.NET 11 Preview 3 Lands With C# Union Types, Zstandard Compression, and Signed Container Images
Microsoft's third .NET 11 preview brings the first in-SDK drop of C# 15 union types, Zstandard support in ASP.NET Core, and cryptographically signed container images.
Overview
Microsoft shipped the third preview of .NET 11 on April 14, pushing several long-discussed features into the hands of developers ahead of the framework’s general availability later this year. According to the .NET Blog, the release spans the runtime, SDK, libraries, ASP.NET Core, Entity Framework Core, .NET MAUI, and the official container images, and includes the first SDK drop of C# 15 union types, Zstandard compression in ASP.NET Core, and cryptographically signed container images.
What We Know
The headline language feature is the arrival of union types, a C# 15 addition that lets developers declare that a value is exactly one of a fixed set of types and get compiler-enforced exhaustive pattern matching. As Microsoft describes in its C# 15 union types post, a declaration as terse as public union Pet(Cat, Dog, Bird); creates a closed set of case types that can be consumed through standard switch expressions, with the compiler warning on any unhandled case if a new type is later added. Union values support implicit conversions from their case types and automatic unwrapping during pattern matching, and unions can themselves carry method bodies and generic parameters. The feature was first introduced in .NET 11 Preview 2 but required developers to declare supporting types in their own projects; Preview 3 is the first build to ship the feature through the SDK itself.
On the web stack, ASP.NET Core gains Zstandard (zstd) support for both response compression and request decompression, extending the existing compression middleware and enabling the algorithm by default, per the .NET Blog. The same release also moves HTTP/3 request processing earlier in the pipeline and updates the Blazor Virtualize component to handle variable-height items at runtime.
Container images published by the .NET team are now cryptographically signed, a change Microsoft positions as a way to verify software integrity in production pipelines, according to ntcompatible. The SDK also gains command-line management of solution filters, multi-file support for top-level programs, environment-variable passing through dotnet run -e, and Aspire integration plus crash recovery in dotnet watch, as detailed in the .NET Blog.
Runtime-side, Microsoft removed the preview API opt-in requirement for the runtime async feature, while the JIT compiler received targeted optimizations for switch statements, bounds checks, and casting operations, according to the .NET Blog. WebAssembly builds picked up WebCIL and debugging improvements. Entity Framework Core introduced a ChangeTracker.GetEntriesForState() helper to avoid unnecessary change detection, removed redundant joins from generated SQL, and added JSON APIs for SQL Server. .NET MAUI added map clustering and styling, a built-in LongPressGestureRecognizer, and preview support for Android 17 / API 37.
What We Don’t Know
Microsoft has not yet confirmed a general-availability date for .NET 11, and the union types feature remains gated behind <LangVersion>preview</LangVersion>, meaning its final surface can still shift before C# 15 is locked. The Zstandard middleware is enabled by default in Preview 3, but Microsoft has not published guidance on how negotiation will interact with existing Brotli and gzip deployments at scale, nor whether signed container images will be mandatory or opt-in for downstream consumers once .NET 11 reaches release. Broader developer reception will depend on how these previews land in Visual Studio 2026 Insiders and the C# Dev Kit over the coming months.