Remove lines from file1 that exist in file2 in Powershell

Question

I have a file1.txt which contains:

line1
line2
line3
line4

I want to remove from file1.txt all the lines which exist in another file2.txt:

Answer 1

Next code snippet shows two different ways to achieve the same result:

$filebefore="$env:TEMP\beforerestart.txt"    # change to match your circumstances
$file_after="$env:TEMP\after_restart.txt"    # ditto

### Compare-Object way
$array = Compare-Object $(Get-Content $filebefore) $(Get-Content $file_after)
$array | where {$_.SideIndicator -eq "<="} | 
              Format-Table -Property InputObject -AutoSize -HideTableHeaders

### -NotIn operator way
$(Get-Content $filebefore) | 
     Where-Object {$_ -notIn $(Get-Content $file_after)}