csharp-developer skill

by Jeffallan·Jeffallan/claude-skills·12k stars·MIT

Use when building C# applications with .NET 8+, ASP.NET Core APIs, or Blazor web apps. Builds REST APIs using minimal or controller-based routing, configures database access with Entity Framework Core, implements async patterns and cancellation, structures applications with CQRS via MediatR, and scaffolds Blazor components with state management. Invoke for C#, .NET, ASP.NET Core, Blazor, Entity Framework, EF Core, Minimal API, MAUI, SignalR.

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Install the csharp-developer skill

A skill is a folder. Copy it into your agent's skills folder and the agent loads it when the task matches its description.

git clone --depth 1 https://github.com/Jeffallan/claude-skills.git /tmp/claude-skills
mkdir -p ~/.claude/skills
cp -r /tmp/claude-skills/skills/csharp-developer ~/.claude/skills/csharp-developer

available in every project

In the Claude apps, zip the folder and upload it from the Skills settings. The folder on GitHub

The instructions your agent would load

SKILL.md as published, without the frontmatter. Read it on GitHub

C# Developer

Senior C# developer with mastery of .NET 8+ and Microsoft ecosystem. Specializes in high-performance web APIs, cloud-native solutions, and modern C# language features.

When to Use This Skill

Building ASP.NET Core APIs (Minimal or Controller-based)
Implementing Entity Framework Core data access
Creating Blazor web applications (Server/WASM)
Optimizing .NET performance with Span, Memory
Implementing CQRS with MediatR
Setting up authentication/authorization

Core Workflow

Analyze solution — Review .csproj files, NuGet packages, architecture
Design models — Create domain models, DTOs, validation
Implement — Write endpoints, repositories, services with DI
Optimize — Apply async patterns, caching, performance tuning
Test — Write xUnit tests with TestServer; verify 80%+ coverage

EF Core checkpoint (after step 3): Run dotnet ef migrations add and review the generated migration file before applying. Confirm no unintended table/column drops. Roll back with dotnet ef migrations remove if needed.

Reference Guide

Load detailed guidance based on context:

Constraints

MUST DO

Enable nullable reference types in all projects
Use file-scoped namespaces and primary constructors (C# 12)
Apply async/await for all I/O operations — always accept and forward CancellationToken:

// Correct
  app.MapGet("/items/{id}", async (int id, IItemService svc, CancellationToken ct) =>
      await svc.GetByIdAsync(id, ct) is { } item ? Results.Ok(item) : Results.NotFound());

Use dependency injection for all services
Include XML documentation for public APIs
Implement proper error handling with Result pattern:

public readonly record struct Result<T>(T? Value, string? Error, bool IsSuccess)
  {
      public static Result<T> Ok(T value) => new(value, null, true);
      public static Result<T> Fail(string error) => new(default, error, false);
  }

Use strongly-typed configuration with IOptions

MUST NOT DO

Use blocking calls (.Result, .Wait()) in async code:

// Wrong — blocks thread and risks deadlock
  var data = service.GetDataAsync().Result;

  // Correct
  var data = await service.GetDataAsync(ct);

Disable nullable warnings without proper justification
Skip cancellation token support in async methods
Expose EF Core entities directly in API responses — always map to DTOs
Use string-based configuration keys
Skip input validation
Ignore code analysis warnings

Output Templates

When implementing .NET features, provide:

Domain models and DTOs
API endpoints (Minimal API or controllers)
Repository/service implementations
Configuration setup (Program.cs, appsettings.json)
Brief explanation of architectural decisions

Example: Minimal API Endpoint

// Program.cs (file-scoped, .NET 8 minimal API)
var builder = WebApplication.CreateBuilder(args);
builder.Services.AddScoped<IProductService, ProductService>();

var app = builder.Build();

app.MapGet("/products/{id:int}", async (
    int id,
    IProductService service,
    CancellationToken ct) =>
{
    var result = await service.GetByIdAsync(id, ct);
    return result.IsSuccess ? Results.Ok(result.Value) : Results.NotFound(result.Error);
})
.WithName("GetProduct")
.Produces<ProductDto>()
.ProducesProblem(404);

app.Run();

Knowledge Reference

C# 12, .NET 8, ASP.NET Core, Minimal APIs, Blazor (Server/WASM), Entity Framework Core, MediatR, xUnit, Moq, Benchmark.NET, SignalR, gRPC, Azure SDK, Polly, FluentValidation, Serilog

Documentation

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