Why Is It Possible to Loop Through a Null Array

Question

Given the following PowerShell code:

$FolderItems = Get-ChildItem -Path "C:\Test"

Write-Host "FolderItems Is Null: $($FolderItems -eq $null)"

foreach ($FolderItem in $FolderItems)
{
    Write-Host "Inside the loop: $($FolderItem.Name)"
}

Write-Host "Done."

When I test it with one file in the C:\Test folder, it outputs this:

FolderItems Is Null: False
Inside the loop: MyFile.txt
Done.

However, when I test it with ZERO files in the folder, it outputs this:

FolderItems Is Null: True
Inside the loop: 
Done."

If $FolderItems is null, then why does it enter the foreach loop?

Jason Shirk · Accepted Answer · 2014-02-13 17:44:19Z

This was an intentional design choice made in V1 and revisited in V3.

In most languages, the foreach statement can only loop over collections of things. PowerShell has always been a little different, and in V1, you could loop over a single value in addition to collections of values.

For example:

foreach ($i in 42) { $i }  # prints 42

In V1, if a value was a collection, foreach would iterate over each element in the collection, otherwise it would enter the loop for just that value.

Note in the above sentence, $null isn't special. It's just another value. From a language design point of view, this is fairly clean and concisely explained.

Unfortunately many people did not expect this behavior and it caused many bugs. I think some confusion arises because people expect the foreach statement to behave almost like the foreach-object cmdlet. In other words, I think people expect the following to work the same:

$null | foreach { $_ }
foreach ($i in $null) { $i }

In V3, we decided that it was important enough to change behavior because we could help scripters avoid introducing bugs in their scripts.

Note that changing the behavior could in theory break existing scripts in unexpected ways. We ultimately decided that most scripts that potentially see $null in the foreach statement already guard the foreach statement with an if, e.g.:

if ($null -ne $c)
{
    foreach ($i in $c) { ... }
}

So in reality, most real world scripts would not see a change in behavior.

mjolinor · Accepted Answer · 2014-02-13 13:48:16Z

0

This was something of an idiosyncracy/bug in ForEach in V1 and V2. It was corrected in the V3 release.

answered Feb 13, 2014 at 13:48

mjolinor

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Comments

EmileKumfa · Accepted Answer · 2014-02-13 14:04:04Z

-1

Seems to me like you need to wrap your foreach within a conditional that checks if $FolderItem != null. This way, it'll never get in the if statement whenever $FolderItems is NULL

If (-NOT $FolderItems -eq $null) {
    foreach ($FolderItem in $FolderItems)
    {
        Write-Host "Inside the loop: $($FolderItem.Name)"
    }
}

This may be of help as well http://bit.ly/1brKRRk

edited Feb 13, 2014 at 14:04

answered Feb 13, 2014 at 13:46

EmileKumfa

4143 silver badges6 bronze badges

2 Comments

arco444 Over a year ago

Was OP not asking why it happens? Not how to avoid it?

rory.ap Over a year ago

Yes, I know how to avoid it.

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