Edit an object non-destructively in PowerShell - powershell

Question

I am trying to format the resulting object without destroying it. But all my efforts and research has failed me. Any tips are welcome.
My code looks like this:
Set-Location 'C:\Temp'
$Files = Get-ChildItem -File | Select-Object FullName, Length
And what I get, is this:
FullName Length
-------- ------
C:\Temp\CleanupScript.txt 10600
C:\Temp\Columns.csv 4214
C:\Temp\Content.html 271034
C:\Temp\Content.txt 271034
C:\Temp\DirSizes.csv 78
What I want is this:
FullName Length
-------- ------
Temp\CleanupScript.txt 10600
Temp\Columns.csv 4214
Temp\Content.html 271034
Temp\Content.txt 271034
Temp\DirSizes.csv 78
When I tried this:
$Files = Get-ChildItem -File | Select-Object FullName, Length | % { $_.FullName.Remove(0, 3) }
I got the right result, but I lost the Length column.
PS C:\Temp> $Files
Temp\CleanupScript.txt
Temp\Columns.csv
Temp\Content.html
Temp\Content.txt
Temp\DirSizes.csv
Please help.
Big thanks
Patrik

Answer 1

The easiest way to do this is to construct the property you want in the Select command, such as:
$Files = Get-ChildItem -File | Select #{l='FullName';e={$_.FullName.Substring(3)}},Length
The format for this is a hashtable with two entries. The keys are lable (or name), and expression. You can shorten them to l (or n), and e. The label entry defines the name of the property you are constructing, and the expression defines the value.
If you want to retain all of the original methods and properties of the objects you should add a property to them rather than using calculated properties. You can do that with Add-Member as such:
$Files = GCI -File | %{Add-Member -inputobject $_ -notepropertyname 'ShortPath' -notepropertyvalue $_.FullName.Substring(3) -PassThru}
Then you can use that property by name like $Files | FT ShortPath,Length -Auto, while still retaining the ability to use the file's methods like Copy() and what not.

Answer 2

I would recommend using a calculated property and Split-Path -NoQualifier; e.g.:
Get-ChildItem -File | Select-Object `
#{Name = "NameNoQualifier"; Expression = {Split-Path $_.FullName -NoQualifier}},
Length
For help on calculated properties, see the help for Select-Object.
(Aside: To correct your terminology a bit, this is not modifying objects non-destructively but rather outputting new objects containing the properties you want formatted how you want them.)