remove-variable and pipeline - null-valued expression? - powershell

Question

I'm trying to remove some variables in my PowerShell script like so:
#create example com object
$Excel = New-Object -ComObject Excel.Application
#do something with COM object, close COM object etc
#clear variable
get-variable | where-object { $_.Value -ne $null -And $_.Value.ToString() -eq $Excel.GetType() } | remove-variable
But it returns:
You cannot call a method on a null-valued expression.
on the get-variable.... line? However, when I test that line like so:
get-variable | where-object { $_.Value -ne $null -And $_.Value.ToString() -eq $Excel.GetType() } | Select -Property Name
It returns the correct data? I get the same behaviour with:
get-variable | where-object { $_.Value -ne $null -And $_.Value.ToString() -eq $Excel.GetType() } | ForEach-Object { remove-variable -Name $_.Name }
I can't figure out why remove-variable doesn't work? The remove-variable cmdlet accepts 'Name' in the pipeline by 'PropertyName' so i think that looks ok to me?
UPDATED
Interestingly, the full error is multiple occurrences of this (interesting because the line should only return 3 results but there are 48 of these error messages):
You cannot call a method on a null-valued expression.
At line:3 char:31
+ ... re-object { $_.Value -ne $null -And $_.Value.ToString() -eq $Excel.Ge ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : InvokeMethodOnNull
ALSO
This works:
$vars = get-variable | where-object { $_.Value -ne $null -And $_.Value.ToString() -eq $Excel.GetType() } | Select-object -ExpandProperty Name
foreach ($var in $vars) {
remove-variable -Name $var
}
But this doesn't?:
get-variable | where-object { $_.Value -ne $null -And $_.Value.ToString() -eq $Excel.GetType() } | foreach { remove-variable -Name $_.Name }

Answer 1

I fixed it in the end by adding more null checks in the where-object part of the pipeline (thanks #ScottHeath):
get-variable | where-object { $null -ne $_ -And $null -ne $_.Value -And $null -ne $Excel -And $_.Value.ToString() -eq $Excel.GetType().ToString() } | foreach { Remove-Variable -Name $_.Name }

Answer 2

Com objects need to be released from memory with:
[System.Runtime.Interopservices.Marshal]::ReleaseComObject($Excel) | Out-Null
[System.GC]::Collect()
[System.GC]::WaitForPendingFinalizers()
After that, remove the variable itself with
$Excel = $null
Remove-Variable -Name Excel
Remove-Variable also has an -Include parameter where you can specify an array of variable names. Wildcards are also permitted there.