Understanding Value Objects in Domain-Driven Design

Value Objects in Domain-Driven Design

• 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.

Definition of Value Objects

• 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.

Characteristics of Value Objects

• 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.

Examples of Value Objects

• 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.

Exercise: Value Objects vs. Entities

• 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.

Learn about Value Objects, a fundamental concept in Domain-Driven Design, and how they're used to model concepts in code. Discover their role in measuring and describing things in your domain.