Powershell IF and not condition not working properly - powershell

Question

I'm struggling to have two conditions met, somehow it seems only one of them works. I'm trying to do :
If a user is connected AND NOT at the lock screen, ask for permission. The commands themselves have been verified and work individually but I must be missing something. Here is what i have:
if (Get-WmiObject –ComputerName $poste –Class Win32_ComputerSystem | Select-Object -expandproperty UserName -and -not (get-process -ComputerName $poste -name logonui)) {
"ask for permission"
}
Right now it just doesn't go in this code, it skips to the lower part where something else is happening.
What is wrong with my syntax ?
I can work around it and make it work this old fashioned way from my CMD days:
Clear-Variable -name statut_user
$statut_user -eq 0
if (Get-WmiObject –ComputerName $poste –Class Win32_ComputerSystem | Select-Object -expandproperty UserName) {$statut_user +=1}
if (-not (get-process -ComputerName $poste -name logonui)) {$statut_user += 1}
if ($statut_user -eq 2) {
"ask for permission"
}
It works, but not as clean as a proper one liner with the two conditions. Thank you for your help!
ANSWER EDIT: Thanks to vonPryz's answer below i ended up using :
$utilisateur = Get-WmiObject –ComputerName $poste –Class Win32_ComputerSystem | Select-Object -expandproperty UserName
$ecran_verr = get-process -ComputerName $poste -name logonui
if( -not ($ecran_verr) -and ($utilisateur)) {
"ask for permission"
}

Answer 1

Aim for clarity, not small codebase size. Instead of making WMI calls in the if statement and piping the results, consider something more readable. Like so,
$cs = gwmi -computername $p Win32_ComputerSystem
$uname = $cs | select-object -expandproperty UserName
$logonui = get-process -ComputerName $p -name logonui
if( -not ($logonui) -and ($uname )) {
# Stuff
}
This approach makes it easy to check that the WMI objects contain sensible values, whatever those may be. Then it should be easier to write a concise conditional statement.

Answer 2

While breaking an expression down into multiple steps is always a good idea for debugging, as demonstrated in vonPryz's helpful answer, sometimes you do want the concision of a single expression without auxiliary variables.
What is wrong with my syntax?
You're missing (...) around the Get-WmiObject ... | Select-Object ... pipeline.
To use a command or pipeline as part of a larger expression, you must always enclose it in (...) A command in PowerShell is a call to an executable - be it a cmdlet, function, alias, or external program.
A simple example:
# !! BROKEN: tokens `-eq 'jdoe'` are interpreted as *arguments for Select-Object*
# !! rather than as operator -eq and RHS 'jdoe'
if (Get-WmiObject Win32_ComputerSystem | Select-Object -Expand UserName -ne 'jdoe') {
'not jdoe'
}
# OK: (...) around the pipeline properly embeds it in the overall expression:
if ((Get-WmiObject Win32_ComputerSystem | Select-Object -Expand UserName) -ne 'jdoe') {
'not jdoe'
}
Here's a fixed version of your original command that fixes that improves other aspects too:
if (
(Get-WmiObject –ComputerName $poste –Class Win32_ComputerSystem).UserName `
-and -not `
(Get-Process -ErrorAction SilentlyContinue -ComputerName $poste -name logonui)
) {
"ask for permission"
}
Given that your Get-WmiObject call only ever outputs 1 object, you can access the .UserName property directly, which is also more efficient than piping to Select-Object -ExpandProperty.
Get-Process outputs a non-terminating error if a process by the given name cannot be found, so -ErrorAction SilentlyContinue suppresses that.
Note the use of ` as a line-continuation character, which allows spreading the conditional across multiple lines, making it much more readable.
Note that the ` must be at the very end of the line.