Why this code passing switch parameters to a PowerShell function fails

Question

This question is about passing switch parameters. Let's see the code. I have this PowerShell 3.0 function:

#test1.ps1
param(
    [switch] $param1 = $false
)

Write-Host "param1: $($param1.IsPresent)"
Write-Host

I have this main PowerShell function that invokes test.ps1 in four different ways:

#Test0.ps1
cls

$param1 = $True

# 1
.\test1.ps1 -param1

# 2
.\test1.ps1 -param1:$true

# 3
$potato = "-param1:`$$($param1)"
Write-Host "Parameter value: $potato" 
.\test1.ps1 $potato

# 4
$command = ".\test1.ps1 -param1:`$$($param1)"
Write-Host "Command: $command" 
iex $command

exit

Why is the 3rd way of doing it failing? I know I can do 4th way but I would love to understand why 3rd is failing.

Here is the output. As result all the parameters should be True but third one is False...

param1: True

param1: True

Parameter value: -param1:$True
param1: False

Command: .\test1.ps1 -param1:$True
param1: True

Mathias R. Jessen · Accepted Answer · 2014-11-17 14:22:35Z

3

What happens is that:

The parser looks at the provided argument: "-param1:$true"
Fails to bind it to parameter param1, since the value you provided is a string, not a switch/bool
No specific parameter type is required at position 0, argument is ignored

If you make param1 positional, you can see how PowerShell fails to bind it properly:

function test-parambinding {param([Parameter(Position=0)][switch]$param1);$param1.IsPresent}
test-parambinding "-param1:`$true"

You'll see a ParameterArgumentTransformationException thrown before anything else happens

answered Nov 17, 2014 at 14:22

Mathias R. Jessen

178k13 gold badges175 silver badges234 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Mathias R. Jessen Over a year ago

Well, yes, a non-empty string will be treated as $True when you cast it as a bool.

Duncan · Accepted Answer · 2014-11-17 14:49:48Z

2

Mathias explains why your 3rd way of passing the parameter fails, but there is another way to pass parameters that lets you do roughly what you seem to be attempting here.

I've used a function here as it's a bit less to type when calling it, but your script file will work just the same:

PS C:\> function test1() {
param(
    [switch] $param1 = $false
)

Write-Host "param1: $($param1.IsPresent)"
Write-Host
}

PS C:\> $param1 = $True
PS C:\> $potato = @{'param1'=$param1}

PS C:\> $potato

Name                           Value                                                                                                  
----                           -----                                                                                                  
param1                         True                                                                                                   



PS C:\> test1 @potato
param1: True

So, instead of passing the argument and value as a single string, create a hash with the argument name as the key and the argument as the value. Then call the function or script using the @ splatting operator. (See help about_Splatting for more detail).

answered Nov 17, 2014 at 14:49

Duncan

96.4k15 gold badges129 silver badges160 bronze badges

1 Comment

Oscar Foley Over a year ago

+1: This is not answering directly to the question (Why it fails) but gives a very interesting alternative

Collectives™ on Stack Overflow

Why this code passing switch parameters to a PowerShell function fails

2 Answers 2

1 Comment

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related