Have you ever needed a method to return different types depending on the situation? Perhaps a payment processor that returns different payment types, an order that can be in various states with different data, or better, a file loader that handles mu...

freeCodeCamp is a nonprofit organization offering free online coding courses and programming tutorials, covering topics such as web development, data science, and machine learning. Learners can gain practical coding skills, build real-world projects, and earn certifications to advance their careers in tech.

freeCodeCamp

OneOf is a C# library that implements discriminated unions, allowing methods to return one of several predefined types in a type-safe manner. Unlike tuples that bundle multiple values together, OneOf represents a choice between types (A or B or C), with compiler-enforced exhaustive handling through the .Match() method. The library provides a cleaner alternative to exception-driven control flow, inheritance hierarchies, and marker interfaces. Common use cases include polymorphic return types without inheritance, state machines with rich data per state, multi-channel notifications, file format handling, and explicit error handling. The approach makes failures explicit in method signatures, improves code readability, and eliminates entire categories of bugs by forcing developers to handle all possible cases at compile time.

How to Implement Type Safe Unions in C# With OneOf

A Solution to Exception-Driven Control Flow