Define custom property sets (with Add-Member?) for use in Select-Object

Question

What I try to do is quite simple: create a custom object with some properties, and then define "groups" of properties (columns) for use in Select-Object. Let me clarify:

$props = @{"Mary"=1;"Jane"=2;"Frank"=3;"John"=5;"Brenda"=6}
$obj = New-Object PSObject $props

I now have a custom object with some bogus data. What I now want to be able to do is

$obj | select Male
$obj | select Female

And what I had thought would do the trick, is something like this:

$obj | Add-Member PropertySet "Male" @("Frank","John")
$obj | Add-Member PropertySet "Female" @("Mary","Jane","Brenda")

It doesn't work - I get this error:

Add-Member : Cannot convert the "System.Object[]" value of type 
"System.Object[]" to type "System.Collections.ObjectModel.Collection`1[System.String]".

I guess I should provide another object type than my array to Add-Member, but I'm unsure how I should do that.

Does anyone have experience with this?

Important note: I'm on Powershell 2, and I read on various sites that it has a bug that doesn't allow setting the default properties. That's not what I want to do - I want to create a custom property set and not a default one - but it could be that this bug also prevents me from getting what I want.

Answer 1

You are very close. The problem is that you're not creating the object correctly. You need to specify the -Property parameter before you specify a hashtable of properties. Without it, you just create a hashtable. This works:

$props = @{"Mary"=1;"Jane"=2;"Frank"=3;"John"=5;"Brenda"=6}
$obj = New-Object -TypeName PSObject -Property $props

$obj | Add-Member PropertySet "Male" @("Frank","John")
$obj | Add-Member PropertySet "Female" @("Mary","Jane","Brenda")

$obj | select male

Frank John
----- ----
    3    5

Why did this happend? If you read the syntax for New-Object using Get-help new-object or Get-Command New-Object -Syntax, you will see that for normal .Net types, the syntax is:

New-Object [-TypeName] <String> [[-ArgumentList] <Object[]>] [-Property <IDictionary>]

Notice that -ArgumentList is the 2nd paramter, not -Property as you expected. So your code actually did:

$obj = New-Object -TypeName PSObject -ArgumentList $props

instead of:

$obj = New-Object PSObject -Property $props

EDIT The solution above only worked in PS3.0 . It's still valid though, as the -Property parameter is required in PS2.0 also. In PS2.0 you need to cast the propertyset-array to string[] (string-array) and not an object-array (object[]) which is the default array. A complete solution for PS2.0 is:

$props = @{"Mary"=1;"Jane"=2;"Frank"=3;"John"=5;"Brenda"=6}
$obj = New-Object -TypeName PSObject -Property $props

$obj | Add-Member PropertySet "Male" ([string[]]@("Frank","John"))
$obj | Add-Member PropertySet "Female" ([string[]]@("Mary","Jane","Brenda"))

$obj | select male

Frank John
----- ----
    3    5