.NET 10 RC 1 is Knocking at the Door: Architectural Impact, C# 14, and Performance

.NET 10 RC 1 is knocking at the door, marking the first release candidate and offering the .NET community a detailed preview of what’s to come in the next LTS cycle. While not the final release, RC 1 is “go-live” supported and represents the feature-complete platform that will soon become .NET 10 LTS. In this article, I’ll try to give a rough overview of the architectural impact of .NET 10 RC 1, focusing on the latest C# 14 features, under-the-hood performance improvements, and strategic considerations for the upcoming LTS.

.NET 10 RC 1 at a Glance: LTS on the Horizon, Key Improvements Now

With .NET 10 RC 1, Microsoft is inviting early adopters to prepare for the next major LTS release (expected with the final GA later this year). RC 1 brings broad improvements across the platform:

• Runtime Enhancements: Smarter JIT compilation (improved inlining, devirtualization, and loop optimizations) and Native AOT advances result in faster code execution. Memory management sees gains with better stack allocation and garbage collection optimizations, including new write barriers on ARM64 for reduced pause times.
• SDK and Tooling: RC 1 introduces the ability for console apps to publish directly to container images, new SDK options for controlling image formats, and CLI improvements like standardized command ordering and tab-completion generation.
• ASP.NET Core & Libraries: ASP.NET Core 10.0 (RC 1) updates include Blazor WebAssembly loading improvements, enhanced minimal APIs, and OpenAPI 3.1 support. Core libraries gain new APIs and performance tweaks—like stricter and more efficient JSON serialization.

The key theme of RC 1: improve performance and developer productivity while ensuring a stable path to LTS for production environments.

C# 14: New Language Features in .NET 10 RC 1

C# 14 ships with .NET 10 RC 1, delivering language enhancements that simplify code and enable new patterns:

• Field-Backed Properties: Easily reference the compiler-generated backing field inside a property accessor using the new field keyword, streamlining property evolution.
• nameof on Generic Types: Supports unbound generics (e.g., nameof(List<>)), handy for logging and diagnostics.
• Implicit Span<T> Conversions: More natural and efficient use of spans for high-performance code.
• Lambda Parameter Modifiers: Use ref, in, out, or scoped modifiers in lambda expressions without explicit types, increasing flexibility for advanced scenarios.
• Partial Constructors and Events: Declare constructors and events as partial, improving code organization in large or generated projects.
• Extension Members via extension Blocks: Group extension methods and properties, even static ones, in a natural way.
• Null-Conditional Assignment: Assign values using the null-conditional operator on the left side, reducing boilerplate.
• User-Defined Compound Operators: Explicitly define +=, ++, etc., for custom types, offering more control to library authors.

All these features are available in .NET 10 RC 1 and will be fully supported at GA.

Performance Gains in .NET 10 RC 1

The performance enhancements in .NET 10 RC 1 are substantial:

• Smarter JIT Compiler: RC 1 improves codegen, method inlining, and virtual call elimination—especially in array and IEnumerable<T> usage.
• Reduced Allocations: More aggressive stack allocation for small arrays and transient objects means fewer GC pauses.
• NativeAOT Improvements: RC 1 expands AOT support for various patterns, resulting in even faster startup and smaller binaries.
• Hardware Intrinsics: Ready for AVX10.2, preparing for next-gen hardware.
• GC Optimizations: More efficient write barriers for Arm64, reducing GC pause times on modern cloud servers.

These changes are available today in RC 1, and will deliver even more value as you roll forward to .NET 10 GA.

Impact on Existing Applications and Breaking Changes

Adopting .NET 10 RC 1 is generally a smooth process for modern .NET applications (especially those already on .NET 6, 7, or 8), but every major release introduces a set of important changes—both breaking changes and subtle behavioral differences. Understanding these up front ensures that architectural decisions and upgrade paths are properly managed.

Obsolete and Deprecated APIs

• ASP.NET Core: Several APIs and features are now marked as obsolete or removed. For example, IActionContextAccessor is deprecated in ASP.NET Core 10. If your solution relied on global access to ActionContext, you should switch to more explicit DI patterns. Similarly, runtime Razor view compilation is no longer supported in the same way; dynamic compilation at runtime is now discouraged for performance and security reasons. Refactor views to use pre-compilation and follow the new guidance for dynamic scenarios.
• Legacy Behaviors Cleaned Up: Some behaviors that were previously tolerated for backward compatibility are now stricter or have been removed altogether. For instance, certain legacy configuration providers or authentication flows in ASP.NET Core may no longer behave as before.

Changes in Core Libraries

• Non-nullable Properties: Properties such as FilePatternMatch.Stem (in file globbing APIs) are now non-nullable. If your code previously checked for null or allowed for missing stems, you’ll need to adjust for the new contract, or you may encounter new compiler warnings or exceptions.
• Async LINQ in BCL: The core libraries now include System.Linq.AsyncEnumerable. If you previously used the community-provided async LINQ NuGet package, you may face type conflicts or ambiguous references. Removing the extra package and using the in-box types resolves this issue.
• Configuration Binding Behavior: Configuration binding now preserves null values by default. Where earlier versions might have ignored explicit nulls in config sources, RC 1 will bind them. This can surface bugs or require logic changes in options classes or startup code.

Platform and Environment Changes

• Container Images Default to Ubuntu: The official .NET 10 RC 1 container images now use Ubuntu 22.04 as the base image by default. Previously, you may have been relying on Debian or Alpine-based images. This change affects container size, security baselines, and possibly compatibility with tools or scripts expecting a different OS environment. If your deployment process or compliance requirements depend on a specific distro, update your Dockerfiles to use the explicit tag (e.g., alpine or debian).
• Disabled HTTP/3 by Default with Trimming: When using trimming (for self-contained, minimal-size deployments), HTTP/3 support is now disabled unless explicitly opted-in. This prevents accidental inclusion of large, unnecessary networking code in trimmed apps but can break scenarios relying on HTTP/3, such as modern gRPC or advanced ASP.NET Core configurations.
• Logging Changes: Console logging is now smarter—by default, it no longer duplicates messages across multiple providers/outputs. If you had custom logging logic or were expecting certain log flows, validate that your log output remains as intended after the upgrade.

NuGet and Dependency Management

• Package Compatibility: Most NuGet packages targeting .NET Standard 2.0+ or .NET 6+ will work with .NET 10 RC 1. However, any packages using reflection or internal runtime APIs might require updates. Check for new versions of critical dependencies, and validate all third-party libraries in your build and test pipelines.
• Framework Targeting: .NET 10 RC 1 is stricter about target framework compatibility. Projects that reference legacy assemblies or unsupported TFMs will generate more explicit build warnings or errors, driving better practices but possibly requiring codebase updates.

Security and Compliance

• Cryptography Baseline Updates: .NET 10 RC 1 updates cryptographic algorithms, strengthens TLS defaults, and adds support for new standards (including post-quantum algorithms and improved AES support). Applications with custom cryptography, or those that interoperate with older protocols, should be tested carefully to ensure continued compatibility and compliance.

Windows Desktop and Other Subsystems

• WinForms/WPF Updates: There are numerous small but impactful updates in Windows desktop stacks (WinForms, WPF), including new APIs, DPI scaling improvements, and better accessibility. Some legacy controls or behaviors may be removed or altered. Review the official breaking changes for desktop-specific notes if you target Windows.

Practical Recommendations

• Review the official breaking changes documentation and filter for all frameworks your solutions depend on (ASP.NET Core, Entity Framework Core, WinForms, WPF, etc.).
• Test Early and Often: Use the RC 1 bits in development or staging to surface behavioral changes. Pay special attention to integration boundaries—especially serialization, networking, and interop code.
• Automate Compatibility Checks: Update your CI pipelines to include warnings-as-errors and run tests with .NET 10 RC 1 to proactively identify issues.

By systematically addressing these changes, you ensure a smooth transition to .NET 10 LTS and avoid common pitfalls as the ecosystem moves forward.

Architectural Considerations for .NET 10 RC 1

With .NET 10 RC 1, architectural planning gains new momentum for modern cloud-native and high-performance development. The direct support for containerization enables streamlined deployment pipelines, allowing teams to produce ready-to-run images straight from their build processes and reducing operational friction in multi-service environments. The expanded capabilities of NativeAOT open new scenarios for applications demanding rapid startup and minimal resource consumption, such as serverless workloads, lightweight microservices, or command-line tools. Performance improvements across the runtime—particularly in JIT optimizations and memory allocation—encourage a more declarative and idiomatic use of C# constructs, enabling cleaner architectures without sacrificing efficiency. Security also receives a boost, with updated cryptography baselines and compliance with emerging standards, which is increasingly relevant for regulated industries and zero-trust environments. At the same time, C# 14’s new features, such as extension members and partial constructors, offer architects and teams greater flexibility in organizing large codebases and adopting modern software design patterns. Altogether, .NET 10 RC 1 provides a compelling foundation for scalable, maintainable, and future-proof solutions, making it an excellent platform for architects who want to maximize developer productivity while ensuring technical resilience and readiness for the next generation of .NET workloads.

Conclusion

.NET 10 RC 1 is a strong signal of where the platform is heading. With architectural and performance improvements, a modernized C# 14, and production-ready containerization support, RC 1 is the ideal launchpad for planning your next step. While this is not yet the final LTS release, it’s fully supported for “go-live” production workloads and gives you the opportunity to validate your apps and infrastructure ahead of time. Leverage the new features and performance enhancements in your pilot projects now, and your transition to .NET 10 GA will be smoother than ever.

In the next few articles, we’ll dive deeper into specific areas of interest, including advanced C# 14 features, best practices for leveraging .NET 10 RC 1 in cloud-native applications, and strategies for optimizing performance in high-load scenarios. Stay tuned!