Microsoft ships .NET 11 with C# 15 union types and broad runtime performance work

Microsoft releases .NET 11 as a two-year STS release

Microsoft has released .NET 11, announcing the update on its .NET Blog on November 10, 2026. The release is a Standard Term Support (STS) version, which entitles it to two years of support. According to the announcement, STS and Long Term Support (LTS) releases carry the same quality and servicing policies and differ only in support length; .NET 10 remains the supported LTS option. Microsoft is marking the launch with .NET Conf, running November 10–13.

C# 15 becomes the default language

C# 15 is the default language version for projects targeting .NET 11. Its headline addition is union types: a value can be one of a declared set of case types, with conversions from those types and exhaustiveness checking in switch expressions. The announcement gives the example of a product lookup that returns either a found product or a not-found record, so callers know exactly which outcomes they must handle without falling back to object or an unrelated base class.

Closed hierarchies arrive alongside unions. A closed class limits direct derivation to its declaring assembly, giving the compiler the information needed to check patterns across the hierarchy. Unions suit combining existing types; closed hierarchies fit models where you own the inheritance relationship.

Smaller features include collection expression arguments, which let developers pass comparer or factory arguments while keeping collection expression syntax, plus extension indexers and labeled break and continue. Memory safety work remains in preview and requires opting in. F# 11 also ships, with lower compiler memory use, fewer allocations in FSharp.Core collection operations, and new ways to bound concurrent asynchronous work.

Leaner SDK and tooling by default

The SDK now enables a Native AOT CLI host by default, so commands such as help and version queries can run without starting the managed CLI, while commands needing managed execution still use it. The MSBuild server also stays warm between invocations by default, cutting repeated startup work, and installers on Linux and macOS are smaller through assembly deduplication.

File-based apps can grow across source files with #:include, and solution filters can be created and edited from the command line. dotnet watch adds Aspire app-host integration, crash recovery and device selection for mobile projects. Teams using Microsoft.Testing.Platform get run-level timeouts, failure limits and clearer progress reporting in dotnet test — switches the announcement notes are not interchangeable with VSTest options. Container publishing gains reproducible image digests and avoids redundant layer uploads, and NuGet source configuration gains a minimum package publication age setting, which Microsoft describes as a policy control rather than a substitute for vulnerability checks.

Runtime optimizations across the board

Performance work touches both deployed code and developer tools. The announcement points to Stephen Toub's deep-dive post documenting a large set of runtime and library optimizations with benchmarks and examples.

JIT improvements remove more redundant bounds checks and branches, improve devirtualization and fold more operations into constants — for instance, optimizing additional SequenceEqual calls when inputs are known at compile time. Native AOT interface dispatch now uses a shared helper that shrinks the code needed at interface call sites and improves throughput for workloads dominated by interface calls. Hardware support expands with FP16 instructions for Half operations on supported x64 and Arm64 chips, plus new vector construction and composition APIs.

On diagnostics, in-process crash report logging — previously available on mobile platforms — now also supports Linux and macOS, capturing managed stack traces, module information and runtime state before a process exits. Microsoft cautions that throughput, startup time, memory use and deployment size depend on the workload and publishing options, so teams should measure their own applications. Runtime Async, which moves asynchronous suspension and resumption into the runtime rather than compiler-generated machinery, is a recurring theme across the release.

Web, AI and cross-platform updates

ASP.NET Core adds asynchronous validation, OpenAPI 3.2 support, and new capabilities for Blazor and SignalR. Experimental Blazor AI components integrate with the Agent User Interaction Protocol (AG-UI) for agentic web experiences.

For AI work more broadly, Microsoft highlights Microsoft.Extensions.AI for model access, MCP for tool integration and the Microsoft Agent Framework for agents and workflows. Agentic developer tooling includes .NET Skills for .NET-specific guidance, .NET MAUI DevFlow for inspecting and testing running apps, and the .NET Upgrade canvas for reviewing modernization progress.

.NET MAUI adopts CoreCLR as its default runtime, with improvements to XAML iteration and native controls. The base libraries add built-in Zstandard compression, simpler process execution, richer JSON support, and new ways to work with in-memory data without copying it first.

Why it matters

.NET 11 keeps Microsoft's annual cadence, and its two-year STS window means adopting teams should plan their next upgrade inside that timeframe. The tooling changes — the Native AOT CLI host and warm MSBuild server enabled by default — deliver faster everyday commands without any action from developers. C# 15's union types and closed hierarchies bring compiler-checked modeling of possible states into the language proper, echoing patterns long familiar from functional languages. Meanwhile, the spread of AI-related features, from MEAI and the Agent Framework to AG-UI Blazor components, signals where Microsoft expects .NET developers to invest next, even as the volume of runtime work shows that raw performance remains the platform's core pitch.