Trellis.DomainDrivenDesign 3.0.0-alpha.99

Domain Driven Design

Building blocks for implementing Domain-Driven Design tactical patterns in C# with functional programming principles.

Installation

dotnet add package Trellis.DomainDrivenDesign

Quick Start

Entity

Objects with unique identity. Equality based on ID.

public partial class CustomerId : RequiredGuid<CustomerId> { }

public class Customer : Entity<CustomerId>
{
    public string Name { get; private set; }
    
    private Customer(CustomerId id, string name) : base(id)
    {
        Name = name;
    }
    
    public static Result<Customer> TryCreate(string name) =>
        name.ToResult()
            .Ensure(n => !string.IsNullOrWhiteSpace(n), Error.Validation("Name required"))
            .Map(n => new Customer(CustomerId.NewUniqueV7(), n));
}

Value Object

Immutable objects with no identity. Equality based on all properties.

public class Money : ValueObject
{
    public decimal Amount { get; }
    public string Currency { get; }
    
    private Money(decimal amount, string currency)
    {
        Amount = amount;
        Currency = currency;
    }
    
    public static Result<Money> TryCreate(decimal amount, string currency = "USD") =>
        (amount, currency).ToResult()
            .Ensure(x => x.amount >= 0, Error.Validation("Amount cannot be negative"))
            .Map(x => new Money(x.amount, x.currency));
    
    protected override IEnumerable<IComparable> GetEqualityComponents()
    {
        yield return Amount;
        yield return Currency;
    }
}

Aggregate

Cluster of entities and value objects treated as a unit. Manages domain events.

public record OrderCreated(OrderId Id, CustomerId CustomerId, DateTime OccurredAt) : IDomainEvent;

public class Order : Aggregate<OrderId>
{
    private readonly List<OrderLine> _lines = [];
    
    public CustomerId CustomerId { get; }
    public OrderStatus Status { get; private set; }
    
    private Order(OrderId id, CustomerId customerId) : base(id)
    {
        CustomerId = customerId;
        Status = OrderStatus.Draft;
        DomainEvents.Add(new OrderCreated(id, customerId, DateTime.UtcNow));
    }
    
    public static Result<Order> TryCreate(CustomerId customerId) =>
        new Order(OrderId.NewUniqueV7(), customerId).ToResult();
    
    public Result<Order> AddLine(ProductId productId, string name, Money price, int qty) =>
        this.ToResult()
            .Ensure(_ => Status == OrderStatus.Draft, Error.Validation("Order not editable"))
            .Ensure(_ => qty > 0, Error.Validation("Quantity must be positive"))
            .Tap(_ => _lines.Add(new OrderLine(productId, name, price, qty)));
}

Specification

Encapsulate business rules as composable, storage-agnostic expression trees.

public class HighValueOrderSpec(decimal threshold) : Specification<Order>
{
    public override Expression<Func<Order, bool>> ToExpression() =>
        order => order.TotalAmount > threshold;
}

// Compose specifications
var spec = new OverdueOrderSpec(now).And(new HighValueOrderSpec(500m));

// Use with EF Core
var orders = await dbContext.Orders.Where(spec).ToListAsync();

// In-memory evaluation
if (spec.IsSatisfiedBy(order))
    // order matches

Domain Events

Publish events after persisting:

if (order.IsSuccess)
{
    await repository.SaveAsync(order.Value);
    
    foreach (var evt in order.Value.UncommittedEvents())
        await eventBus.PublishAsync(evt);
    
    order.Value.AcceptChanges();
}

Core Types

Best Practices

1. Use entities when identity matters — Customer : Entity<CustomerId>
2. Keep aggregates small — Include only what's needed for invariants
3. Reference other aggregates by ID — public CustomerId CustomerId { get; } not public Customer Customer { get; }
4. Use Maybe<T> for optional properties — Maybe<Url> Website instead of Url?
5. Enforce invariants in aggregate root — Use Result<T> for validation
6. Use domain events for side effects — Not direct service calls
7. Make value objects immutable — No setters

Related Packages

• Trellis.Results — Core Result<T> type
• Trellis.Primitives — RequiredString, RequiredGuid, EmailAddress, and more
• Trellis.Asp — ASP.NET Core integration

License

MIT — see LICENSE for details.