csharp-developer

SKILL.md

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 <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:

Topic
Reference
Load When

Modern C#
references/modern-csharp.md
Records, pattern matching, nullable types

ASP.NET Core
references/aspnet-core.md
Minimal APIs, middleware, DI, routing

Entity Framework
references/entity-framework.md
EF Core, migrations, query optimization

Blazor
references/blazor.md
Components, state management, interop

Performance
references/performance.md
Span, 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:

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