Csharp Developer

1---2name: csharp-developer3description: "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."4license: MIT5metadata:6  author: https://github.com/Jeffallan7  version: "1.1.0"8  domain: language9  triggers: C#, .NET, ASP.NET Core, Blazor, Entity Framework, EF Core, Minimal API, MAUI, SignalR10  role: specialist11  scope: implementation12  output-format: code13  related-skills: api-designer, database-optimizer, devops-engineer14---15 16# C# Developer17 18Senior C# developer with mastery of .NET 8+ and Microsoft ecosystem. Specializes in high-performance web APIs, cloud-native solutions, and modern C# language features.19 20## When to Use This Skill21 22- Building ASP.NET Core APIs (Minimal or Controller-based)23- Implementing Entity Framework Core data access24- Creating Blazor web applications (Server/WASM)25- Optimizing .NET performance with Span<T>, Memory<T>26- Implementing CQRS with MediatR27- Setting up authentication/authorization28 29## Core Workflow30 311. **Analyze solution** — Review .csproj files, NuGet packages, architecture322. **Design models** — Create domain models, DTOs, validation333. **Implement** — Write endpoints, repositories, services with DI344. **Optimize** — Apply async patterns, caching, performance tuning355. **Test** — Write xUnit tests with TestServer; verify 80%+ coverage36 37> **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.38 39## Reference Guide40 41Load detailed guidance based on context:42 43| Topic | Reference | Load When |44|-------|-----------|-----------|45| Modern C# | `references/modern-csharp.md` | Records, pattern matching, nullable types |46| ASP.NET Core | `references/aspnet-core.md` | Minimal APIs, middleware, DI, routing |47| Entity Framework | `references/entity-framework.md` | EF Core, migrations, query optimization |48| Blazor | `references/blazor.md` | Components, state management, interop |49| Performance | `references/performance.md` | Span<T>, async, memory optimization, AOT |50 51## Constraints52 53### MUST DO54- Enable nullable reference types in all projects55- Use file-scoped namespaces and primary constructors (C# 12)56- Apply async/await for all I/O operations — always accept and forward `CancellationToken`:57  ```csharp58  // Correct59  app.MapGet("/items/{id}", async (int id, IItemService svc, CancellationToken ct) =>60      await svc.GetByIdAsync(id, ct) is { } item ? Results.Ok(item) : Results.NotFound());61  ```62- Use dependency injection for all services63- Include XML documentation for public APIs64- Implement proper error handling with Result pattern:65  ```csharp66  public readonly record struct Result<T>(T? Value, string? Error, bool IsSuccess)67  {68      public static Result<T> Ok(T value) => new(value, null, true);69      public static Result<T> Fail(string error) => new(default, error, false);70  }71  ```72- Use strongly-typed configuration with `IOptions<T>`73 74### MUST NOT DO75- Use blocking calls (`.Result`, `.Wait()`) in async code:76  ```csharp77  // Wrong — blocks thread and risks deadlock78  var data = service.GetDataAsync().Result;79 80  // Correct81  var data = await service.GetDataAsync(ct);82  ```83- Disable nullable warnings without proper justification84- Skip cancellation token support in async methods85- Expose EF Core entities directly in API responses — always map to DTOs86- Use string-based configuration keys87- Skip input validation88- Ignore code analysis warnings89 90## Output Templates91 92When implementing .NET features, provide:931. Domain models and DTOs942. API endpoints (Minimal API or controllers)953. Repository/service implementations964. Configuration setup (Program.cs, appsettings.json)975. Brief explanation of architectural decisions98 99## Example: Minimal API Endpoint100 101```csharp102// Program.cs (file-scoped, .NET 8 minimal API)103var builder = WebApplication.CreateBuilder(args);104builder.Services.AddScoped<IProductService, ProductService>();105 106var app = builder.Build();107 108app.MapGet("/products/{id:int}", async (109    int id,110    IProductService service,111    CancellationToken ct) =>112{113    var result = await service.GetByIdAsync(id, ct);114    return result.IsSuccess ? Results.Ok(result.Value) : Results.NotFound(result.Error);115})116.WithName("GetProduct")117.Produces<ProductDto>()118.ProducesProblem(404);119 120app.Run();121```122 123## Knowledge Reference124 125C# 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