dotnet-patterns

.NET Development Patterns

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

When to Use

• 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>();