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

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

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

The other answers address the main thrust of the question  but just to comment on this part      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    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 gt      PS gt     The array is still there  even if you can t see its elements     PS gt      GetType    IsPublic IsSerial Name                                     BaseType -------- -------- ----                                     -------- True     True     Object                                   System Array   PS gt      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 gt   foo -eq  null PS gt    foo -eq  null  GetType    IsPublic IsSerial Name                                     BaseType -------- -------- ----                                     -------- True     True     Object                                   System Array   PS gt    foo -eq  null  Length 0 PS gt    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 gt   bar      null  PS gt   bar -eq  null PS gt    bar -eq  null  GetType    IsPublic IsSerial Name                                     BaseType -------- -------- ----                                     -------- True     True     Object                                   System Array   PS gt    bar -eq  null  Length 1 PS gt    bar -eq  null  GetValue 0  PS gt   null -eq   bar -eq  null  GetValue 0  True PS gt    bar -eq  null  GetValue 0  -eq  null True PS gt    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 gt     null  PS gt     null  GetType    IsPublic IsSerial Name                                     BaseType -------- -------- ----                                     -------- True     True     Object                                   System Array   PS gt     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

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

Examples related to powershell

Examples related to null