Pass array value to a function

Question

I have created the following to get understanding of how a variable array is passed to a function:

# Define array
[array]$Global:P_sourceHostName = @()
[array]$Global:P_destinationHostName = @()

# Add string values to source array
$global:P_sourceHostName = "ABC"
$global:P_sourceHostName += "DEF"
$global:P_sourceHostName += "GHI"

# add string values to destination array
$global:P_destinationHostName = "zzz"
$global:P_destinationHostName += "yyy"

function test {
    Param(
        [string]$paramA="",
        [string]$paramB=""
    )

    Write-Host "test function > paramA: $paramA"
    Write-Host "test function > paramB: $paramB"
}

$i = 0
# Pass the individual value to a function
test ($Global:P_sourceHostName[$i],$Global:P_destinationHostName[$i])

#Pass the individual value to a function with an additional text
test ("AAA $Global:P_sourceHostName[$i]", "BBB $Global:P_destinationHostName[$i]")

What resulted is:

test function > paramA: ABC zzz
test function > paramB: 
test function > paramA: AAA ABC DEF GHI[0] BBB zzz yyy[0]
test function > paramB:

Question:

Why the first call of test function, it resulted with a blank "paramB"?
Why the second call of test function, it combines the text but does not resulted in the correct array value?

Ansgar Wiechers · Accepted Answer · 2017-10-24 07:59:46Z

Why the first call of test function, it resulted with a blank "paramB"?

Because the array is passed as a single argument to paramA. You need to use splatting to have the elements of an array passed to individual parameters.

$params = $global:P_sourceHostName[$i], $global:P_destinationHostName[$i]
test @params

Otherwise use distinct arguments (pass the parameters without a comma between them):

test $global:P_sourceHostName[$i] $global:P_destinationHostName[$i]

or named parameters:

test -paramA $global:P_sourceHostName[$i] -paramB $global:P_destinationHostName[$i]

Why the second call of test function, it combines the text but does not resulted in the correct array value?

Because you put your variables in a string and PowerShell does only simple variable expansion inside strings. More complex things like index operators or dot-access are ignored. An expression like this

$a = 'a', 'b', 'c'
"$a[0]"

effectively becomes

$a = 'a', 'b', 'c'
($a -join ' ') + '[0]'

hence the output is

a b c[0]

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