What is dotnet-modern-csharp?

Guidance for writing current C# code in .NET projects while checking which language features the project's framework and C# version support.

Use it when choosing C# features, upgrading language versions, reviewing feature usage, or keeping code compatible across .NET repositories.

It helps avoid using newer C# features that the project's target framework or compiler settings cannot handle.

dotnet-modern-csharp — Skill by managedcode

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods

npx agentmods add skills/managedcode/prompterone/dotnet-modern-csharp

Any agent

npx skills add managedcode/PrompterOne --skill dotnet-modern-csharp

Clone the repo

git clone --depth 1 https://github.com/managedcode/PrompterOne

Made for: Claude Code, Codex.

Per session 53 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 1,186 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. Scan, not verified.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5	$0.00053	$0.01186
Opus 5	$0.00026	$0.00593
Sonnet 5	$0.00011	$0.00237
Haiku 4.5	$0.00005	$0.00119

Measured yesterday against content hash 981b52bb59a5, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

dotnet-modern-csharp scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured yesterday.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

Origin

Copies of this mod

2 near-identical copies found in the catalogue:

mcaf-dotnet-features — 95% identical, 16 lines differ
mcaf-dotnet-features — 95% identical, 16 lines differ

.codex/skills/dotnet-modern-csharp/SKILL.md · 120 lines

How it starts

The opening of the file, as written. The whole thing — 120 lines — stays where its author put it; the contents beside it link to each section on GitHub.

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

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

Workflow

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

Bootstrap When Missing

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

Read the full file on GitHub · 120 lines

Files

What ships with it

4 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

yesterday First seen · 120 lines · 53 tokens per session scan A 981b52bb59a5

Subscribe to this mod's changes

dotnet-modern-csharp is a skill published in the GitHub repository managedcode/PrompterOne (42 stars, last pushed 3mo ago), licensed MIT. It adds 53 tokens to every session and 1,186 once invoked, about $0.0003 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

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