PowerShell: Function doesn't have proper return value

Question

I wrote a powershell script to compare the content of two folders:

$Dir1 ="d:\TEMP\Dir1"
$Dir2 ="d:\TEMP\Dir2"

function Test-Diff($Dir1, $Dir2) {
    $fileList1 = Get-ChildItem $Dir1 -Recurse | Where-Object {!$_.PsIsContainer} | Get-Item | Sort-Object -Property Name
    $fileList2 = Get-ChildItem $Dir2 -Recurse | Where-Object {!$_.PsIsContainer} | Get-Item | Sort-Object -Property Name

    if($fileList1.Count -ne $fileList2.Count) {
        Write-Host "Following files are different:"
        Compare-Object -ReferenceObject $fileList1 -DifferenceObject $fileList2 -Property Name -PassThru | Format-Table FullName
        return $false
    }

    return $true
}

$i = Test-Diff $Dir1 $Dir2

if($i) { 
    Write-Output "Test OK" 
} else { 
    Write-Host "Test FAILED" -BackgroundColor Red
}

If I set a break point on Compare-Object, and I run this command in console, I get the list of differences. If I run the whole script, I don't get any output. Why?

I'm working in PowerGUI Script Editor, but I tried the normal ps console too.

EDIT:

The problem is the check on the end of the script.

$i = Test-Diff $Dir1 $Dir2

if($i) { 
   Write-Output "Test OK" 
...

If I call Test-Diff without $i = check, it works!

Test-Diff returns with an array of objects and not with an expected bool value:

[DBG]: PS D:\>> $i | ForEach-Object { $_.GetType() } | Format-Table -Property Name
 Name                                                                                                                        
 ----                                                                                                                        
 FormatStartData                                                                                                             
 GroupStartData                                                                                                              
 FormatEntryData                                                                                                             
 GroupEndData                                                                                                                
 FormatEndData                                                                                                               
 Boolean         

If I comment out the line with Compare-Object, the return value is a boolean value, as expected.

The question is: why?

Answer 1

I've found the answer here: http://martinzugec.blogspot.hu/2008/08/returning-values-from-fuctions-in.html

Functions like this:

Function bar {
 [System.Collections.ArrayList]$MyVariable = @()
 $MyVariable.Add("a")
 $MyVariable.Add("b")
 Return $MyVariable
}

uses a PowerShell way of returning objects: @(0,1,"a","b") and not @("a","b")

To make this function work as expected, you will need to redirect output to null:

Function bar {
 [System.Collections.ArrayList]$MyVariable = @()
 $MyVariable.Add("a") | Out-Null
 $MyVariable.Add("b") | Out-Null
 Return $MyVariable
}

In our case, the function has to be refactored as suggested by Koliat.

Answer 2

An alternative to adding Out-Null after every command but the last is doing this:

$i = (Test-Diff $Dir1 $Dir2 | select -last 1)

PowerShell functions always return the result of all the commands executed in the function as an Object[] (unless you pipe the command to Out-Null or store the result in a variable), but the expression following the return statement is always the last one, and can be extracted with select -last 1.