LDM Notes for February 9th, 2026

[333fred]

https://github.com/dotnet/csharplang/blob/main/meetings/2026/LDM-2026-02-09.md

Agenda

• Closed hierarchies open questions
  • Confirming API shape
  • Blocking subtyping from other languages
  • Multiple CompilerFeatureRequired attributes
  • Same module restriction
  • Permit explicit use of abstract modifier
  • Subtype metadata

[ds5678]

A closed class is implicitly abstract.

Why must the root of a closed class hierarchy be abstract? That seems like a needless restriction.

[TahirAhmadov]

Does it mean that closeness should propagate to derived types, at least by default?

[HaloFour]

Does it mean that closeness should propagate to derived types, at least by default?

I don't think it needs to. Exhaustiveness doesn't require it, as long as you cover those cases. And you'd need another keyword to re-open the hierarchy, which feels awkward.

[TahirAhmadov]

// project A
public closed (abstract) class Shape { }
public class Circle : Shape { }
public class Rectangle : Shape { }

// project B
public class Square : Rectangle { } // is this allowed?

I can't see how Square can be allowed in project B, without breaking the exhaustiveness guarantee. close has to propagate and in fact it can't even be optional. Or am I missing anything?
PS. On the other hand, we can say that as far Shape is concerned, Rectangle is the option - Square has to satisfy its contract. I guess that's the answer to my own question.

[HaloFour]

It doesn't break the exhaustiveness check, you have to check for Rectangle, and checking for Rectangle covers Square.

[CyrusNajmabadi]

Does it mean that closeness should propagate to derived types, at least by default?

Definitely not.

I can't see how Square can be allowed in project B, without breaking the exhaustiveness guarantee.

Simply, when switching on a 'Shape', because it is 'closed', all the direct subtypes of it are known to be in the same assembly. So consuming code knows it must be Circle/Rectangle.

So any switch must cover those cases. Once you cover Circle/Rectangle, you've exhaustively covered all the possibilities. That the Rectangle may itself be a Square doesn't change that as Rectangle subsumes Squares (as well as any other possible subtypes.

[huoyaoyuan]

There was also a suggestion about whether constructors of closed types should be required to have private protected accessibility, to further express the intent that these constructors are only for use by known subtypes. However, this was considered more trouble than it is worth: private protected is a niche accessibility, primary constructors pick up the accessibility of their containing type, and synthesized constructors would need special handling.

Isn't private protected already the current practice?

A concern is about difference in reference assembly. private protected member don't need to exist in reference assembly.