The snippet for putting parameters into a hashtable is subtly mangling arrays

[briantist]

This was really difficult to track down. I noticed it when using the xADGroup resource, which works perfectly fine until you use any of the Members* attributes, which are all [string[]]. Then it fails with:

Failed to serialize properties into CimInstance

Which is an error you usually see with composite resources (a known limitation of Invoke-DscResource).

It turns out that the snippet I use (from my own blog 😓 ) to put the parameters into a hashtable results in array parameters reverting to [object[]] even if they were originally more strongly typed.

I traced it back to this line:

$val = Get-Variable -Name $key -ErrorAction Stop | Select-Object -ExpandProperty Value -ErrorAction Stop

Using Select-Object -ExpandProperty Value turns it into [object[]] even though it was originally strongly typed as [string[]].

Replacing this with:

$val = Get-Variable -Name $key -ValueOnly -ErrorAction Stop

Should fix the issue (and it's clearer anyway).

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For most code, this is indistinguishable and wouldn't cause a problem.

But of course Invoke-DscResource bombs out on it spectacularly, which an unbelievably unhelpful error message.