.NET 11 Preview 3: Union Types, Zstandard, and Aspire Integration
Here is a breakdown of the most impactful updates in the latest preview.
C# 15 and Language Evolution
The language features in .NET 11 continue to focus on reducing boilerplate and enforcing stricter type safety.
Union Types: Preview 3 introduces support for union types, one of the most requested features in the C# ecosystem. This allows developers to define types that can hold one of several distinct, strongly typed values, leading to cleaner domain modeling and more robust compile-time checks. Collection Expression Arguments: First introduced in Preview 1, developers can now pass arguments (like capacity or comparers) directly within collection expression brackets using a with() element (e.g., [with(capacity: 10), .. values]).
Runtime and Performance Optimizations
.NET 11 is heavily focused on pushing execution speed and throughput limits, particularly for asynchronous and heavy computational workloads.
Native Zstandard Compression: Zstandard has been formally moved into System.IO.Compression. ASP.NET Core leverages this directly to provide built-in Zstandard response compression and request decompression out of the box. Runtime Async: The new runtime-level infrastructure designed to optimize heavily asynchronous workloads has officially dropped its preview-API opt-in requirement. JIT Enhancements: The Just-In-Time (JIT) compiler includes fresh optimizations that improve the speed of switch statements, bounds checks, and type casts. WebAssembly Capabilities: Browser and WebAssembly targets now support WebCIL and feature an improved debugging experience.
SDK and Developer Experience
Tooling improvements in Preview 3 are aimed at reducing friction in day-to-day inner loop development.
Aspire Integration for Hot Reload: The dotnet watch command now natively integrates with .NET Aspire app hosts. This enables seamless hot-reload workflows across entire multi-service application models, alongside automatic crash recovery. Streamlined CLI Commands: Developers can now pass environment variables directly via the command line using dotnet run -e KEY=VALUE. Additionally, solution filters (.slnf) can now be created and edited directly from the CLI. File-Based App Splitting: File-based apps now support an #:include directive, allowing developers to organize shared helpers into separate files without abandoning the file-based workflow. Signed Containers: All .NET container images are now signed by default to enhance security and supply chain integrity.
#### Reduced SDK Install Sizes
The .NET SDK installers on Linux and macOS are now significantly smaller due to assembly deduplication using symbolic links. Windows deduplication is planned for a future preview.
| Platform / Artifact | .NET 10 Size | .NET 11 Preview Size | Reduction | | :--- | :--- | :--- | :--- | | linux-x64 tarball | 230 MB | 189 MB | 17.8% | | linux-x64 deb | 164 MB | 122 MB | 25.6% | | linux-x64 rpm | 165 MB | 122 MB | 26.0% |
Entity Framework Core and Data Access
EF Core continues to evolve, minimizing overhead and expanding support for modern database features.
ChangeTracker Optimizations: The ChangeTracker.GetEntriesForState() method has been optimized to avoid unnecessary change detection overhead, improving performance in data-heavy operations. Advanced SQL Server Support: EF Core now includes support for SQL Server JSON APIs, JSON_CONTAINS(), and DiskANN vector indexes (VECTOR_SEARCH()) to better support AI-driven data retrieval.
Looking Ahead
While .NET 11 has sparked some debate regarding its increasing syntax complexity and its strategic push toward "Agentic AI" (allowing .NET applications to act as tools for AI agents via the Model Context Protocol), the under-the-hood performance wins and tooling upgrades in Preview 3 represent a substantial step forward.
Developers can test these features today by downloading the .NET 11 Preview 3 SDK or updating to the latest Visual Studio 2026 Insiders build.