C# 15 Union Types and Closed Hierarchies: Key Features

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Explore C# 15's new type features like union types and closed hierarchies in .NET 11, along with its updated unsafe model and improvements for everyday coding efficiency.

C# 15 Union Types and Closed Hierarchies: Key Features - bimakale.com

27 Ağustos 2026 Perşembe - 16:41 (3 Gün önce) 3 dk okuma

With the release of C# 15 alongside .NET 11, the language introduces two new constructs to its type system, directly enhancing developers' coding experience. Union types and closed hierarchies improve type safety and code readability, especially in large-scale applications.

Multi-Type Support with Union Types

Union types allow a variable to hold values of multiple possible types simultaneously. In traditional C#, similar scenarios required using object or dynamic, which sacrifice type safety and can lead to runtime errors. With union types, you can define a type like int | string, and the compiler ensures no values outside these types are assigned.

Practical Use Cases

API Responses: A service can return either a successful data object or an error message. Union types combine these possibilities into a single type, enabling safe differentiation with switch statements.
Configuration Values: Many configuration files allow settings to be either numeric or text-based. Union types reflect this ambiguity in the type definition, preventing incorrect type usage.

This feature also enhances test coverage, as each possible type scenario can be validated individually in unit tests.

Limiting Class Hierarchies with Closed Hierarchies

Closed hierarchies specify that a class or interface can only be extended by a predefined subset of types. Similar to Java's sealed classes, this feature ensures the compiler ignores any derivations outside the defined hierarchy.

Security and Performance Benefits

The compiler knows all possible subtypes of a closed hierarchy in advance. This knowledge enables exhaustive case lists in switch statements, eliminating the need for a default branch. Additionally, JIT optimizations can be applied more aggressively since type diversity is limited.

Closed hierarchies are particularly useful in domain-driven design (DDD) projects, where they technically support the "aggregate root" concept.

Preview of the Updated Unsafe Model

C# 15 introduces a fresh perspective on unsafe code blocks. Previous versions limited unsafe code to fixed statements and pointer manipulations. The new model offers a more controlled API, enabling safer memory access. However, the announcement did not detail the full scope or performance metrics, emphasizing its preview status.

What’s Changed?

Clearer lifetime management for pointer types.
More flexible support for stack-alloc usage without restrictions.
New span-based helper methods within unsafe blocks to enhance type safety.

These improvements could help system-level programming teams establish a stronger presence in the .NET ecosystem.

Enhancements for Everyday Coding

The announcement highlighted "small but impactful changes" aimed at streamlining developers' daily workflows. For example, using statements no longer require semicolons, and some compiler warnings have been made more descriptive.

Such details improve code readability and provide a smoother autocomplete experience in IDEs.

In conclusion, C# 15's type innovations and unsafe model improvements create a secure and performance-focused programming environment. Developers gain type safety and fewer "null objects" with union types, while closed hierarchies enforce design decisions at the compiler level. These features reduce maintenance costs in large codebases and offer a clearer architectural framework for new projects. The unsafe model improvements also serve as a bridge for teams bringing low-level optimizations into the .NET world. Together, these advancements demonstrate .NET's continued response to modern programming needs.

Source: .NET Blog

Kaynak: .NET Blog

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