Microsoft Releases .NET 11 and C# 15 with New Type Safety Features and Built-in AI Agent Support
Microsoft launched .NET 11 at .NET Conf 2026, introducing C# 15 Union Types and Closed Hierarchies for better code safety, enabling Native AOT for faster CLI startup, and including built-in MCP server templates to streamline AI agent development.
US technology company Microsoft has officially released .NET 11 at the .NET Conf 2026 developer conference. This version defaults to the C# 15 programming language. .NET is Microsoft's cross-platform framework widely used for web, desktop, and mobile applications. The update focuses on enhancing language safety, execution efficiency, and native support for AI agent development.
In terms of language features, C# 15 introduces "Union Types" and "Closed Hierarchies." Union types allow developers to explicitly declare which specific types a value can belong to—for example, a query result might be either "product data" or a "no-product response." The compiler can then check if the code handles all possible cases, reducing errors caused by missed branches. Closed hierarchies restrict class inheritance to within the same assembly, giving the compiler full visibility of subclass structures to further prevent logical gaps.
Regarding execution performance, the .NET 11 SDK now defaults to using Native AOT (Ahead-of-Time compilation) for its Command Line Interface (CLI) startup programs. Common commands like checking versions or viewing help are executed directly as native code, bypassing the traditional managed .NET runtime and significantly reducing startup time. Additionally, the MSBuild server remains active between commands, minimizing repeated initialization overhead during frequent builds and tests.
Addressing the growing trend in AI application development, Microsoft has prioritized AI agent support in the .NET roadmap. The .NET 11 SDK includes built-in templates for MCP (Model Context Protocol) server projects, allowing developers to quickly create tool services callable by AI agents. Blazor frameworks have also added AI interface components supporting streaming conversations and tool execution results, along with tools like .NET Skills to assist AI agents in coding and testing.