Powershell "special" switch parameter

Question

I have the powershell function below

Function Test
{
    Param
    (               
        [Parameter()]
        [string]$Text = "default text"
    )

    Write-Host "Text : $($Text)"
}

And I would like to be able to call this function like below :

Test -Text : should display the default text on the host

Test -Text "another text" : should display the provided text on the host

My issue is that the first syntax is not allowed in powershell ..

Any ideas of how I can achieve this goal ? I would like a kind of 'switch' parameter that can take values other than boolean.

Thanks

Answer 1

The problem you're running into is with parameter binding. PowerShell is seeing [string] $Text and expecting a value. You can work around this like so:

function Test {
    param(
        [switch]
        $Text,

        [Parameter(
            DontShow = $true,
            ValueFromRemainingArguments = $true
        )]
        [string]
        $value
    )

    if ($Text.IsPresent -and [string]::IsNullOrWhiteSpace($value)) {
        Write-Host 'Text : <default text here>'
    }
    elseif ($Text.IsPresent) {
        Write-Host "Text : $value"
    }
}

Note: this is a hacky solution and you should just have a default when parameters aren't passed.

Answer 2

tl;dr

• PowerShell does not support parameters with optional values.

• A workaround is possible, but only for a single parameter.

---

Maximilian Burszley's helpful answer provides a workaround for a single parameter, via a catch-all parameter that collects all positionally passed arguments via the ValueFromRemainingArguments parameter property.

Fundamentally, though, what you're asking for is unsupported in PowerShell:

PowerShell has no support for parameters with optional values as of 7.2 - except for [switch] parameters, which are limited to [bool] values.

That is:

• Any parameter you declare with a type other than [switch] invariably requires a value (argument).

• The only other option is to indiscriminately collect any unbound positional arguments in a ValueFromRemainingArguments-tagged parameter, but you won't be able to associate these with any particular other bound parameter.

In other words:

• If you happen to need just one optional-argument parameter, the ValueFromRemainingArguments can work for you (except that you should manually handle the case of mistakenly receiving multiple values), as shown in Maximilian Burszley's answer.

• If you have two or more such parameters, the approach becomes impractical: you'd have to know in which order the parameters were passed (which PowerShell doesn't tell you) in order to associate the remaining positional arguments with the right parameters.

---

With [switch] parameters (using an imagined -Quiet switch as an example):

• The default value - if you just pass -Quiet -is $true.
• $false is typically indicated by simply not specifying the switch at all (that is, omitting -Quiet)

However, you may specify a value explicitly by following the switch name with :, followed by the Boolean value:

• -Quiet:$true is the same as just -Quiet

• -Quiet:$false is typically the same as omitting -Quiet; in rare cases, though, commands distinguish between an omitted switch and one with an explicit $false value; notably, the common -Confirm parameter allows use of -Confirm:$false - as opposed to omission of -Confirm - to override the value of the $ConfirmPreference preference variable.

While : as the separator between the parameter name and its argument (as opposed to the usual space char.) is supported with all parameters, with [switch] parameters it is a must so as to unequivocally signal that what follows is an argument for the switch parameter (which by default needs no argument) rather than an independent, positional argument.

---

The above tells us that PowerShell already has the syntax for general support of optional-argument parameters, so at some point in the future it could support them with any data type, as suggested in GitHub issue #12104.

Answer 3

I like @Maximilian Burszley's answer (and his name!) for String, I tweaked it for Ints:

function Optional-SwitchValue {
  [CmdletBinding()]
  param (
    [Switch]
    $Bump,

    [Int]
    $BumpAmount
  )

  Begin {
    # nifty pattern lifted from https://stackoverflow.com/questions/58838941/powershell-special-switch-parameter
    # default Bump to 1
    if ($Bump.IsPresent -and -not $BumpAmount) {
      $BumpAmount = 1
    }
  }
  
  Process {
    if($Bump) {
        #use $BumpAmount in some way
    }
  }
}