.NET 11 RC1: Production Ready with C# 15 & F# 11

The .NET 11 RC1 Leap

Microsoft's release of .NET 11 RC1 marks a significant milestone, bringing C# 15 and F# 11 to the forefront with go-live support, signaling production readiness for a wide array of developers. The stabilization of key C# 15 features like union types, collection expression arguments, and non-virtual static interface members, alongside F# 11's stabilized record spreads and direct delegate construction, addresses long-standing developer requests and paves the way for more robust and expressive code. The performance improvements noted, such as string interpolation compiling to String.Concat in F#, are crucial for modern development workflows, especially with trimming and Native AOT. Furthermore, the enhanced dotnet test capabilities for cross-platform development and improved container publishing streamline the CI/CD pipeline, making this release highly attractive for immediate adoption. The integration with Visual Studio 2026 Insiders and VS Code with C# Dev Kit ensures a smooth developer experience. The community's positive reception, with reports of easy migration, underscores the practicality and maturity of this release candidate. The go-live license is a particularly strong signal for enterprises considering upgrades.

However, the article touches upon a critical limitation: the reported issue with union types in C# 15 not correctly round-tripping through System.Text.Json due to a missing type discriminator in the serialized output. This is a substantial concern for applications heavily reliant on JSON serialization for data exchange, particularly in distributed systems or microservices architectures. While the release notes indicate stabilization, this specific interoperability issue with a foundational serialization library could be a showstopper for some teams. The fact that 'Unsafe Evolution' remains a preview feature, even with the allowance of await inside an unsafe context, suggests that certain advanced language features are still undergoing refinement. The experimental status of the 'Device Bound Session Credentials' package also indicates areas where .NET 11 is still evolving. These points, while not detracting from the overall strength of the release, highlight areas where developers might need to exercise caution or await further updates before fully leveraging these specific functionalities in production.

Key Points

• .NET 11 Release Candidate 1 (RC1) is now available with go-live support, enabling production use.
• C# 15 becomes the default language version, stabilizing features like union types, collection expression arguments, and non-virtual static interface members.
• F# 11 is also the default, with stabilized record spreads, direct delegate construction, and improved string interpolation for trimming and Native AOT.
• SDK improvements include enhanced dotnet test for cross-platform targets and optimized container publishing.
• ASP.NET Core enhances SignalR authentication refresh APIs and marks OpenAPI operations as deprecated.
• .NET MAUI brings faster Android builds, TabbedPage badges, and themed splash screens.
• Community feedback is largely positive, citing easy migration and the value of the go-live license.
• A key concern raised is the lack of type discrimination in System.Text.Json serialization of C# union types.

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📖 Source: Microsoft Releases .NET 11 RC1 with Go-Live Support, C# 15 and F# 11 as Default Language Versions