Domain-Driven Design Aggregates in C#

What is an Aggregate?

An aggregate is a cluster of domain objects treated as a single unit for data changes. It has a root entity that controls all access to the cluster.

Aggregate Root

public class Order : AggregateRoot
{
    private readonly List<OrderLine> _lines = [];
    public IReadOnlyList<OrderLine> Lines => _lines.AsReadOnly();
    public OrderStatus Status { get; private set; }

    public void AddLine(Product product, int qty)
    {
        if (Status != OrderStatus.Draft)
            throw new DomainException("Cannot modify a confirmed order.");
        _lines.Add(new OrderLine(product.Id, product.Price, qty));
        AddDomainEvent(new OrderLineAdded(Id, product.Id, qty));
    }
}

Value Objects

public record Money(decimal Amount, string Currency)
{
    public Money Add(Money other)
    {
        if (Currency != other.Currency) throw new InvalidOperationException();
        return this with { Amount = Amount + other.Amount };
    }
}

Domain Events

public class OrderConfirmed(Guid orderId) : IDomainEvent
{
    public Guid OrderId { get; } = orderId;
    public DateTime OccurredAt { get; } = DateTime.UtcNow;
}