Domain-Driven Design Patterns

Core Principles

• Aggregate roots are consistency boundaries — all changes go through the root
• Value objects are immutable and compared by structural equality
• Domain events communicate side effects across aggregates
• Strongly-typed IDs prevent primitive obsession and accidental ID swaps
• Repository per aggregate root, not per entity
• Rich domain model: behavior lives on entities, not in services

Patterns

Strongly-Typed IDs

public readonly record struct OrderId(Guid Value)
{
    public static OrderId New() => new(Guid.NewGuid());
    public override string ToString() => Value.ToString();
}

public readonly record struct CustomerId(Guid Value)
{
    public static CustomerId New() => new(Guid.NewGuid());
}

// Prevents accidental: GetOrder(customerId) when OrderId expected

Value Objects

public sealed record Money(decimal Amount, string Currency)
{
    public static Money Zero(string currency) => new(0, currency);

    public Money Add(Money other)
    {
        if (Currency != other.Currency)
            throw new DomainException("Cannot add different currencies");
        return this with { Amount = Amount + other.Amount };
    }

    public Money Multiply(int factor) =>
        this with { Amount = Amount * factor };
}

public sealed record Address(
    string Street, string City, string State, string ZipCode, string Country)
{
    // Value objects validate on construction
    public Address
    {
        if (string.IsNullOrWhiteSpace(Street))
            throw new DomainException("Street is required");
        if (string.IsNullOrWhiteSpace(City))
            throw new DomainException("City is required");
    }
}

Aggregate Root Base Class

public abstract class AggregateRoot<TId> where TId : struct
{
    public TId Id { get; protected set; }

    private readonly List<IDomainEvent> _domainEvents = [];
    public IReadOnlyList<IDomainEvent> DomainEvents =>
        _domainEvents.AsReadOnly();

    protected void RaiseDomainEvent(IDomainEvent domainEvent) =>
        _domainEvents.Add(domainEvent);

    public void ClearDomainEvents() => _domainEvents.Clear();
}

public interface IAggregateRoot { }

public interface IDomainEvent : INotification
{
    DateTimeOffset OccurredAt { get; }
}

Rich Aggregate Root

public sealed class Order : AggregateRoot<OrderId>, IAggregateRoot
{
    public CustomerId CustomerId { get; private set; }
    public Money Total { get; private set; } = Money.Zero("USD");
    public OrderStatus Status { get; private set; } = OrderStatus.Draft;

    private readonly List<OrderLine> _lines = [];
    public IReadOnlyList<OrderLine> Lines => _lines.AsReadOnly();

    private Order() { } // EF Core

    // Factory method — the only way to create an Order
    public static Order Create(CustomerId customerId)
    {
        var order = new Order
        {
            Id = OrderId.New(),
            CustomerId = customerId
        };
        order.RaiseDomainEvent(new OrderCreatedEvent(order.Id));
        return order;
    }

    public void AddLine(ProductId productId, int quantity, Money unitPrice)
    {
        Guard.Against.NegativeOrZero(quantity);

        if (Status != OrderStatus.Draft)
            throw new DomainException("Can only add lines to draft orders");

        var line = new OrderLine(productId, quantity, unitPrice);
        _lines.Add(line);
        RecalculateTotal();
    }

    public void RemoveLine(ProductId productId)
    {
        var line = _lines.FirstOrDefault(l => l.ProductId == productId)
            ?? throw new DomainException("Line not found");
        _lines.Remove(line);
        RecalculateTotal();
    }

    public void Submit()
    {
        if (Status != OrderStatus.Draft)
            throw new DomainException("Only draft orders can be submitted");
        if (_lines.Count == 0)
            throw new DomainException("Cannot submit empty order");

        Status = OrderStatus.Submitted;
        RaiseDomainEvent(new OrderSubmittedEvent(Id, Total));
    }

    public void Cancel(string reason)
    {
        if (Status == OrderStatus.Shipped)
            throw new DomainException("Cannot cancel shipped orders");

        Status = OrderStatus.Cancelled;
        RaiseDomainEvent(new OrderCancelledEvent(Id, reason));
    }

    private void RecalculateTotal()
    {
        Total = _lines.Aggregate(
            Money.Zero("USD"),
            (sum, line) => sum.Add(line.LineTotal));
    }
}

Entity (Non-Root)

public sealed class OrderLine
{
    public Guid Id { get; private set; } = Guid.NewGuid();
    public ProductId ProductId { get; private set; }
    public int Quantity { get; private set; }
    public Money UnitPrice { get; private set; }
    public Money LineTotal => UnitPrice.Multiply(Quantity);

    private OrderLine() { } // EF Core

    internal OrderLine(ProductId productId, int quantity, Money unitPrice)
    {
        ProductId = productId;
        Quantity = quantity;
        UnitPrice = unitPrice;
    }
}

Domain Events

public abstract record DomainEvent : IDomainEvent
{
    public DateTimeOffset OccurredAt { get; } = DateTimeOffset.UtcNow;
}

public sealed record OrderCreatedEvent(OrderId OrderId) : DomainEvent;

public sealed record OrderSubmittedEvent(
    OrderId OrderId, Money Total) : DomainEvent;

public sealed record OrderCancelledEvent(
    OrderId OrderId, string Reason) : DomainEvent;

Domain Exception

public sealed class DomainException : Exception
{
    public DomainException(string message) : base(message) { }
    public DomainException(string message, Exception inner) : base(message, inner) { }
}

Bounded Context Communication

// Each bounded context has its own models
// OrderManagement context
namespace {Company}.{Domain}.OrderManagement.Domain;
public sealed class Order : AggregateRoot<OrderId> { /* ... */ }

// Shipping context — different model for the same concept
namespace {Company}.{Domain}.Shipping.Domain;
public sealed class Shipment : AggregateRoot<ShipmentId>
{
    public Guid OrderReference { get; private set; } // not OrderId
}

// Anti-corruption layer translates between contexts
public sealed class ShippingAntiCorruptionLayer(IOrderService orderService)
{
    public async Task<ShipmentRequest> TranslateOrderAsync(
        Guid orderId, CancellationToken ct)
    {
        var order = await orderService.GetOrderSummaryAsync(orderId, ct);
        return new ShipmentRequest(
            OrderReference: order.Id,
            Address: MapToShippingAddress(order.ShippingAddress));
    }
}

Anti-Patterns

• Anemic domain model: entities with only getters/setters, no behavior
• Public setters on aggregate properties (allows bypassing invariants)
• Business logic in application services instead of domain entities
• Repository per entity instead of per aggregate root
• Primitive obsession: using Guid everywhere instead of typed IDs
• Exposing internal collections as List<T> instead of IReadOnlyList<T>

Detect Existing Patterns

1. Look for AggregateRoot, Entity, ValueObject base classes
2. Check for strongly-typed IDs (record structs wrapping Guid)
3. Look for IDomainEvent or DomainEvent types
4. Check for rich entities with behavior methods (not just properties)
5. Look for private setters and factory methods (Create, Register)
6. Check for Guard.Against or similar invariant enforcement

Adding to Existing Project

1. Introduce base classes — AggregateRoot<TId>, IDomainEvent
2. Add typed IDs to key entities — start with the most-used aggregate
3. Move behavior into entities — encapsulate state transitions
4. Add private setters and factory methods to enforce invariants
5. Define bounded context boundaries before splitting further
6. Add domain events for cross-aggregate side effects

Decision Guide

Concept	When to Use
Value Object	Immutable, identity-less (Money, Address, DateRange)
Entity	Has identity, mutable within aggregate boundary
Aggregate Root	Consistency boundary, entry point for changes
Domain Event	Side effect that other aggregates need to know about
Strongly-typed ID	Every aggregate root and key entity ID
Factory Method	When creation involves invariants or events

References

• https://learn.microsoft.com/en-us/dotnet/architecture/microservices/microservice-ddd-cqrs-patterns/
• https://www.domainlanguage.com/ddd/reference/