C# Metadata Questions

[InsertCreativityHere]

I've been working on #117 (improve metadata validation).
For my PR, I'm trying to keep the current behavior as much as possible (to avoid changing too much in one PR),
but I've across a few oddities that don't seem right to me. I intend to fix these in a follow-up PR, but wanted to list them here,
in case anyone had any thoughts/explanations to offer.

1. We support SortedList as an argument to cs:generic on dictionaries:
   This seems nonsensical to me, why would you map a dictionary to a list, instead of using a sequence?
   Also, in the documentation, we only list SortedDictionary, not SortedList.
   EDIT: @pepone points out that SortedList is actually an IDictionary, so we should keep support for it.

2. We only support cs:implements on structs and interfaces. Why don't we support this on classes?
   Note that in the documentation, we state that it's supported on struct, class, and interface.
   It's not clear if the documentation is outdated, or if this is a bug in the current validation logic.
   EDIT: after looking at it further, this metadata seems useless and will be removed instead.

3. Should we allow cs:attribute on enumerators?
   If you put this metadata on an enumerator, we don't reject it, but we also don't seem to generate code for it.
   It's just silently ignored. Is there a reason we'd disallow this? Or is this just missing functionality.
   I intend to change slice2cs to generate attributes on enumerators like we do for everything else.

4. Should we allow cs:property on exceptions?
   The documentation says it's only supported on structs and classes, but the validation logic allowed it through:

   ice/cpp/src/slice2cs/CsUtil.cpp

   Lines 2015 to 2017 in 2c85ea7

   	else if (dynamic_pointer_cast<ClassDecl>(cont) || dynamic_pointer_cast<Exception>(cont))
   	{
   	if (directive == "cs:property" && arguments.empty())

   I intend to allow this metadata on exceptions, unless there's a reason not to

[pepone]

• cs:implements I think we should consider dropping this, in favor of defining this in a partial. See Drop support for cs:implements metadata #3247

[pepone]

I intend to change slice2cs to generate attributes on enumerators like we do for everything else.

Seems the right thing to do.

[InsertCreativityHere]

Should we allow cs:property on exceptions?

Apparently we actually do this in some of our tests, so I'm going to add this to my PR, instead of doing it after-the-fact.
Otherwise, we'd get build warnings on main.

[pepone]

We support SortedList as an argument to cs:generic on dictionaries:

Because sorted list is a collection of key/value pairs and it implements IDictionary, see https://learn.microsoft.com/en-us/dotnet/api/system.collections.sortedlist?view=net-9.0

[InsertCreativityHere]

Because sorted list is a collection of key/value pairs and it implements IDictionary

Yes, you're right. I should of looked closer at SortedList, although that seems like a pretty bad name to describe what it actually is. But yes, we should keep this support, and we should also make sure to document it somewhere too... It's pretty crazy we don't document or test it anywhere, since we've been supporting it for over 17 years now:
1d27c59#diff-88cc14de1ecb3a23b3990783d7e7c41dcd87f24f315607c3cbb6622dcea5d00c

[InsertCreativityHere]

Currently, we allow cs:attribute on modules, we should remove this since C# doesn't support attributes on namespaces.