skills/ Jeffallan/claude-skills

csharp-developer

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 sca

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SKILL.md

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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

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

EF Core checkpoint (after step 3): Run dotnet ef migrations add <Name> 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:

TopicReferenceLoad When
Modern C#references/modern-csharp.mdRecords, pattern matching, nullable types
ASP.NET Corereferences/aspnet-core.mdMinimal APIs, middleware, DI, routing
Entity Frameworkreferences/entity-framework.mdEF Core, migrations, query optimization
Blazorreferences/blazor.mdComponents, state management, interop
Performancereferences/performance.mdSpan, async, memory optimization, AOT

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<T>

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:

  1. Domain models and DTOs
  2. API endpoints (Minimal API or controllers)
  3. Repository/service implementations
  4. Configuration setup (Program.cs, appsettings.json)
  5. 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

Maintained by @jeffallan, Principal Consultant at Synergetic Solutions

Documentation

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57.9 KB

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