CODEWITHSUNDEEP
powershellparametersswitch-statement
Riana
Riana

Reputation: 689

Powershell "special" switch parameter

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

Upvotes: 5

Views: 2843

Answers (3)

mklement0
mklement0

Reputation: 437698

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 PowerShell (Core) 7 v7.4.x - 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) when targeted by name in a given invocation.

  • 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.

Upvotes: 4

Max Cascone
Max Cascone

Reputation: 833

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
    }
  }
}

Upvotes: 0

Maximilian Burszley
Maximilian Burszley

Reputation: 19664

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.

Upvotes: 6

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