Understanding Value Objects in Domain-Driven Design

Study Notes

Value Objects are a fundamental building block in Domain-Driven Design, used to model concepts of the Ubiquitous Language in code.
A Value Object is not just a thing in your Domain, but rather a measure, quantifies, or describes something.
Value Objects are small, simple objects whose equality is not based on identity, but instead on the content held.
Examples of Value Objects include money, date ranges, numbers, dates, monies, and strings.
Value Objects have a trivial memory footprint, and new instance creation is favored over reference reuse, even when being used to represent the same value.
Equality is checked based on the comparability of the fields of both instances.

Ward Cunningham defines a Value Object as a measure or description of something.
Martin Fowler defines a Value Object as a small object that follows value semantics rather than reference semantics.
Value Objects are equal if all their fields are equal, and their identity is based on their state rather than on their object identity.

They are small objects used widely.
Their identity is based on their state rather than on their object identity.
They follow value semantics rather than reference semantics.
They are typically immutable, and if you want to change a Value Object, you should replace the object with a new one.

Numbers, text strings, dates, times, a person's full name, currencies, colors, phone numbers, and postal addresses.
Dollar bills are Value Objects when people exchange them, but they are Entities when the Federal Reserve is concerned about each unique bill.
Seats are Value Objects for airlines that do not distinguish between every seat, but they are Entities for airlines that distinguish each seat uniquely.

Consider the context where an address (street, number, zip code, and so on) could be modeled as an Entity and not as a Value Object.
Discuss your findings with a peer.

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