How to test for null array in PowerShell

Question

I m using an array variable in PowerShell 2 0   If it does not have a value  it will be  null  which I can test for successfully   PS C   gt   array  foo    null PS C   gt   foo -eq  null True   But when I give it a value  the test for  null does not return anything   PS C   gt   array  foo      bar   PS C   gt   foo -eq  null PS C   gt    How can  -eq  null  give no results   It s either  null or it s not   What is the correct way to determine if an array is populated vs   null

User · Answer

The other answers address the main thrust of the question, but just to comment on this part...

PS C:\> [array]$foo = @("bar")
PS C:\> $foo -eq $null
PS C:\>
How can "-eq $null" give no results? It's either $null or it's not.

It's confusing at first, but that is giving you the result of $foo -eq $null, it's just that the result has no displayable representation.

Since $foo holds an array, $foo -eq $null means "return an array containing the elements of $foo that are equal to $null". Are there any elements of $foo that are equal to $null? No, so $foo -eq $null should return an empty array. That's exactly what it does, the problem is that when an empty array is displayed at the console you see...nothing...

PS> @()
PS>

The array is still there, even if you can't see its elements...

PS> @().GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Object[]                                 System.Array


PS> @().Length
0

We can use similar commands to confirm that $foo -eq $null is returning an array that we're not able to "see"...

PS> $foo -eq $null
PS> ($foo -eq $null).GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Object[]                                 System.Array


PS> ($foo -eq $null).Length
0
PS> ($foo -eq $null).GetValue(0)
Exception calling "GetValue" with "1" argument(s): "Index was outside the bounds of the array."
At line:1 char:1
+ ($foo -eq $null).GetValue(0)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : IndexOutOfRangeException

Note that I am calling the Array.GetValue method instead of using the indexer (i.e. ($foo -eq $null)[0]) because the latter returns $null for invalid indices and there's no way to distinguish them from a valid index that happens to contain $null.

We see similar behavior if we test for $null in/against an array that contains $null elements...

PS> $bar = @($null)
PS> $bar -eq $null
PS> ($bar -eq $null).GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Object[]                                 System.Array


PS> ($bar -eq $null).Length
1
PS> ($bar -eq $null).GetValue(0)
PS> $null -eq ($bar -eq $null).GetValue(0)
True
PS> ($bar -eq $null).GetValue(0) -eq $null
True
PS> ($bar -eq $null).GetValue(1)
Exception calling "GetValue" with "1" argument(s): "Index was outside the bounds of the array."
At line:1 char:1
+ ($bar -eq $null).GetValue(1)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : IndexOutOfRangeException

In this case, $bar -eq $null returns an array containing one element, $null, which has no visual representation at the console...

PS> @($null)
PS> @($null).GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Object[]                                 System.Array


PS> @($null).Length
1

User · Answer

If your solution requires returning 0 instead of true false  I ve found this to be useful   PS C   gt   array  foo    null PS C   gt    foo   Measure-Object  Count 0   This operation is different from the count property of the array  because Measure-Object is counting objects  Since there are none  it will return 0

User · Answer

if  foo -eq  null     yes    else    no     help about comparison operators    displays help and includes this text      All comparison operators except the   containment operators  -contains    -notcontains  and type operators  -is  -isnot  return a Boolean value when the input to the operator  the value   on the left side of the operator  is a   single value  a scalar   When the   input is a collection of values  the   containment operators and the type   operators return any matching values    If there are no matches in a   collection  these operators do not   return anything  The containment   operators and type operators always   return a Boolean value

User · Answer

It s an array  so you re looking for Count to test for contents  I d recommend  foo count -gt 0  The  quot why quot  of this is related to how PSH handles comparison of collection objects

User · Answer

How do you want things to behave   If you want arrays with no elements to be treated the same as unassigned arrays  use    array  foo        example where we d want TRUE to be returned    foo  Count -eq 0   If you want a blank array to be seen as having a value  albeit an empty one   use    array  foo        example where we d want FALSE to be returned  foo PSObject -eq  null   If you want an array which is populated with only null values to be treated as null    array  foo    null  null    foo        PSObject   Count -eq 0    NB  In the above I use    PSObject over    to avoid  bool  false   int 0   string     etc from being filtered out  since here we re focussed solely on nulls

User · Answer

You can reorder the operands    null -eq  foo   Note that -eq in PowerShell is not an equivalence relation

[powershell] How to test for $null array in PowerShell

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