new-object PSObject causes null-valued expression error - powershell

Question

My PowerShell script:
Param([string]$Computers) #Must supply a comma seperated list of servers
$Threshold = 20 #Only show CPU over this number
$NoP = 20 #Number of processes to list
$NoRS = 4 #Number of result sets
If (! $Computers) {
Write-Host "Connection to server failed - please specify a server name." -ForegroundColor Red
Break
} Else {
$ComputerList = $Computers -Split " "#,[StringSplitOptions]'RemoveEmptyEntries')
}
$Credential = $host.ui.PromptForCredential("Need credentials", "Please enter your user name and password.", "", "NetBiosUserName")
If (! $Credential) {
Write-Host "Authentication failed - please verify your username and password." -ForegroundColor Red
Break
}
$UserName = $Credential.Username
$Password = $Credential.GetNetworkCredential().Password
$CurrentDomain = "LDAP://" + ([ADSI]"").distinguishedName
$Domain = New-Object System.DirectoryServices.DirectoryEntry($CurrentDomain,$UserName,$Password)
If ($Domain.Name -eq $null){
Write-Host "Authentication failed - please verify your username and password." -ForegroundColor Red
Break
}
ForEach ($ComputerName In $ComputerList) {
$LoadPercentage = $Processors.LoadPercentage
If (!$LoadPercentage) {$LoadPercentage = 0}
Write-Host "Server: $ComputerName (CPU load $LoadPercentage%)" -NoNewline
$Processors = Get-WmiObject win32_processor -ComputerName $ComputerName -Credential $Credential
$i = 1
$TopProcess = #()
$PercentComplete = 0
Do{
$PercentComplete = [Math]::Floor($i/$NoRS*100)
Write-Progress -Activity $ComputerName -Status "$PercentComplete% Complete:" -PercentComplete $PercentComplete
$ProcessList = gwmi Win32_PerfFormattedData_PerfProc_Process -ComputerName $ComputerName -Credential $Credential |
Select IDProcess,Name,PercentProcessorTime |
Where {$_.Name -ne "_Total" -and $_.Name -ne "Idle"} |
Sort PercentProcessorTime -Descending |
Select -First $NoP
ForEach ($Process In $ProcessList) {
$row = New-Object PSObject -Property #{
Id = $Process.IDProcess
Name = $Process.Name
User = (gwmi Win32_Process -ComputerName $ComputerName -Credential $Credential | Where {$_.ProcessId -eq $Process.IDProcess}).GetOwner().User
CPU = $Process.PercentProcessorTime/$Processors.NumberOfLogicalProcessors -f {P}
Description = (gwmi Win32_Process -ComputerName $ComputerName -Credential $Credential | Where {$_.ProcessId -eq $Process.IDProcess}).Description
}
$TopProcess += $row
}
$i++
} While ($i -lt $NoRS + 1)
Write-Progress -Activity $ComputerName -Completed
$Group = $TopProcess | Where {$_.CPU -gt $Threshold} | Group 'ID' | Where Count -eq $NoRS
If (!$Group) {
Write-Host " has no processes persistently above $Threshold percent CPU usage."
} Else {
$Processes = #()
ForEach ($Groupee In $Group) {
$Ungroup = $Groupee | Select -ExpandProperty Group
$CPU = 0
ForEach ($ugr in $Ungroup) {
$CPU += $ugr.CPU
}
$row = new-object PSObject -Property #{
Id = $Ungroup.Id | Select -First 1
Name = $Ungroup.Name | Select -First 1
CPU = $CPU/$NoRS
User = $Ungroup.User | Select -First 1
Description = $Ungroup.Description | Select -First 1
}
$Processes += $row
}
$Processes | Format-Table #{Expression={$_.User};Label="User Name";width=25},#{Expression={$_.CPU};Label="CPU";width=5},#{Expression={$_.Id};Label="ID";width=8},#{Expression={$_.Description};Label="Description";width=48}
}
}
intermittantly gives the following error:
You cannot call a method on a null-valued expression. At C:\Users\Jasons1\CPUusage.ps1:41 char:4
$row = new-object PSObject -Property #{
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
CategoryInfo : InvalidOperation: (:) [], RuntimeException
FullyQualifiedErrorId : InvokeMethodOnNull
which I fail to understand as it is within a loop and should either work or get skipped as there is a test for null.

Answer 1

Pretty sure that your issues are stemming from this line:
User = (gwmi Win32_Process -ComputerName $ComputerName -Credential $Credential | Where {$_.ProcessId -eq $Process.IDProcess}).GetOwner().User
Specifically from .GetOwner(). Your where clause must not be finding a matching process for that item while it is in the loop. I realize there is not much time elapsed but WMI queries are not the fastest things out there.
What is happening is likely a result of a process queried earlier in $ProcessList = gwmi Win32_PerfFormattedData_PerfProc_Process and then later when you are using gwmi Win32_Process the list of processes changed. You need to account for this as well. Time has elapsed and threads do not live forever.
$queryResult = gwmi Win32_Process -ComputerName $ComputerName -Credential $Credential | Where {$_.ProcessId -eq $Process.IDProcess}
$owner = if($queryResult){$queryResult.GetOwner().User}else{"Process DNE"}
#...
User = $owner
Not very pretty but accounts for the potential of a null return from the wmi query.