Confused about Nulls in Powershell

Question

I'm new to Powershell and I'm confused about something, more specifically null and $null.

For example. let's say you have a function:

function myFunction([ref]$foo){ if($foo -ne $null){ ...do stuff } }

And when you call this function, you do so like:

[ref]$foo = $null myFunction $foo

If you execute the code above, the condition if($foo -ne $null) will return true.

However, if you call the function like:

$foo = $null myFunction $foo

the condition if($foo -ne $null) will return false.

Why is this? Why is it that when you assign a ref variable $null, it isn't considered null when it is checked?

@jpmc26 Um, I can't remember....

BlackHatSamurai
– BlackHatSamurai

2014-01-06 22:43:20 +00:00
Commented Jan 6, 2014 at 22:43 — BlackHatSamurai
– BlackHatSamurai, Commented Jan 6, 2014 at 22:43

jpmc26 · Accepted Answer · 2013-05-31 07:53:43Z

PowerShell seems to cast $null to some non-null value when you attempt to store it in a strongly typed variable. I believe this is the cause. Notice the following:

PS C:\> [ref]$foo = $null PS C:\> $foo Value ----- PS C:\> $foo.GetType() IsPublic IsSerial Name BaseType -------- -------- ---- -------- True False PSReference System.Object

It appears that PowerShell is casting $null to a PSReference. I've seen similar behavior with strings:

PS C:\> [String]$str = $null PS C:\> $str -eq $null False PS C:\> $str -eq [String]::Empty True

It confused the heck out of me when I had a [String] parameter on a function that I was defaulting to $null and my null check was never true.

Why would they do this? I wonder the same thing.

For strings, I ended up using [String]::IsNullOrEmpty (although there may be a more PowerShell-ish way). For PSReference, Value might suit your needs for the null check:

PS C:\> [ref]$foo = $null PS C:\> $foo.Value -eq $null True

Note that we have just proven you won't run into a null reference error by accessing Value.

Start-Automating · Accepted Answer · 2013-05-31 03:23:17Z

Because $null is a special variable which is actually null, where as [ref]$foo is a strongly typed variable containing a reference to a location, which just so happens to be null.

So of course $ref -ne $null. $null is the null, $ref is a reference which could be null later.

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