dotnet-patterns

SKILL.md

.NET Development Patterns

Idiomatic C# and .NET patterns for building robust, performant, and maintainable applications.

When to Activate

Writing new C# code

Reviewing C# code

Refactoring existing .NET applications

Designing service architectures with ASP.NET Core

Core Principles

1. Prefer Immutability

Use records and init-only properties for data models. Mutability should be an explicit, justified choice.
// Good: Immutable value object
public sealed record Money(decimal Amount, string Currency);

// Good: Immutable DTO with init setters
public sealed class CreateOrderRequest
{
    public required string CustomerId { get; init; }
    public required IReadOnlyList<OrderItem> Items { get; init; }
}

// Bad: Mutable model with public setters
public class Order
{
    public string CustomerId { get; set; }
    public List<OrderItem> Items { get; set; }
}

2. Explicit Over Implicit

Be clear about nullability, access modifiers, and intent.

// Good: Explicit access modifiers and nullability
public sealed class UserService
{
    private readonly IUserRepository _repository;
    private readonly ILogger<UserService> _logger;

    public UserService(IUserRepository repository, ILogger<UserService> logger)
    {
        _repository = repository ?? throw new ArgumentNullException(nameof(repository));
        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
    }

    public async Task<User?> FindByIdAsync(Guid id, CancellationToken cancellationToken)
    {
        return await _repository.FindByIdAsync(id, cancellationToken);
    }
}

3. Depend on Abstractions

Use interfaces for service boundaries. Register via DI container.

// Good: Interface-based dependency
public interface IOrderRepository
{
    Task<Order?> FindByIdAsync(Guid id, CancellationToken cancellationToken);
    Task<IReadOnlyList<Order>> FindByCustomerAsync(string customerId, CancellationToken cancellationToken);
    Task AddAsync(Order order, CancellationToken cancellationToken);
}

// Registration
builder.Services.AddScoped<IOrderRepository, SqlOrderRepository>();

Async/Await Patterns

Proper Async Usage

// Good: Async all the way, with CancellationToken
public async Task<OrderSummary> GetOrderSummaryAsync(
    Guid orderId,
    CancellationToken cancellationToken)
{
    var order = await _repository.FindByIdAsync(orderId, cancellationToken)
        ?? throw new NotFoundException($"Order {orderId} not found");

    var customer = await _customerService.GetAsync(order.CustomerId, cancellationToken);

    return new OrderSummary(order, customer);
}

// Bad: Blocking on async
public OrderSummary GetOrderSummary(Guid orderId)
{
    var order = _repository.FindByIdAsync(orderId, CancellationToken.None).Result; // Deadlock risk
    return new OrderSummary(order);
}

Parallel Async Operations

// Good: Concurrent independent operations
public async Task<DashboardData> LoadDashboardAsync(CancellationToken cancellationToken)
{
    var ordersTask = _orderService.GetRecentAsync(cancellationToken);
    var metricsTask = _metricsService.GetCurrentAsync(cancellationToken);
    var alertsTask = _alertService.GetActiveAsync(cancellationToken);

    await Task.WhenAll(ordersTask, metricsTask, alertsTask);

    return new DashboardData(
        Orders: await ordersTask,
        Metrics: await metricsTask,
        Alerts: await alertsTask);
}

Options Pattern

Bind configuration sections to strongly-typed objects.

public sealed class SmtpOptions
{
    public const string SectionName = "Smtp";

    public required string Host { get; init; }
    public required int Port { get; init; }
    public required string Username { get; init; }
    public bool UseSsl { get; init; } = true;
}

// Registration
builder.Services.Configure<SmtpOptions>(
    builder.Configuration.GetSection(SmtpOptions.SectionName));

// Usage via injection
public class EmailService(IOptions<SmtpOptions> options)
{
    private readonly SmtpOptions _smtp = options.Value;
}

Result Pattern

Return explicit success/failure instead of throwing for expected failures.

public sealed record Result<T>
{
    public bool IsSuccess { get; }
    public T? Value { get; }
    public string? Error { get; }

    private Result(T value) { IsSuccess = true; Value = value; }
    private Result(string error) { IsSuccess = false; Error = error; }

    public static Result<T> Success(T value) => new(value);
    public static Result<T> Failure(string error) => new(error);
}

// Usage
public async Task<Result<Order>> PlaceOrderAsync(CreateOrderRequest request)
{
    if (request.Items.Count == 0)
        return Result<Order>.Failure("Order must contain at least one item");

    var order = Order.Create(request);
    await _repository.AddAsync(order, CancellationToken.None);
    return Result<Order>.Success(order);
}

Repository Pattern with EF Core

public sealed class SqlOrderRepository : IOrderRepository
{
    private readonly AppDbContext _db;

    public SqlOrderRepository(AppDbContext db) => _db = db;

    public async Task<Order?> FindByIdAsync(Guid id, CancellationToken cancellationToken)
    {
        return await _db.Orders
            .Include(o => o.Items)
            .AsNoTracking()
            .FirstOrDefaultAsync(o => o.Id == id, cancellationToken);
    }

    public async Task<IReadOnlyList<Order>> FindByCustomerAsync(
        string customerId,
        CancellationToken cancellationToken)
    {
        return await _db.Orders
            .Where(o => o.CustomerId == customerId)
            .OrderByDescending(o => o.CreatedAt)
            .AsNoTracking()
            .ToListAsync(cancellationToken);
    }

    public async Task AddAsync(Order order, CancellationToken cancellationToken)
    {
        _db.Orders.Add(order);
        await _db.SaveChangesAsync(cancellationToken);
    }
}

Middleware and Pipeline

// Custom middleware
public sealed class RequestTimingMiddleware
{
    private readonly RequestDelegate _next;
    private readonly ILogger<RequestTimingMiddleware> _logger;

    public RequestTimingMiddleware(RequestDelegate next, ILogger<RequestTimingMiddleware> logger)
    {
        _next = next;
        _logger = logger;
    }

    public async Task InvokeAsync(HttpContext context)
    {
        var stopwatch = Stopwatch.StartNew();
        try
        {
            await _next(context);
        }
        finally
        {
            stopwatch.Stop();
            _logger.LogInformation(
                "Request {Method} {Path} completed in {ElapsedMs}ms with status {StatusCode}",
                context.Request.Method,
                context.Request.Path,
                stopwatch.ElapsedMilliseconds,
                context.Response.StatusCode);
        }
    }
}

Minimal API Patterns

// Organized with route groups
var orders = app.MapGroup("/api/orders")
    .RequireAuthorization()
    .WithTags("Orders");

orders.MapGet("/{id:guid}", async (
    Guid id,
    IOrderRepository repository,
    CancellationToken cancellationToken) =>
{
    var order = await repository.FindByIdAsync(id, cancellationToken);
    return order is not null
        ? TypedResults.Ok(order)
        : TypedResults.NotFound();
});

orders.MapPost("/", async (
    CreateOrderRequest request,
    IOrderService service,
    CancellationToken cancellationToken) =>
{
    var result = await service.PlaceOrderAsync(request, cancellationToken);
    return result.IsSuccess
        ? TypedResults.Created($"/api/orders/{result.Value!.Id}", result.Value)
        : TypedResults.BadRequest(result.Error);
});

Guard Clauses

// Good: Early returns with clear validation
public async Task<ProcessResult> ProcessPaymentAsync(
    PaymentRequest request,
    CancellationToken cancellationToken)
{
    ArgumentNullException.ThrowIfNull(request);

    if (request.Amount <= 0)
        throw new ArgumentOutOfRangeException(nameof(request.Amount), "Amount must be positive");

    if (string.IsNullOrWhiteSpace(request.Currency))
        throw new ArgumentException("Currency is required", nameof(request.Currency));

    // Happy path continues here without nesting
    var gateway = _gatewayFactory.Create(request.Currency);
    return await gateway.ChargeAsync(request, cancellationToken);
}

Anti-Patterns to Avoid

Anti-Pattern
Fix

async void methods
Return Task (except event handlers)

.Result or .Wait()
Use await

catch (Exception) { }
Handle or rethrow with context

new Service() in constructors
Use constructor injection

public fields
Use properties with appropriate accessors

dynamic in business logic
Use generics or explicit types

Mutable static state
Use DI scoping or ConcurrentDictionary

string.Format in loops
Use StringBuilder or interpolated string handlers