I've tried using the following code to set up a new Dictionary in my Powershell code. I want the Dictionary to use a tuple as the key, and an Arraylist as the value (so that I can add items to the value over time).
# Create dictionary
$mydict = [Collections.Generic.Dictionary[[Tuple[int,int]],Collections.ArrayList]]::new()
# Add an empty Arraylist to a key
$mydict.Add([Tuple]::Create(0,0),[Collections.ArrayList]::new())
# Try to add an item to this new arraylist
$mydict[(0,0)].Add(1)
MethodInvocationException: Exception calling "Add" with "1" argument(s): "Collection was of a fixed size."
# And sure enough:
$mydict[(0,0)].IsFixedSize
True
$mydict[(0,0)].GetType()
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True Object[] System.Array
Anyone know why Powershell is ignoring my requests to add ArrayLists as members, and for some reason silently converting the objects to System.Array instead?
Just like you used [Tuple]::Create(0,0), i.e explicit creation of a tuple in the .Add() call, you must use the same in trying to access an entry:
$null = $mydict[[Tuple]::Create(0,0)].Add(1)
The reason that (0,0) didn't work as a lookup key is that it is of type [object[]] and PowerShell's index operator ([…]) by design supports passing an array of indices, whose elements are individually interpreted as indices to look up.
E.g., (1, 2, 3, 4)[1, -1] yields 2, 4, i.e. the second (1) and the last (-1) element of the input array.
Since the [object[]] type of (0, 0) didn't match the actual key type of $mydict, $mydict was in effect treated like a single-element array, it was something akin to the intrinsic array indexing of scalar types[1] that kicked in, meaning that both instances of 0 returned the $mydict dictionary as a whole, implicitly resulting in a two-element array to which the .Add() method was applied, and - because a .NET array (whose base type is always System.Array) is of fixed size - the Collection was of a fixed size. error was emitted.
[1] Note that PowerShell generally treats objects that implement the System.Collections.IDictionary interface as single objects (scalars) in its pipeline, even though they are enumerable, namely as key-value pairs. However, with a key type that is an implementation of System.Collections.ICollection other than an array, such as System.Tuple`2 in your case, numeric indices, which on their own do not match such a key type, seemingly treat the dictionary instance like a single-element array when intrinsic indexing is applied, so that even multiple indices can be applied, and each index with value 0 or -1 (to select the one and only "element" from the start or the end) returns the dictionary itself.