Given a list with some duplicate items, I can use Group-Object -AsHashtable and I get a hash table that seems to have keys with the item names, and the value is something I don't recognize.
So, given
$array = [System.Collections.Generic.List[String]]@('A', 'B', 'C', 'B', "c", 'C')
$grouped = $array | Group-Object -AsHashTable
$grouped will be
Name Value
---- -----
A {A}
B {B, B}
C {C, c, C}
At which point I would have thought that $grouped['B'].Count or $grouped.'B'.Count would produce the correct count of 2. But I am getting 0. What am I missing?
You're seeing a bug in Windows PowerShell that has since been fixed in PowerShell (Core) 7+ - see GitHub issue #6933 for details.
Workaround: In Windows PowerShell, always combine -AsHashTable with -AsString, even if the input objects or grouping property values already are strings. Tip of the hat to Abraham Zinala.
# Note: .B.Count is equivalent to ['B'].Count
PS> ('A', 'B', 'C', 'B', "c", 'C' | Group-Object -AsHashTable -AsString).B.Count
2 # OK, thanks to -AsString (not necessary in PowerShell 7+)
This bypasses the bug, which otherwise manifests as follows: the strings that are meant to become the hashtable keys are unexpectedly wrapped in invisible [psobject] wrappers, which in turn prevents string-based key lookup.
(('A', 'B', 'C', 'B', "c", 'C' | Group-Object -AsHashTable).B yields $null in Windows PowerShell.)
Note: The implication is that you cannot use -AsHashtable in Windows PowerShell if you want the keys to be of a data type other than string (e.g. [int]), because for such data types the invisible [psobject] wrapping cannot be bypassed.