter accepts the $false. You are saying -Confirm and also passing an (separate) argument to the cmdlet with value $false.

Since Confirm is a switch, just the presence of -Confirm means it is true. Absence of -Confirm means it is false.

Let me give you a script example:

param([switch]$test)

write-host Test is $test

If you just run the script without any arguments / paramters i.e .\script.ps1 you get output:

Test is False

If you run it as .\script.ps1 -test, the output is

Test is True

If you run it as .\script.ps1 -test $false, the output is

Test is True

If you run it as .\script.ps1 -test:$false the output is

Test is False

It is in scenarios where the value for a switch variable itself has to be determined from another variable that the : is used.

For example, consider the script:

param ([boolean]$in)

function func([switch] $test){
write-host Test is $test
}

func -test:$in

Here if you run it as .\script.ps1 -in $false, you get

Test is false

If you weren't able to use the :, you would have had to write it as:

if($in){  func -test}
else { func }

Solution 2:

The colon can be used with every parameter value but is more special in the case of switch parameters. Switch parameters don't take values, they are either present ($true) or absent ($false).

Imagine you have a function like this:

function test-switch ([string]$name,[switch]$force) { ... }

And you call it like so:

test-switch -force $false

Since switch parameters are either present or not, $false would actually bind to the Name parameter. So, how do you bind a value to a switch parameter? With the colon: