Null element in ArrayList causing 'Cannot bind argument to parameter' error

Question

Problem

When calling function f $array in code below (PowerShell v2), I am getting an error:

f : Cannot bind argument to parameter 'csvObject1' because it is null.

My Code

$hash1 = @{
    dependent_name = 'ADDM-Fun-3';
}
$obj1  = New-Object -TypeName PSObject -Property $hash1
$array = [System.Collections.ArrayList]@( $obj1, $null )

function f () {
    Param(
        [parameter(Position=0, Mandatory=$true)]
        [System.Collections.ArrayList]$array
    )
    "Hello"
}

f $array

Question

Why does Powershell do this? It seams to me to be a design flaw - but maybe I am not seeing the big picture.

Comments

I believe this error is occurring because of the second line in the ArrayList is $null. I am slightly shocked by this 'finding' because:

It has taken my about 4 hours to track down the issue.
This seams to imply that using strong type defintions in the function is a bad idea because it causes PowerShell to check every element in the array which is an unexepected overhead.
If I remove [System.Collections.ArrayList] from the function definition, the problem goes away.

Paweł Dyl · Accepted Answer · 2016-08-16 05:48:05Z

3

You should use AllowNullAttribute as stated in about_Functions_Advanced_Parameters help topic

Param
(
    [Parameter(Position=0, Mandatory=$true)]
    [AllowNull()]
    [System.Collections.ArrayList]$array
)

It is some kind of defensive programming. PS automatically unwraps arrays and hastables when pipelined. Think globally - you don't want empty server name in list of other server names when you pass bunch of them to function.

answered Aug 16, 2016 at 5:48

Paweł Dyl

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1 Comment

Kiwi Over a year ago

Thanks. Both answers above are great. Coming from a Java and .NET background, it has taken me a while to get used to this "Automatic Unwrapping". I really don't like using the magic comma on return values. I have started passing parameters into functions and return values out out of functions using Properties on a PSObject. It is slightly more overhead - but at least I get predictable results.

Micky Balladelli · Accepted Answer · 2016-08-16 05:39:11Z

3

Not sure why PowerShell generates an exception as null is a valid member for ArrayList. However, you can force your parameter validation by allowing nulls.

Like this:

[parameter(Position=0, Mandatory=$true)][AllowNull()][System.Collections.ArrayList] $array

Then there is no exception generated.

answered Aug 16, 2016 at 5:39

Micky Balladelli

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