FluentAPI

FluentAPI is a programming API design pattern that uses method chaining to create a fluent, readable domain-specific language for configuring objects or constructing complex instances. The approach emphasizes producing code that reads like natural language, with each call returning a builder or related object to continue the chain.

Fluent APIs typically use descriptive method names and return types that enable chaining. They may implement an internal or external domain-specific language and are often paired with the builder pattern to assemble configurations or objects piece by piece. Design considerations include whether methods return the same instance (mutable builders) or new instances (immutable builders).

Common applications include configuration of frameworks and libraries, object construction, and query building. In many ecosystems the term is associated with Fluent API configurations in ORMs and data mappers, such as configuring entity mappings in Entity Framework Core with methods like HasKey, HasMany, and ToTable, often in lambda-based configuration blocks.

Benefits include improved readability, discoverability through IDEs, and a clearer separation between configuration and usage. Drawbacks can include overly long chains that are hard to debug, potential for steep learning curves, and tighter coupling to a fluent chaining style that may hinder refactoring.

Fluent interfaces emerged in the late 1990s and gained prominence in the 2000s, popularized by discussions of the pattern and its use in modern programming languages. The term Fluent API is now used broadly to describe libraries that expose chainable, expressive configuration methods. See also Fluent interface and Builder pattern.