Microsoft Ships .NET 11, a Two-Year STS Release With C# 15 Unions and a Faster SDK

Microsoft released .NET 11 on November 10 as a Standard Term Support (STS) release with two years of support. .NET 10 remains a supported LTS option, and Microsoft says both tracks share the same quality and servicing policies. Only the length of support differs.

The timing adds pressure. .NET 8 and .NET 9 reached end of support on November 10, the same day .NET Conf 2026 opened. The conference runs November 10–13, with product-team sessions on the first two days and community sessions afterward.

Teams still on either version need to move to a supported release to keep receiving fixes. The real decision is whether to land on .NET 11 for the newest features or on .NET 10 for a longer runway.

What’s New in C# 15

C# 15 is the default language version for projects targeting net11.0.

Union types are the headline addition. A value can be one of a declared set of case types, with conversions from those types and exhaustiveness checking in switch expressions. In practice, a method that returns “found” or “not found” no longer needs object or an unrelated base class just to put both outcomes behind a single return type.

Closed hierarchies arrive alongside them. A closed class limits direct derivation to its declaring assembly, which gives the compiler enough information to check patterns over the whole hierarchy. Microsoft positions unions for combining existing types, and closed hierarchies for models where you own the inheritance relationship.

Smaller additions include:

• Collection expression arguments, for example passing a comparer to a HashSet initializer without giving up collection-expression syntax
• Extension indexers
• Labeled break and continue

None of these require the preview language version. The separate memory-safety work remains in preview.

SDK, Runtime, and Tooling

The SDK now enables a Native AOT CLI host by default. Commands such as help and version queries can run without starting the managed CLI, though commands that need managed execution still use it. The MSBuild server also stays warm between CLI invocations, and the Linux and macOS installers are smaller thanks to assembly deduplication.

For teams on Microsoft.Testing.Platform, dotnet test gains run-level timeouts, failure limits, and per-module results directories. Microsoft notes that these switches are not interchangeable with VSTest options.

Runtime Async remains a preview feature. You opt in per project, without enabling EnablePreviewFeatures. It moves asynchronous suspension and resumption into the runtime instead of relying on compiler-generated state machines. One of its most useful results is a clearer live call stack in profilers and debuggers.

AI tooling

.NET 11 gives AI development more concrete entry points:

• MCP server template: dotnet new mcpserver
• Microsoft.Extensions.AI: model abstractions such as IChatClient
• Microsoft Agent Framework: agents and workflows

Two pieces are explicitly not stable yet. The AG-UI hosting integration is prerelease, and the Blazor AI components package stays experimental throughout .NET 11.

Before You Upgrade

A few changes deserve attention before you retarget:

• .NET MAUI makes CoreCLR its default runtime across platforms. MAUI also follows its own support policy, separate from the STS/LTS lifecycle.
• XAML Incremental Hot Reload is a preview feature, enabled by default in Debug builds.
• Microsoft.Maui.Controls.Compatibility no longer ships, so apps that reference it must migrate.
• EF Core 11 requires .NET 11.
• Breaking changes include updated minimum CPU requirements.

Installing the SDK alone does not retarget an app. The upgrade path runs through your global.json pins, target frameworks, packages, and workloads. After that, test against your actual publishing mode and container base images.

For tooling, Microsoft points to Visual Studio 2026 or Visual Studio Code with the C# Dev Kit.