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In general, is there a convenient way to figure out which lines of a PowerShell script/function are returning values (are "uncaptured")? I was hoping there was a way to query the current state of the to-be-returned value while debugging. I can check it after each line to see which lines add to it.

I have some scripts at work and some lines are turning my return value into an Object[]. I usually pipe such lines to Out-Null to fix the situation. I only want one object returned (the one I pick at the end of the function).

Some of the lines are Cmdlet calls, some are calls to other functions, and some are function calls on .NET objects.

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2 Answers 2

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6

I suppose you can use Set-PsDebug -trace 1 to see which line is returning them.

Example:

Consider the script below:

function f{
1..10
}

function g{
f
return 11
}

g

After doing Set-PsDebug -trace 1, the trace would be something like below:

DEBUG:    1+  <<<< .\test.ps1
DEBUG:    1+ function f <<<< {
DEBUG:    5+ function g <<<< {
DEBUG:   10+  <<<< g
DEBUG:    6+  <<<< f
DEBUG:    2+ 1.. <<<< 10
1
2
3
4
5
6
7
8
9
10
DEBUG:    7+ return <<<<  11
11

Clearly, you can see where the output is coming from. But if the output is captured, you wouldn't get this.

Also, if you don't care about the other objects that are being returned and only want to get the last one that you returned with the return statement, you can always do something like (func)[-1] or func | select -last 1 ( as pointed out in the comment) to get the last one.

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12 Comments

func | select -last 1 is an alternative method to retrieve the last item. They both give you the same item, but I think that the -1 array syntax can be confusing to someone who isn't as well versed in powershell.
@Thebigcheeze - For some one like me coming from Python background and other scripting languages, it makes sense, but yes, select -last 1 maybe the "POSH" way of doing it.
I've never used negative indices, interesting. These two methods look like they require the caller to add extra code? I'm not seeing the info I need in the Set-PsDebug -trace 1 debug output. Line 137 is the offending line: [DBG]>>> Hit Line breakpoint on 'D:\Build\WiXConversion\WiXTransforms\000_LaunchConditions.ps1:134' DEBUG: 135+ <<<< $previousInstallCheckFragment.appendChild($WiXElement) | Out-Null DEBUG: 136+ $fragmentElement = <<<< $previousInstallCheckFragment.CreateElement('Fragment') DEBUG: 137+ $WiXElement.AppendChild <<<< ($fragmentElement)
@VimesWatch - Can you update your question with a sample script and the DBG output that you have pasted above. Can't really make out anything from it.
@VimesWatch in order to avoid the issue of the caller needing extra code, you could wrap your current function in a new function, call the inner function with the appropriate input parameters, then pipe the output of that to select -last 1. This will then allow whoever consumes the function to continue calling it without additional code, and it returns the object as you need.
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3

To complete the possible answer, I'd like to add 2 notes:

First, if you use func | select -last 1, you have to wrap returned object to array, if you return array itself. Why? Look at a failing sample:

function MyOutputs {
    $list = new-Object Collections.ArrayList
    $list.Add('first')
    $list.Add('second')
    $list
}
myoutputs | Select -last 1 #doesn't work

Second, if you don't know exactly what commands return output, you can Out-Null them all like this:

function MyOutputs {
    . { 
        $list = new-Object Collections.ArrayList
        $list.Add('first')  # returns index
        $list.Add('second') # returns index
    } | Out-Null
    write-Host Returning...
    $list
}
$a = MyOutputs
Write-Host Result is
$a

Just try to put the ArrayList code outside the scriptblock and you will see what it does. Running it inside scriptblock with . notation means, that the scriptblock is executed in current scope. Out-Null just eats the output from Add methods.

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6 Comments

So in the first example $list in the last line of MyOutputs should be replaced with ,$list
Your second example is unnecessarily complex IMO. Because the . notation for script block is not pertinent and your explanation seems to make it pertinent to the question of finding which lines are returning output. I believe you are doing that only to have $list in scope outside.
@manojlds the point is that if someone doesn't want to use a lot of Out-Nulls, he can use only one. I added this, because I felt from the question that VimesWatch wants to find the lines with unwanted output and discard the output.
I understand that, but I thought the example could have been simpler. I liked the script block out null idea, and +1'd your answer :)
The script-block-to-out-null is a pretty slick way to eliminate the problem, with just a little boiler plate code in my functions. I'll consider using that. As for the original question, it's sounding like there probably isn't a "convenient" way, unless I'm messing up the Set-PsDebug -trace 1 approach (could be).
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