Why is the value of “switch” type parameter passed to a string parameter?

问题

The test code (test.ps1):

Param(
    [string]$content = $null,
    [switch]$flag = $false
)
Write-Host $content

Output:

PS C:\Users\powershell> .\test.ps1

PS C:\Users\powershell> .\test.ps1 -flag $true    
True
PS C:\Users\powershell> .\test.ps1 -flag $false
False
PS C:\Users\powershell> .\test.ps1 -flag $true -content "lalala"
lalala

No matter I set $content to $null, or "", or without any default value, the output is the same.

So why does $content take the value from $flag?

回答1:

@PetSerAl already indicated what your misunderstanding is, but maybe it was a bit too brief, so I'll elaborate a little more.

A [switch] parameter doesn't take a value like regular parameters. You normally set it to true or false respectively by providing or omitting the parameter:

PS C:\> Get-Content .\test.ps1
Param(
    [string]$Content = $null,
    [switch]$Flag
)
$flag.IsPresent
$Content
PS C:\> .\test.ps1
False

PS C:\> .\test.ps1 -Flag
True

Alternatively you can explicitly pass a value by putting a colon between switch parameter and value:

PS C:\> .\test.ps1 -Flag:$false
False

PS C:\> .\test.ps1 -Flag:$true
True

Whitespace after the colon is allowed, but not before (-Flag: $false passes $false as the switch value, -Flag :$false doesn't).

If you try to assign the switch value without the colon the value is actually passed to the next parameter in line, in your case the -Content parameter:

PS C:\> .\test.ps1 -Flag $false
True         # value of the parameter $Flag
False        # value of the (positional) parameter $Content
PS C:\> .\test.ps1 -Flag -Content $false
True         # value of the parameter $Flag
False        # value of the (named) parameter $Content

If you use -Flag $true (without the colon) and pass a value to the (named) parameter -Content the value $true is passed as an unnamed third parameter (accessible via the automatic variable $args), same as if you put the value at the end of the statement:

PS C:\> Get-Content .\test2.ps1
Param(
    [string]$Content = $null,
    [switch]$Flag
)
$flag.IsPresent
$Content
$args[0]
PS C:\> .\test2.ps1 -Flag $false -Content 'something'
True
something
False        # $false passed as unnamed 3rd parameter
PS C:\> .\test2.ps1 -Flag:$false -Content 'something'
False        # $false passed as explicit value of switch parameter $Flag
something
PS C:\> .\test2.ps1 -Flag -Content 'something' $false
True
something
False        # $false passed as unnamed 3rd parameter

回答2:

Give this test code a run, calling it as you did in the question. Add this test:

C:\Users\powershell> .\test.ps1 -flag

Script will illustrate the following points:

As per 4c74356b41's comment, switches are $false unless specified on the command line. So -flag sets the $flag switch to true; -flag $true sets the flag to true and passes the argument $true to content.
From this, no need to specify $flag=$false in script params
Added a test so that you can see when $content is empty/null as there is also no need for $content=$null

param(
    [string]$content,
    [switch]$flag
)

$test = [string]::IsNullOrEmpty($content)

Write-Output "content: $content"
Write-Output "flag   : $flag"
Write-Output "Is content null: $test"

回答3:

I will add to this that you can assign the value via splatting as well, changing the value via the splat parameter assignment from $true to $false will alter your switch 'detection'. This is useful when you are using another value or property to determine if a switch should be used or not.

function FooBar {
    param (
        [switch]
        $Foo
    )
    if ($Foo){
        "Foo"
    }
    else{
        "Bar"
    }
}
$splat = @{
    Foo = $False
}
FooBar @splat

来源：https://stackoverflow.com/questions/45691877/why-is-the-value-of-switch-type-parameter-passed-to-a-string-parameter

标签

powershell

parameter-passing