abp-ddd

---
name: abp-ddd
description: ABP DDD patterns - Entities, Aggregate Roots, value objects, Repositories, Domain Services, Domain Events, Specifications. Use when designing domain layer, creating entities, repositories, or domain services in ABP projects.
---

# ABP DDD Patterns

> **Docs**: https://abp.io/docs/latest/framework/architecture/domain-driven-design

## Anti-Patterns to Avoid

- **Anemic entities**: public setters with no behavior — use private setters + methods that enforce invariants
- **Repository for child entities**: only aggregate roots get repositories — access child entities through their root
- **Generating GUID in entity constructor**: use `IGuidGenerator` from outside and pass `id` parameter
- **Navigation properties to other aggregates**: reference by `Id` only, never add full navigation properties across aggregates
- **Domain service depending on current user**: accept values from the application layer instead

## Rich Domain Model vs Anemic Domain Model

ABP promotes **Rich Domain Model** pattern where entities contain both data AND behavior:

| Anemic (Anti-pattern) | Rich (Recommended) |
|----------------------|-------------------|
| Entity = data only | Entity = data + behavior |
| Logic in services | Logic in entity methods |
| Public setters | Private setters with methods |
| No validation in entity | Entity enforces invariants |

**Encapsulation is key**: Protect entity state by using private setters and exposing behavior through methods.

## Entities

### Entity Example (Rich Model)
```csharp
public class OrderLine : Entity<Guid>
{
    public Guid ProductId { get; private set; }
    public int Count { get; private set; }
    public decimal Price { get; private set; }

    protected OrderLine() { } // For ORM

    internal OrderLine(Guid id, Guid productId, int count, decimal price) : base(id)
    {
        ProductId = productId;
        SetCount(count); // Validates through method
        Price = price;
    }

    public void SetCount(int count)
    {
        if (count <= 0)
            throw new BusinessException("Orders:InvalidCount");
        Count = count;
    }
}
```

## Aggregate Roots

Aggregate roots are consistency boundaries that:
- Own their child entities
- Enforce business rules
- Publish domain events

```csharp
public class Order : AggregateRoot<Guid>
{
    public string OrderNumber { get; private set; }
    public Guid CustomerId { get; private set; }
    public OrderStatus Status { get; private set; }
    public ICollection<OrderLine> Lines { get; private set; }

    protected Order() { } // For ORM

    public Order(Guid id, string orderNumber, Guid customerId) : base(id)
    {
        OrderNumber = Check.NotNullOrWhiteSpace(orderNumber, nameof(orderNumber));
        CustomerId = customerId;
        Status = OrderStatus.Created;
        Lines = new List<OrderLine>();
    }

    public void AddLine(Guid lineId, Guid productId, int count, decimal price)
    {
        // Business rule: Can only add lines to created orders
        if (Status != OrderStatus.Created)
            throw new BusinessException("Orders:CannotModifyOrder");

        Lines.Add(new OrderLine(lineId, productId, count, price));
    }

    public void Complete()
    {
        if (Status != OrderStatus.Created)
            throw new BusinessException("Orders:CannotCompleteOrder");

        Status = OrderStatus.Completed;

        // Publish events for side effects
        AddLocalEvent(new OrderCompletedEvent(Id));           // Same transaction
        AddDistributedEvent(new OrderCompletedEto { OrderId = Id }); // Cross-service
    }
}
```

### Domain Events
- `AddLocalEvent()` - Handled within same transaction, can access full entity
- `AddDistributedEvent()` - Handled asynchronously, use ETOs (Event Transfer Objects)

### Entity Best Practices
- **Encapsulation**: Private setters, public methods that enforce rules
- **Primary constructor**: Enforce invariants, accept `id` parameter
- **Protected parameterless constructor**: Required for ORM
- **Initialize collections**: In primary constructor
- **Virtual members**: For ORM proxy compatibility
- **Reference by Id**: Don't add navigation properties to other aggregates
- **Don't generate GUID in constructor**: Use `IGuidGenerator` externally

## Repository Pattern

### When to Use Custom Repository
- **Generic repository** (`IRepository<T, TKey>`): Sufficient for simple CRUD operations
- **Custom repository**: Only when you need custom query methods

### Interface (Domain Layer)
```csharp
// Define custom interface only when custom queries are needed
public interface IOrderRepository : IRepository<Order, Guid>
{
    Task<Order> FindByOrderNumberAsync(string orderNumber, bool includeDetails = false);
    Task<List<Order>> GetListByCustomerAsync(Guid customerId, bool includeDetails = false);
}
```

### Repository Best Practices
- **One repository per aggregate root only** - Never create repositories for child entities
- Child entities must be accessed/modified only through their aggregate root
- Creating repositories for child entities breaks data consistency (bypasses aggregate root's business rules)
- In ABP, use `AddDefaultRepositories()` without `includeAllEntities: true` to enforce this
- Define custom repository only when custom queries are needed
- ABP handles `CancellationToken` automatically; add parameter only for explicit cancellation control
- Single entity methods: `includeDetails = true` by default
- List methods: `includeDetails = false` by default
- Don't return projection classes
- Interface in Domain, implementation in data layer

```csharp
// ✅ Correct: Repository for aggregate root (Order)
public interface IOrderRepository : IRepository<Order, Guid> { }

// ❌ Wrong: Repository for child entity (OrderLine)
// OrderLine should only be accessed through Order aggregate
public interface IOrderLineRepository : IRepository<OrderLine, Guid> { } // Don't do this!
```

## Domain Services

Use domain services for business logic that:
- Spans multiple aggregates
- Requires repository queries to enforce rules

```csharp
public class OrderManager : DomainService
{
    private readonly IOrderRepository _orderRepository;
    private readonly IProductRepository _productRepository;

    public OrderManager(
        IOrderRepository orderRepository,
        IProductRepository productRepository)
    {
        _orderRepository = orderRepository;
        _productRepository = productRepository;
    }

    public async Task<Order> CreateAsync(string orderNumber, Guid customerId)
    {
        // Business rule: Order number must be unique
        var existing = await _orderRepository.FindByOrderNumberAsync(orderNumber);
        if (existing != null)
        {
            throw new BusinessException("Orders:OrderNumberAlreadyExists")
                .WithData("OrderNumber", orderNumber);
        }

        return new Order(GuidGenerator.Create(), orderNumber, customerId);
    }

    public async Task AddProductAsync(Order order, Guid productId, int count)
    {
        var product = await _productRepository.GetAsync(productId);
        order.AddLine(productId, count, product.Price);
    }
}
```

### Domain Service Best Practices
- Use `*Manager` suffix naming
- No interface by default (create only if needed)
- Accept/return domain objects, not DTOs
- Don't depend on authenticated user - pass values from application layer
- Use base class properties (`GuidGenerator`, `Clock`) instead of injecting these services

## Domain Events

### Local Events
```csharp
// In aggregate
AddLocalEvent(new OrderCompletedEvent(Id));

// Handler
public class OrderCompletedEventHandler : ILocalEventHandler<OrderCompletedEvent>, ITransientDependency
{
    public async Task HandleEventAsync(OrderCompletedEvent eventData)
    {
        // Handle within same transaction
    }
}
```

### Distributed Events (ETO)
For inter-module/microservice communication:
```csharp
// In Domain.Shared
[EventName("Orders.OrderCompleted")]
public class OrderCompletedEto
{
    public Guid OrderId { get; set; }
    public string OrderNumber { get; set; }
}
```

## Specifications

Reusable query conditions:
```csharp
public class CompletedOrdersSpec : Specification<Order>
{
    public override Expression<Func<Order, bool>> ToExpression()
    {
        return o => o.Status == OrderStatus.Completed;
    }
}

// Usage
var orders = await _orderRepository.GetListAsync(new CompletedOrdersSpec());
```