# modern-csharp

Teaches the agent C# 14 syntax: primary constructors, collection expressions, the field keyword, extension blocks. Useful when you write new C# or modernize old code.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 2900 installs · ★ 789
- Author: codewithmukesh
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/codewithmukesh/dotnet-claude-kit/tree/HEAD/skills/modern-csharp/SKILL.md
- Updated: 2026-08-07
- HTML: https://affpapa.org/en/nevibecode/skills/modern-csharp

## What it does

The skill gives the agent a baseline for modern C#: primary constructors, collection expressions `[]` with spread, records and `readonly record struct`, pattern matching, `Span<T>` slicing, raw string literals, and the `required` modifier. Inside there is a quick-reference table of features, an anti-patterns section, and a decision table — which construct to pick for a DTO, which for a DI service, which for a small value object.

Two sections stand on their own: the `field` keyword from C# 14 and `extension` blocks. The first removes manual backing fields, so validation, lazy loading and `INotifyPropertyChanged` live right in the accessors. The second adds extension properties and static extension members that classic extension methods never had. The old `this`-parameter syntax still works and coexists with the new blocks.

The target is C# 14 and .NET 10, so on a project pinned to an older language version some examples will not compile at all. The skill is meant to load as the baseline for every agent, which means you wire it up once and it colours all C# generation afterwards.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add codewithmukesh/dotnet-claude-kit --skill modern-csharp -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/modern-csharp/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/modern-csharp/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add codewithmukesh/dotnet-claude-kit --skill modern-csharp -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/modern-csharp/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add codewithmukesh/dotnet-claude-kit --skill modern-csharp -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/modern-csharp/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install codewithmukesh/dotnet-claude-kit modern-csharp
```

2. **Manually: put the files in place**
- File: `.github/skills/modern-csharp/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add codewithmukesh/dotnet-claude-kit --skill modern-csharp -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/codewithmukesh/dotnet-claude-kit --path skills/modern-csharp
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/modern-csharp/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add codewithmukesh/dotnet-claude-kit --skill modern-csharp -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/codewithmukesh/dotnet-claude-kit/tree/HEAD/skills/modern-csharp/SKILL.md

## When to use it
- write DTOs and value objects as records instead of tuples
- drop manual backing fields from properties with validation
- build lists and arrays with collection expressions
- slice strings through `Span<T>` without extra allocations
- move old code onto primary constructors and `required`

## Example prompts
- Rewrite this class for C# 14: primary constructor instead of fields, DI through the constructor
- Replace the manual backing fields with the `field` keyword and a value check
- Add a `TotalWithTax` extension property to `Order` using an `extension` block

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: f05d4efd209a1d49.

BEGIN THIRD-PARTY FILE f05d4efd209a1d49
```text
---
name: modern-csharp
description: >
  Modern C# language features for .NET 10 and C# 14. Covers primary constructors,
  collection expressions, the field keyword, extension members, records, pattern
  matching, spans, and raw string literals.
  Load this skill when writing any new C# code, reviewing existing code for
  modernization, using "modern C#", "C# 14", "primary constructor", "collection
  expression", "records", "pattern matching", "span", "field keyword", or
  "extension members". Always loaded as the baseline for all agents.
---

# Modern C# (C# 14 / .NET 10)

## Core Principles

1. **Use the newest stable features** — C# 14 is the target. Prefer language-level constructs over library workarounds.
2. **Readability over cleverness** — Pattern matching and expression-bodied members improve readability when used appropriately; deeply nested patterns do not.
3. **Value types where possible** — Prefer `record struct`, `Span<T>`, and stack allocation to reduce GC pressure.
4. **Immutability by default** — Use `record`, `readonly`, `init`, and `required` to make illegal states unrepresentable.

## Patterns

### Well-Known Features Quick Reference

| Feature | Usage | Example |
|---------|-------|---------|
| Primary constructors | DI injection, eliminate field assignments | `public class OrderService(IOrderRepo repo, TimeProvider clock) { }` |
| Collection expressions | `[]` for all collection types + spread | `List<string> names = ["Alice", "Bob"];` / `int[] all = [..a, ..b, 99];` |
| Records | DTOs, value objects, immutable data | `public record CreateOrderRequest(string CustomerId, List<OrderItem> Items);` |
| `readonly record struct` | Small stack-allocated value types | `public readonly record struct Money(decimal Amount, string Currency);` |
| Pattern matching | Switch expressions, list/property patterns | `order switch { { Total: > 1000 } => "Premium", _ => "Standard" };` |
| List patterns | Deconstruct arrays/lists | `items switch { [] => "Empty", [var x] => $"One: {x}", [var f, .., var l] => $"{f}..{l}" };` |
| `Span<T>` | Zero-allocation slicing | `ReadOnlySpan<char> trimmed = input.Trim(); int.TryParse(trimmed[4..], out id);` |
| Raw string literals | Multi-line SQL, JSON, XML | `var sql = """ SELECT ... """;` / interpolated: `$$""" {"id": "{{id}}"} """;` |
| `required` members | Enforce initialization | `public required string ConnectionString { get; init; }` |
```
END THIRD-PARTY FILE f05d4efd209a1d49

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