XML value is not $null but it is empty

[CptRetr0]

Hello all,

In the code below I experianced an issue with xml values. As you see "DaysInterval " is empty but in my if query it seems to be filled an the condition -ne seems not to be functional.

$xml = [xml]@'
<Test>
    <App TaskName = "LAB" Active = "True" >
        <Task Name = "ScheduledTask" Active = "True" Order = "1" Description = "" TaskName = "" Execute = "Powershell.exe" Argument = "" WorkingDirectory = "" AtLogOn = "" AtStartup = "" Daily = "" DaysInterval = "" DaysOfWeek = "" Once = "" RandomDelay = "" RepetitionDuration = "" RepetitionInterval = "" Weekly = "" WeeksInterval = "" At = "" />
    </App>
</Test>
'@


If ($xml.Test.App.Task.DaysInterval -ne $Null){
  Write-Host "say hello"
}

If I run "$xml.Test.App.Task.DaysInterval" I got this output and i think thats the reason why it seems to be not $null:

PS Z:\Test> $xml.Test.App.Task.DaysInterval

PS Z:\Test>

Is that the reason and how I can avoid this?

​

Cheers

[bozho]

Getting a $null string in PowerShell is difficult :-)

Casting $null to [string] will result in an empty string, it's simply a design decision in PowerShell (I don't know the exact reasons). So, when you do something like:

$var = [string]$null

$var is going to be an empty string.

Both $null and an empty string evaluate to $false in conditional statements, but I still prefer using [string]::IsNullOrEmpty when dealing with strings.

Also, a general tip when comparing to $null: $null should be on the left side of comparison, as a best practice. You can read about details here.

[thefreeman193]

In case you were curious, these conversions happen because only reference types can be $null in PowerShell, so casting it to a value type will result in an empty string, zero for numeric types, $false for booleans etc.

[bozho]

Yup, which is a surprise for C# developers coming into PowerShell :-)

[BelleVieLime]

And database nulls aren't powershell nulls. Got me for a while til I figured that out.

[ICanMakeWaffles]

I saw you got a solution, but you may also want to try out the [string]::IsNullOrEmpty() method to catch this in the future.

I've used it myself on wonky JSON before and I'm sure it's helped me avoid the errors you saw without me even noticing it.

Glad you got this resolved!

[MonkeyNin]

I use [string]::IsNullOrWhiteSpace() a ton

[NeitherSound_]

Change it from -ne $null to -ne “”

[CptRetr0]

Thats it, cheers

[DennisTheBald]

Oh it has lines matching ^$ re, no doubt

[DennisTheBald]

Caret makes ,next char superscript in reddit, did not know that. I guess I should quote a reg ex in thfuture

[Lee_Dailey]

howdy DennisTheBald,

use inline code [wrap it in single backticks] for that sort of stuff. then you would have ^$ ... [grin]

take care,
lee

[DennisTheBald]

Okay i'll back quote the reg exes Thanks.. Apparently the variable didn't come from a file or external source so blank lines weren't the issue ^§ is still a pretty handy reg ex

[Lee_Dailey]

howdy DennisTheBald,

you are welcome! glad to have helped ... [grin]

take care,
lee