Seal Type Hierarchies

1. When to use this skill

Use this skill when:

• A package declares an abstract base type whose subtypes are all defined in the same library, but the base type is not marked sealed.
• Code branches over the members of such a hierarchy with is checks or a non-exhaustive switch, so adding a subtype later fails silently at runtime instead of loudly at compile time.
• Reviewing a new type hierarchy and deciding whether it should be open to external subtyping.

When NOT to use (Abstention Guardrails)

Do NOT seal a type when: