ddd

领域驱动设计（DDD）

核心原则

1. 聚合定义一致性边界 — 聚合是由实体和值对象组成的集群，作为一个整体处理数据变更。聚合内的所有不变量都在单个事务中得到保证。跨聚合的一致性是最终一致的。
2. 优先使用值对象，而非基本类型 — 用值对象替代对基本类型的过度依赖。Money、EmailAddress、OrderNumber 不是字符串，它们承载验证、相等性和行为。使用 C# 记录类型创建不可变值对象。
3. 领域事件解耦副作用 — 当领域中发生有意义的事情时（OrderPlaced、PaymentReceived），触发领域事件。副作用（发送邮件、更新读取模型、通知另一个聚合）订阅这些事件。聚合专注于自身规则。
4. 聚合根是唯一入口 — 外部代码只能通过聚合根实体访问聚合。子实体绝不单独加载或修改。聚合根负责保证整个聚合的不变量。
5. 仓储持久化聚合，而非实体 — 每个聚合根对应一个仓储。仓储将整个聚合作为一个整体加载和保存。子实体不设仓储。Infrastructure 实现内部使用 DbContext——这是用于定义聚合边界的 DDD 战术模式，而非通用 CRUD 包装器。

模式

聚合根

聚合根拥有对子实体的所有访问权限，并负责保证不变量：

// 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);
    }
}

作为记录类型的值对象

使用 C# 记录类型创建具有结构相等性的不可变值对象：

// 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

使用 EF Core 转换器实现强类型 ID

防止混淆不同实体的 GUID：

// 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();
    }
}

分发领域事件

在聚合中触发事件，在 SaveChangesAsync 中分发：

// 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;
    }
}

领域服务

用于处理不属于单个聚合的逻辑：

// 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);
    }
}

反模式

过大的聚合

// 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
}

将领域事件用于聚合内逻辑

// 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

带有身份标识的值对象

// 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);

贫血聚合

// 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

决策指南

场景	建议
何时使用 DDD	业务规则超出 CRUD 范畴的复杂领域
何时使用值对象	任何具有验证规则，或基于属性而非身份标识进行相等性判断的概念
聚合大小	保持精简——通常由 1 个根实体和 0-3 个子实体组成。每次都加载整个聚合
领域事件与集成事件	领域事件：在有界上下文内，通过同一事务处理。集成事件：跨上下文，通过消息总线传递
强类型 ID	对跨越边界的聚合根 ID 始终使用。子实体 ID 可选
何时不使用 DDD	简单 CRUD、设置、审计日志、读取模型——使用普通实体
仓储与 DbContext	复杂聚合使用每个聚合根一个仓储；较简单的查询使用 IAppDbContext
领域服务	仅当逻辑需要涉及多个聚合，或需要聚合不应了解的外部数据时使用