Value object

A value object is a building block of domain-driven design that represents a value in the domain — such as a date, a price, or an address — defined entirely by its attributes rather than by any identity of its own. Two value objects with identical attributes are considered equal and interchangeable, in contrast to an entity, whose identity persists even as its attributes change.

Value objects are treated as immutable. Once constructed, their attributes never change. Where an entity is modified in place, a value object is instead replaced wholesale by a new instance carrying the new value. Immutability also lets a value object validate its own invariants once, at construction, since there is no later mutation that could put it back into an invalid state.

A value object may be composed of other value objects, but never of an entity. An entity’s state is typically described using value objects, and not the reverse. An entity’s attributes, such as a customer’s name and address, are natural candidates to model as value objects even where the entity itself needs a stable identity.