Best way to check if an PowerShell Object exist?

问题

I am looking for the best way to check if a Com Object exists.

Here is the code that I have; I'd like to improve the last line:

$ie = New-Object -ComObject InternetExplorer.Application
$ie.Navigate("http://www.stackoverflow.com")
$ie.Visible = $true

$ie -ne $null #Are there better options?

回答1:

I would stick with the $null check since any value other than '' (empty string), 0, $false and $null will pass the check: if ($ie) {...}.

回答2:

You can also do

if ($ie) {
    # Do Something if $ie is not null
}

回答3:

In your particular example perhaps you do not have to perform any checks at all. Is that possible that New-Object return null? I have never seen that. The command should fail in case of a problem and the rest of the code in the example will not be executed. So why should we do that checks at all?

Only in the code like below we need some checks (explicit comparison with $null is the best):

# we just try to get a new object
$ie = $null
try {
    $ie = New-Object -ComObject InternetExplorer.Application
}
catch {
    Write-Warning $_
}

# check and continuation
if ($ie -ne $null) {
    ...
}

回答4:

What all of these answers do not highlight is that when comparing a value to $null, you have to put $null on the left-hand side, otherwise you may get into trouble when comparing with a collection-type value. See: https://github.com/nightroman/PowerShellTraps/blob/master/Basic/Comparison-operators-with-collections/looks-like-object-is-null.ps1

$value = @(1, $null, 2, $null)
if ($value -eq $null) {
    Write-Host "$value is $null"
}

The above block is (unfortunately) executed. What's even more interesting is that in Powershell a $value can be both $null and not $null:

$value = @(1, $null, 2, $null)
if (($value -eq $null) -and ($value -ne $null)) {
    Write-Host "$value is both $null and not $null"
}

So it is important to put $null on the left-hand side to make these comparisons work with collections:

$value = @(1, $null, 2, $null)
if (($null -eq $value) -and ($null -ne $value)) {
    Write-Host "$value is both $null and not $null"
}

I guess this shows yet again the power of Powershell !

回答5:

Type-check with the -is operator returns false for any null value. In most cases, if not all, $value -is [System.Object] will be true for any possible non-null value. (In all cases, it will be false for any null-value.)

My value is nothing if not an object.

回答6:

I had the same Problem. This solution works for me.

$Word = $null
$Word = [System.Runtime.InteropServices.Marshal]::GetActiveObject('word.application')
if ($Word -eq $null)
{
    $Word = new-object -ComObject word.application
}

https://msdn.microsoft.com/de-de/library/system.runtime.interopservices.marshal.getactiveobject(v=vs.110).aspx

回答7:

Incase you you're like me and you landed here trying to find a way to tell if your PowerShell variable is this particular flavor of non-existent:

COM object that has been separated from its underlying RCW cannot be used.

Then here's some code that worked for me:

function Count-RCW([__ComObject]$ComObj){
   try{$iuk = [System.Runtime.InteropServices.Marshal]::GetIUnknownForObject($ComObj)}
   catch{return 0}
   return [System.Runtime.InteropServices.Marshal]::Release($iuk)-1
}

example usage:

if((Count-RCW $ExcelApp) -gt 0){[System.Runtime.InteropServices.Marshal]::FinalReleaseComObject($ExcelApp)}

mashed together from other peoples' better answers:

RCW & reference counting when using COM interop in C#
RCW Reference Counting Rules != COM Reference Counting Rules
“COM object that has been separated from its underlying RCW cannot be used” with .NET 4.0

and some other cool things to know:

COM object that has been separated from its underlying RCW cannot be used
Custom Marshaling
Advanced COM Interoperability

来源：https://stackoverflow.com/questions/4430067/best-way-to-check-if-an-powershell-object-exist

标签

powershell

object

null

powershell-2.0