Union Types Are Finally Coming to C#

This article explores the long-awaited addition of union types to C# 15 (shipping with .NET 11). It explains the problem of modeling methods that return one of several types, such as a user or a 'not found' error, and how developers previously faked this with marker interfaces, abstract classes, tuples, nullable returns, exceptions, or third-party libraries like OneOf. The new union type syntax is simple and concise, enabling closed sets of types with implicit conversions and compile-time checking. Pattern matching works seamlessly with unions, ensuring exhaustive handling. The article includes examples of declaring unions, creating values, and consuming them via switch expressions, highlighting how this feature reduces ceremony and improves code clarity for backend developers.