Using OutVariable Creates ArrayList

Question

I'm sure there is a valid reason why this happens but I don't know what it is. I have the following code

$Deleted = $Items[0].ParentNode.RemoveChild($Items[0])
Write-Output $Deleted

If I call this code using

Do-Something -OutVariable Test

I get the output on the screen and in the variable as expected, but the variable is an ArraryList with a single element that is of type XmlElement

If Instead I run

$Test = Do-Something

I don't get the output on the screen, as expected, and the returned output is a single object of type XmlElement

Why does OutVariable return an ArrayList rather than return the single item the function returns?

Thanks

Answer 1

As of PowerShell [Core] 7.0:

• PowerShell unexpectedly does not unwrap a command's single-object output with
  -OutVariable: That is, even if a command outputs only one object, that object is unexpectedly stored as an array (array list) in the target variable whose name is passed to the common parameter -OutVariable parameter (or its alias, -ov).

• Additionally, the type of the value stored in the target variable is always [System.Collections.ArrayList], an array list, which is inconsistent with capturing multi-object output in a variable by regular assignment, which results in System.Object[], a regular array.

See this GitHub issue.

Here's a concise test that demonstrates the problem:

$null = Get-Date -OutVariable ov; $ov.GetType().FullName

As of the versions listed above, this outputs:

System.Collections.ArrayList

even though the output is (conceptually) only a single [datetime] ([System.DateTime]) instance.

Contrast this with a regular assignment: $v = Get-Date; $v.GetType().FullName yields System.DateTime.

---

The workaround is to wrap the output-variable reference in the subexpression operator ($(...)), which gives you the same output as via the regular output (success) stream:

• a single-element collection is unwrapped (only outputs the element itself)
• a multi-element collection is returned as a System.Object[] array.

# Note the $(...) around $ov
PS> $null = Get-Date -OutVariable ov; $($ov).GetType().FullName
System.DateTime  # 1-element System.Collections.ArrayList unwrapped

Of course, you could also use $ov[0] if you only ever expect a single output object.

A demonstration that the workaround is effective with multi-element output too:

PS> $null = Get-ChildItem / -OutVariable ov; $($ov).GetType().FullName
System.Object[] # multi-element System.Collections.ArrayList converted to array