Dotnet Development

.NET Development Skill

Execute this process for any .NET implementation task:

1. Analysis Phase (Required)

Before writing code, perform these steps:

1.1. Identify domain concepts: List all aggregates, entities, and value objects involved in this change 1.2. Determine affected layer: Specify whether changes target Domain, Application, or Infrastructure 1.3. Map SOLID principles: Document which principles apply and how they guide the design 1.4. Assess security requirements: Identify authorization rules and data protection needs

2. Architecture Review (Required)

Verify the approach against these criteria:

2.1. Check aggregate boundaries: Confirm they preserve transactional consistency 2.2. Apply Single Responsibility: Ensure each class has exactly one reason to change 2.3. Enforce Dependency Inversion: Verify dependencies point inward (Infrastructure → Application → Domain) 2.4. Validate domain encapsulation: Confirm business logic resides in domain objects, not services

3. Implementation

Execute with these standards:

3.1. Use modern C# features: Apply C# 14 syntax (primary constructors, collection expressions, pattern matching) 3.2. Implement async correctly: Use async/await for all I/O operations, propagate CancellationToken 3.3. Apply constructor injection: Inject all dependencies via primary constructors 3.4. Validate at boundaries: Check inputs at application layer entry points, trust internal calls 3.5. Encapsulate business rules: Place all domain logic in aggregate methods, not services

4. Testing (Required)

Write tests following these guidelines:

4.1. Apply naming convention: Use MethodName_Condition_ExpectedResult pattern 4.2. Structure with AAA: Organize tests into Arrange, Act, Assert sections 4.3. Test domain invariants: Cover all business rules with unit tests 4.4. Verify events: Assert that correct domain events are raised

[Fact]
public void CalculateTotal_WithDiscount_ReturnsReducedAmount()
{
    // Arrange
    var order = new Order();
    order.ApplyDiscount(0.1m);

    // Act
    var total = order.CalculateTotal();

    // Assert
    Assert.Equal(90m, total);
}

Core Principles

Domain-Driven Design

SOLID Principles

• SRP: One reason to change per class
• OCP: Open for extension, closed for modification
• LSP: Subtypes substitutable for base types
• ISP: No forced dependency on unused methods
• DIP: Depend on abstractions

C# Conventions

Naming:

• PascalCase: Types, methods, public members, properties
• camelCase: Private fields, local variables
• Prefix interfaces with I (e.g., IUserService)

Formatting:

• File-scoped namespaces
• Newline before opening braces
• Pattern matching and switch expressions preferred
• Use nameof over string literals

Nullability:

• Enable nullable reference types
• Use is null / is not null (not == null)
• Validate at entry points, trust annotations internally

Layer Responsibilities

Examples ordered by complexity (Easy → Medium → Hard):

// Aggregate root with encapsulated business logic
public class Order : AggregateRoot
{
    private readonly List<OrderLine> _lines = [];

    public IReadOnlyCollection<OrderLine> Lines => _lines.AsReadOnly();

    public void AddLine(Product product, int quantity)
    {
        if (quantity <= 0)
            throw new DomainException("Quantity must be positive");

        _lines.Add(new OrderLine(product, quantity));
        AddDomainEvent(new OrderLineAddedEvent(Id, product.Id, quantity));
    }
}

// Repository implementation with EF Core
public class OrderRepository(AppDbContext db) : IOrderRepository
{
    public async Task<Order?> GetByIdAsync(Guid id, CancellationToken ct) =>
        await db.Orders
            .Include(o => o.Lines)
            .FirstOrDefaultAsync(o => o.Id == id, ct);

    public async Task SaveAsync(Order order, CancellationToken ct)
    {
        db.Orders.Update(order);
        await db.SaveChangesAsync(ct);
    }
}

// Application service orchestrates domain operations
public class OrderService(
    IOrderRepository orders,
    IProductRepository products,
    IEventPublisher events)
{
    public async Task<OrderDto> AddLineAsync(
        Guid orderId,
        AddLineCommand command,
        CancellationToken ct = default)
    {
        // Validate input at boundary
        ArgumentNullException.ThrowIfNull(command);

        var order = await orders.GetByIdAsync(orderId, ct)
            ?? throw new NotFoundException($"Order {orderId} not found");

        var product = await products.GetByIdAsync(command.ProductId, ct)
            ?? throw new NotFoundException($"Product {command.ProductId} not found");

        // Execute domain logic (business rules in aggregate)
        order.AddLine(product, command.Quantity);

        // Persist and publish events
        await orders.SaveAsync(order, ct);
        await events.PublishAsync(order.DomainEvents, ct);

        return order.ToDto();
    }
}

REST API Patterns

var orders = app.MapGroup("/api/orders")
    .WithTags("Orders")
    .RequireAuthorization();

orders.MapPost("/{orderId:guid}/lines", async (
    Guid orderId,
    AddLineCommand command,
    OrderService service,
    CancellationToken ct) =>
{
    var result = await service.AddLineAsync(orderId, command, ct);
    return Results.Ok(result);
})
.WithName("AddOrderLine")
.Produces<OrderDto>()
.ProducesProblem(StatusCodes.Status404NotFound);

[ApiController]
[Route("api/[controller]")]
public class OrdersController(OrderService orderService) : ControllerBase
{
    /// <summary>
    /// Adds a line item to an existing order.
    /// </summary>
    [HttpPost("{orderId:guid}/lines")]
    [ProducesResponseType<OrderDto>(StatusCodes.Status200OK)]
    [ProducesResponseType<ProblemDetails>(StatusCodes.Status404NotFound)]
    public async Task<IActionResult> AddLine(
        Guid orderId,
        AddLineCommand command,
        CancellationToken ct)
    {
        var result = await orderService.AddLineAsync(orderId, command, ct);
        return Ok(result);
    }
}

Complexity Limits

• Maximum 50 lines per method (excluding blank lines and comments)
• Cyclomatic complexity <10 per method
• Maximum 7 dependencies per class (constructor parameters)
• Aggregate size <1MB serialized
• Collection properties limited to 1000 items maximum

Code Quality Standards

• All public APIs must have XML documentation
• No compiler warnings in production code
• Nullable reference types enabled and enforced
• No magic strings - use constants or nameof()

Data Protection

• Never log sensitive data (passwords, tokens, PII)
• Use parameterized queries exclusively (EF Core handles this)
• Implement proper authorization checks before data access
• Hash passwords using BCrypt or Argon2, never store plaintext

API Security

• Require authentication on all endpoints except explicitly public ones
• Implement rate limiting on public endpoints
• Validate JWT tokens with proper issuer and audience checks
• Use HTTPS exclusively in production

Architectural Compliance

• Domain layer has zero infrastructure dependencies
• All aggregate modifications go through aggregate root methods
• No business logic in controllers or infrastructure layer
• All async operations use CancellationToken

Review Checklist

• SOLID principles applied correctly
• Aggregate boundaries maintain consistency
• Domain events capture all significant state changes
• Error handling follows ProblemDetails (RFC 7807)
• Tests follow MethodName_Condition_ExpectedResult naming

Reference Documentation

For detailed patterns and checklists, see:

• DDD Patterns: Aggregate design, domain events, specifications
• API Patterns: Validation, error handling, versioning, documentation
• Testing Patterns: Test categories, mocking strategies, coverage requirements

Quick Reference

Monetary Values

• Use decimal for all financial calculations
• Implement currency-aware value objects
• Handle rounding per financial standards
• Maintain precision through calculation chains

Error Handling

// Global exception handler middleware
app.UseExceptionHandler(error => error.Run(async context =>
{
    var exception = context.Features.Get<IExceptionHandlerFeature>()?.Error;
    
    var problem = exception switch
    {
        NotFoundException e => new ProblemDetails
        {
            Status = 404,
            Title = "Not Found",
            Detail = e.Message
        },
        DomainException e => new ProblemDetails
        {
            Status = 400,
            Title = "Business Rule Violation",
            Detail = e.Message
        },
        _ => new ProblemDetails
        {
            Status = 500,
            Title = "Internal Server Error"
        }
    };
    
    context.Response.StatusCode = problem.Status ?? 500;
    await context.Response.WriteAsJsonAsync(problem);
}));

Dependency Injection Setup

// Program.cs
builder.Services.AddScoped<IOrderRepository, OrderRepository>();
builder.Services.AddScoped<OrderService>();
builder.Services.AddDbContext<AppDbContext>(options =>
    options.UseSqlServer(builder.Configuration.GetConnectionString("Default")));