Domain-Driven Design (DDD)

Core Principles

1. Aggregates define consistency boundaries — An aggregate is a cluster of entities and value objects treated as a single unit for data changes. All invariants within an aggregate are enforced in a single transaction. Cross-aggregate consistency is eventual.
2. Value objects over primitives — Replace primitive obsession with value objects. Money, EmailAddress, OrderNumber are not strings — they carry validation, equality, and behavior. Use C# records for immutable value objects.
3. Domain events decouple side effects — When something meaningful happens in the domain (OrderPlaced, PaymentReceived), raise a domain event. Side effects (send email, update read model, notify another aggregate) subscribe to these events. The aggregate stays focused on its own rules.
4. Aggregate root is the sole entry point — External code accesses an aggregate only through its root entity. Child entities are never loaded or modified independently. The root enforces all invariants for the entire aggregate.
5. Repositories persist aggregates, not entities — One repository per aggregate root. The repository loads and saves the entire aggregate as a unit. No repository for child entities. The Infrastructure implementation uses DbContext internally — this is a DDD tactical pattern for aggregate boundaries, not a generic CRUD wrapper.

Patterns

Aggregate Root

The aggregate root owns all access to its children and enforces invariants:

// Domain/Orders/Order.cs
public sealed class Order : AggregateRoot
{
    private readonly List<OrderLine> _lines = [];

    private Order() { } // EF Core

    public OrderNumber Number { get; private set; } = null!;
    public CustomerId CustomerId { get; private set; }
    public Money Total { get; private set; } = Money.Zero("USD");
    public OrderStatus Status { get; private set; }
    public DateTimeOffset PlacedAt { get; private set; }
    public IReadOnlyList<OrderLine> Lines => _lines.AsReadOnly();

    public static Order Place(CustomerId customerId, OrderNumber number, DateTimeOffset now)
    {
        var order = new Order
        {
            Id = Guid.CreateVersion7(),
            CustomerId = customerId,
            Number = number,
            Status = OrderStatus.Placed,
            PlacedAt = now
        };

        order.RaiseDomainEvent(new OrderPlaced(order.Id, customerId, now));
        return order;
    }

    public Result AddLine(ProductId productId, int quantity, Money unitPrice)
    {
        if (Status is not OrderStatus.Placed)
            return Result.Failure("Cannot modify a confirmed or cancelled order");

        if (quantity <= 0)
            return Result.Failure("Quantity must be positive");

        var existing = _lines.FirstOrDefault(l => l.ProductId == productId);
        if (existing is not null)
        {
            existing.IncreaseQuantity(quantity);
        }
        else
        {
            _lines.Add(new OrderLine(productId, quantity, unitPrice));
        }

        RecalculateTotal();
        return Result.Success();
    }

    public Result Confirm()
    {
        if (Status is not OrderStatus.Placed)
            return Result.Failure("Only placed orders can be confirmed");

        if (_lines.Count == 0)
            return Result.Failure("Cannot confirm an order with no lines");

        Status = OrderStatus.Confirmed;
        RaiseDomainEvent(new OrderConfirmed(Id));
        return Result.Success();
    }

    private void RecalculateTotal()
    {
        Total = _lines.Aggregate(Money.Zero(Total.Currency), (sum, line) => sum + line.Subtotal);
    }
}

Value Objects as Records

Use C# records for immutable value objects with structural equality:

// Domain/Common/Money.cs
public sealed record Money
{
    public decimal Amount { get; }
    public string Currency { get; }

    public Money(decimal amount, string currency)
    {
        ArgumentOutOfRangeException.ThrowIfNegative(amount);
        ArgumentException.ThrowIfNullOrWhiteSpace(currency);

        Amount = amount;
        Currency = currency.ToUpperInvariant();
    }

    public static Money Zero(string currency) => new(0, currency);

    public static Money operator +(Money left, Money right)
    {
        if (left.Currency != right.Currency)
            throw new InvalidOperationException($"Cannot add {left.Currency} and {right.Currency}");
        return new Money(left.Amount + right.Amount, left.Currency);
    }
}

// Other value objects (EmailAddress, OrderNumber, etc.) follow the same pattern:
// sealed record, constructor validation, no public setters

Strongly-Typed IDs with EF Core Converters

Prevent mixing up GUIDs from different entities:

// Domain/Common/StronglyTypedId.cs
public readonly record struct CustomerId(Guid Value)
{
    public static CustomerId New() => new(Guid.CreateVersion7());
    public override string ToString() => Value.ToString();
}

public readonly record struct ProductId(Guid Value)
{
    public static ProductId New() => new(Guid.CreateVersion7());
}

public readonly record struct OrderNumber(string Value)
{
    public override string ToString() => Value;
}

// Infrastructure/Persistence/Configurations/OrderConfiguration.cs
public class OrderConfiguration : IEntityTypeConfiguration<Order>
{
    public void Configure(EntityTypeBuilder<Order> builder)
    {
        builder.HasKey(o => o.Id);

        builder.Property(o => o.CustomerId)
            .HasConversion(id => id.Value, value => new CustomerId(value));

        builder.Property(o => o.Number)
            .HasConversion(n => n.Value, value => new OrderNumber(value))
            .HasMaxLength(50);

        builder.ComplexProperty(o => o.Total, money =>
        {
            money.Property(m => m.Amount).HasColumnName("Total").HasPrecision(18, 2);
            money.Property(m => m.Currency).HasColumnName("Currency").HasMaxLength(3);
        });

        builder.HasMany(o => o.Lines).WithOne().HasForeignKey("OrderId");
        builder.Navigation(o => o.Lines).AutoInclude();
    }
}

Domain Event Dispatching

Raise events in the aggregate, dispatch in SaveChangesAsync:

// Domain/Common/AggregateRoot.cs
public abstract class AggregateRoot : Entity
{
    private readonly List<IDomainEvent> _domainEvents = [];

    public IReadOnlyList<IDomainEvent> DomainEvents => _domainEvents.AsReadOnly();

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

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

// INotification comes from the MIT-licensed Mediator package (not MediatR —
// see the packages rule). Use a plain marker interface if you don't use a mediator.
public interface IDomainEvent : INotification
{
    DateTimeOffset OccurredAt { get; }
}

// Domain/Orders/Events/OrderPlaced.cs
public sealed record OrderPlaced(Guid OrderId, CustomerId CustomerId, DateTimeOffset PlacedAt) : IDomainEvent
{
    public DateTimeOffset OccurredAt => PlacedAt;
}

// Infrastructure/Persistence/AppDbContext.cs — publisher injected via primary constructor
public class AppDbContext(DbContextOptions<AppDbContext> options, IPublisher publisher)
    : DbContext(options)
{
    public override async Task<int> SaveChangesAsync(CancellationToken ct = default)
    {
        var aggregates = ChangeTracker.Entries<AggregateRoot>()
            .Where(e => e.Entity.DomainEvents.Count > 0)
            .Select(e => e.Entity)
            .ToList();

        var events = aggregates.SelectMany(a => a.DomainEvents).ToList();

        var result = await base.SaveChangesAsync(ct);

        foreach (var @event in events)
            await publisher.Publish(@event, ct);

        foreach (var aggregate in aggregates)
            aggregate.ClearDomainEvents();

        return result;
    }
}

Domain Services

For logic that does not belong to a single aggregate:

// Domain/Orders/Services/PricingService.cs
// Coordinates logic across aggregates — takes domain interfaces, returns value objects
public sealed class PricingService(IDiscountPolicy discountPolicy)
{
    public Money CalculatePrice(ProductId productId, int quantity, Money unitPrice, CustomerId customerId)
    {
        var subtotal = new Money(unitPrice.Amount * quantity, unitPrice.Currency);
        var discount = discountPolicy.GetDiscount(customerId, productId, quantity);
        return new Money(subtotal.Amount * (1 - discount), subtotal.Currency);
    }
}

Anti-patterns

Oversized Aggregates

// BAD — Customer aggregate owns everything the customer touches
public class Customer : AggregateRoot
{
    public List<Order> Orders { get; } = [];        // should be separate aggregate
    public List<Payment> Payments { get; } = [];     // should be separate aggregate
    public List<Address> Addresses { get; } = [];    // might be OK as child
    public ShoppingCart Cart { get; set; }            // should be separate aggregate
}

// GOOD — small, focused aggregates linked by ID
public class Customer : AggregateRoot
{
    public CustomerName Name { get; private set; }
    public EmailAddress Email { get; private set; }
    // Orders, Payments, Cart are separate aggregates referencing CustomerId
}

Domain Events for Intra-Aggregate Logic

// BAD — using events for logic within the same aggregate
order.RaiseDomainEvent(new OrderLineAdded(line));
// Then a handler recalculates the total... but you're in the same aggregate!

// GOOD — just call the method directly within the aggregate
_lines.Add(line);
RecalculateTotal();  // private method, no event needed

Value Objects with Identity

// BAD — value object with an Id (it's an entity then!)
public record Address
{
    public Guid Id { get; init; }  // value objects don't have identity
    public string Street { get; init; }
}

// GOOD — value objects are defined by their attributes, not an Id
public record Address(string Street, string City, string PostalCode, string Country);

Anemic Aggregates

// BAD — aggregate is just a data bag, service does all the work
public class Order : AggregateRoot
{
    public OrderStatus Status { get; set; }  // public setter!
    public List<OrderLine> Lines { get; set; } = [];
}

// Service directly manipulates order state
order.Status = OrderStatus.Confirmed;  // no invariant check!
order.Lines.Add(newLine);              // no validation!

// GOOD — aggregate encapsulates rules (see Aggregate Root pattern above)
order.Confirm();  // validates status, raises event
order.AddLine(productId, quantity, unitPrice);  // validates, recalculates

Decision Guide