When would you model something as a record, a normal class, or a record struct in C#?

Records do not automatically create a valid domain model: generated equality includes selected state, constructors still need invariants, and record inheritance has runtime-type equality rules that may surprise a hierarchy. Structs always have a default zeroed value, so I avoid a record struct when that default cannot represent a valid or safely detectable state. Large structs increase copy cost and can box through object or interface boundaries. In production I check serializer and ORM behavior, because tools may bypass constructors or expect mutable members. I choose the simplest type whose equality, ownership, and update semantics remain correct across collections, persistence, and API contracts.