Modern C# (10+) Expert Skill

Trigger On

• The user asks to write, debug, refactor, or review C# code.
• The project directory contains *.cs, *.csproj, or single-file C# scripts.
• The target framework is .NET 6.0 (LTS) and above.
• Code modernization is needed (e.g., converting legacy nested namespaces to File-scoped namespaces).

Workflow

1. Perceive (Version Awareness):

• Check .csproj to identify TargetFramework and determine the syntax upper limit:
• net6.0: C# 10 (File-scoped namespaces, Global usings).
• net7.0: C# 11 (Raw string literals, Required members).
• net8.0: C# 12 (Primary Constructors, Collection Expressions).
• net9.0: C# 13 (params collections, Lock object).
• net10.0: C# 14 (Extension members, field keyword).

1. Reason (Planning Phase):

• Evaluate the modernization level of the current code to determine the refactoring strategy.
• In lower version environments (like .NET 6), avoid using higher version syntax (like Primary Constructors), but prioritize using File-scoped namespaces.
• In higher version environments, actively adopt new features to reduce boilerplate code.

1. Act (Execution Phase):

• Write high-quality code, prioritizing "syntactic sugar" to improve readability.
• Implement strong typing and immutable data structures (record, record struct).
• Utilize Interpolated string handlers and Span<T> to optimize performance-sensitive paths.

1. Validate (Standard Validation):

• Validate the safety of NRT (Nullable Reference Types).
• Check if asynchronous operations correctly handle cancellation tokens (CancellationToken).
• Ensure naming conventions comply with official .NET recommendations.

Feature Roadmap (C# 10 - 14)

C# 10 & 11 (Foundation)

• File-scoped Namespaces: Reduce indentation levels.
• Global Using Directives: Centralize common namespaces management.
• Raw String Literals: """...""" easily handle multi-line and special characters.
• Required Members: Ensure required properties during object initialization.
• List Patterns: if (list is [1, 2, ..]) powerful collection matching.

C# 12 & 13 (Productivity)

• Primary Constructors: Simplify class-level dependency injection.
• Collection Expressions: Uniformly use [1, 2, 3] to initialize collections.
• `params` Collections: Method parameters support various collection types.
• Implicit Span Conversion: Handle memory-safe code more naturally.

C# 14+ (Version-Specific Features)

• Extension Members: Extension properties, operators, and static members.
• `field` Keyword: Directly access backing fields in property logic.
• Null-conditional Assignment: target?.Property = value;.
• Scripting: Support dotnet run file.cs for direct execution.

Coding Standards

• Clean Structure: Prioritize using File-scoped namespaces.
• Immutability: Use record for Data Transfer Objects (DTO).
• Performance: Use Span<T> and ReadOnlySpan<char> in critical paths.
• Async Safety: Always pass CancellationToken, avoid .Result or .Wait().

Deliver

• Version-Optimized Code: Provide the most appropriate modernized syntax code based on the target C# version.
• Modernization Insights: Provide specific refactoring suggestions for upgrading from older C# syntax to new features (e.g., from nested namespaces to File-scoped namespaces).
• Syntax Explanations: Clearly explain the design intent and syntax advantages behind the new C# features used.

Validate

• Ensure the provided code complies with the syntax specifications of the target C# version.
• Validate whether the code follows C# strong typing and Null safety (NRT) principles.
• Confirm the code has good readability and best practices at the pure C# language level (e.g., proper use of Span<T>, record, etc.).

Documentation

Official References

• What's new in C# 14 (Extension Members, field keyword)
• What's new in C# 13 (params collections, Lock object)
• What's new in C# 12 (Primary Constructors, Collection Expressions)
• What's new in C# 11 (Raw String Literals, Required Members)
• What's new in C# 10 (File-scoped Namespaces, Global Usings)
• Nullable Reference Types

Internal References

• C# Coding Style and Naming Conventions Guide
• C# Anti-Patterns and Best Practices
• Modern C# Patterns Guide