Modern C# for .NET

Trigger On

• the repo wants more modern idiomatic C# code
• a change depends on language-version compatibility
• the team is upgrading or reviewing C# feature usage across versions
• you need to know whether a C# 13 or C# 14 feature is safe to use

Value

• produce a concrete project delta: code, docs, config, tests, CI, or review artifact
• reduce ambiguity through explicit planning, verification, and final validation skills
• leave reusable project context so future tasks are faster and safer

Do Not Use For

• non-C# .NET languages such as F# or VB
• analyzer-only or formatter-only setup with no language feature choice

Inputs

• target TFM or TFMs
• explicit LangVersion, if any
• current SDK version
• team style rules in .editorconfig and AGENTS.md

Quick Start

1. Read the nearest AGENTS.md and confirm scope and constraints.
2. Run this skill's Workflow through the Ralph Loop until outcomes are acceptable.
3. Return the Required Result Format with concrete artifacts and verification evidence.

Workflow

1. Detect the real language ceiling from the repo's target framework and explicit LangVersion.
2. Prefer stable features that the current repo actually supports.
3. Use modern syntax when it reduces ceremony, improves correctness, or makes invariants clearer.
4. Do not mass-rewrite a codebase into newer syntax unless the repo wants that churn.
5. Treat preview features as opt-in only. Never assume preview because the current machine has a newer SDK.
6. Pay special attention to C# 13 and C# 14:
   • C# 13 is the stable language for .NET 9
   • C# 14 is the stable language for .NET 10
7. When feature selection changes architecture, style rules, or generated-code patterns, coordinate with:
   • dotnet
   • analyzer-config
   • architecture
8. After feature-driven refactors, run the repo's .NET quality pass through dotnet.

Current Upstream Notes

• .NET runtime v10.0.11 is servicing. It should not by itself justify language-feature rewrites.
• .NET SDK v10.0.400 is the current 10.0.4xx feature band and expands file-based app behavior, but language features still depend on the compiler selected by the repository. Do not use C# 13 or C# 14 syntax in a repo pinned to an older toolset unless it explicitly configures a compatible compiler and LangVersion.

Bootstrap When Missing

If the requested C# feature depends on SDK or language support the repo does not have yet:

1. Detect current state:
   • dotnet --list-sdks
   • rg -n "TargetFramework|LangVersion|TargetFrameworks" -g '*.csproj' -g 'Directory.Build.*' .
2. Confirm whether the repo wants to stay on the current stable language level or intentionally upgrade.
3. If the feature requires a newer supported SDK or target framework, upgrade the repo toolchain deliberately instead of relying on the local machine by accident.
4. If the repo needs explicit LangVersion, record it in project or shared MSBuild config.
5. Run dotnet build SOLUTION_OR_PROJECT after the feature or toolchain change and return status: configured or status: improved.
6. If the repo intentionally stays below the required language level, return status: not_applicable.

Deliver

• modern C# code that fits the repo's real language version
• fewer obsolete patterns when a newer stable feature is clearer
• no accidental preview or unsupported-language drift

Validate

• the chosen syntax is supported by the repo's TFM and LangVersion
• the feature improves clarity, correctness, or maintainability
• preview-only features are used only when the repo explicitly opted in
• style and analyzer rules still agree with the new syntax

Ralph Loop

Use the Ralph Loop for every task, including docs, architecture, testing, and tooling work.

1. Plan first (mandatory):
   • analyze current state
   • define target outcome, constraints, and risks
   • write a detailed execution plan
   • list final validation skills to run at the end, with order and reason
2. Execute one planned step and produce a concrete delta.
3. Review the result and capture findings with actionable next fixes.
4. Apply fixes in small batches and rerun the relevant checks or review steps.
5. Update the plan after each iteration.
6. Repeat until outcomes are acceptable or only explicit exceptions remain.
7. If a dependency is missing, bootstrap it or return status: not_applicable with explicit reason and fallback path.

Required Result Format

• status: complete | clean | improved | configured | not_applicable | blocked
• plan: concise plan and current iteration step
• actions_taken: concrete changes made
• validation_skills: final skills run, or skipped with reasons
• verification: commands, checks, or review evidence summary
• remaining: top unresolved items or none

For setup-only requests with no execution, return status: configured and exact next commands.

Load References

• references/patterns.md
• references/migration.md
• references/csharp-modern-features.md

Example Requests

• "Make this C# code more modern."
• "Which features can we use on .NET 9?"
• "Review this repo for C# 13 or C# 14 opportunities."