dotnet-patterns

name dotnet-patterns description Idiomatic C# and .NET patterns, conventions, dependency injection, async/await, and best practices for building robust, maintainable .NET applications. Use when writing or reviewing C# / .NET code — DI, async, or general conventions. metadata {"origin":"ECC"} .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