@jeffallan/csharp-developer

---
name: csharp-developer
description: "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."
license: MIT
metadata:
  author: https://github.com/Jeffallan
  version: "1.1.0"
  domain: language
  triggers: C#, .NET, ASP.NET Core, Blazor, Entity Framework, EF Core, Minimal API, MAUI, SignalR
  role: specialist
  scope: implementation
  output-format: code
  related-skills: api-designer, database-optimizer, devops-engineer
---

# 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<T>, Memory<T>
- 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:

| 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<T>, 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`:
  ```csharp
  // 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:
  ```csharp
  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:
  ```csharp
  // 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

```csharp
// 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