dotnet-ddd

Domain-Driven Design guidance for .NET domain code. Makes DDD principles explicit and enforceable for the coder and code-reviewer agents.

When to Activate

• Creating or modifying domain models (entities, value objects, aggregates)
• Adding repositories, factories, or domain services
• Defining or refactoring bounded contexts
• Reviewing domain code for DDD compliance
• Refactoring anemic models toward rich behavior
• Enforcing aggregate invariants or encapsulation rules

Provenance

Content is synthesized from established DDD literature:

• Eric Evans, Domain-Driven Design: Tackling Complexity in the Heart of Software (2003) — "Blue Book"
• Vaughn Vernon, Implementing Domain-Driven Design (2013) — "Red Book"
• Microsoft .NET Architecture guidance (eShop, microservices e-book, CQRS docs)
• Martin Fowler, bliki articles on Anemic Domain Model, Domain Event, etc.

This is a practitioner synthesis, not a verbatim reproduction. Where sources disagree, the Red Book's pragmatic guidance takes precedence for implementation details.

Core Principles

1. The domain model is the software. Business logic lives in domain objects, not in services or controllers. (Evans, ch. 1)
2. Ubiquitous Language. Code names match the domain expert's vocabulary. No translation layer between business and code. (Evans, ch. 2)
3. Bounded Contexts define boundaries. Each context owns its model. Shared types across contexts are a design smell. (Evans, ch. 14)
4. Aggregates enforce invariants. All mutations go through the aggregate root. External code never reaches inside. (Vernon, ch. 10)
5. Value Objects are immutable and identity-free. They describe characteristics, not things. (Evans, ch. 5)
6. Repositories return aggregates, not tables. Persistence is an infrastructure concern. (Vernon, ch. 12)

Tactical Patterns (summary)

Pattern	One-liner	Reference
Entity	Has identity; equality by ID, not attributes	tactical-patterns.md § Entity
Value Object	No identity; immutable; equality by attributes	tactical-patterns.md § Value Object
Aggregate + Root	Consistency boundary; root is sole entry point	tactical-patterns.md § Aggregate
Repository	Collection-like abstraction over aggregates	tactical-patterns.md § Repository
Factory	Encapsulates complex creation logic	tactical-patterns.md § Factory
Specification	Reusable, composable query/predicate logic	tactical-patterns.md § Specification
Domain Service	Stateless logic that doesn't belong to an entity	tactical-patterns.md § Domain Service
Rich vs Anemic	Behavior lives with data, not in external services	tactical-patterns.md § Rich vs Anemic

Full definitions, GOOD/BAD examples, and per-pattern checklist lines: tactical-patterns.md

Strategic Design (summary)

Concept	One-liner	Reference
Bounded Context	Explicit boundary around a model + language	strategic-design.md § Bounded Context
Ubiquitous Language	Shared vocabulary within a context	strategic-design.md § Ubiquitous Language
Anti-Corruption Layer	Translation boundary shielding your model from external models	strategic-design.md § ACL
Context Mapping	Relationships between contexts (Shared Kernel, Customer/Supplier, etc.)	strategic-design.md § Context Mapping
Domain Events	Immutable record of something that happened in the domain	strategic-design.md § Domain Events
CQRS	Separate read/write models (deep enforcement defers to dotnet-cop)	strategic-design.md § CQRS

Full definitions, GOOD/BAD examples, and per-concept checklist lines: strategic-design.md

Review Checklist

The code-reviewer agent applies review-checklist.md when reviewing domain code. It is a terse, file:line-oriented list — one item per pattern/concept. Load it on demand during review.

Relationship to Other Skills

Skill	Relationship
[[dotnet-clean-architecture]]	Provides hexagonal layering (ports/adapters, module isolation). This skill provides the domain-modeling rules that live inside Core.
[[dotnet-cop]]	Pre-merge review enforcement. Its optional-cqrs and optional-event-sourcing sub-pages handle deep CQRS/ES review. This skill links to them rather than duplicating.
[[coding-standards]]	General TypeScript/JS standards. Not .NET-specific; no overlap.

Quick-Start Checklist (for coder)

When writing new domain code:

• Identify the bounded context this code belongs to
• Name types using the context's ubiquitous language
• Model entities with identity + behavior (not just data)
• Extract value objects for descriptive attributes (immutable, no ID)
• Group related entities into aggregates; designate one root
• Enforce invariants inside the aggregate root (constructor + methods)
• Expose mutations only through the aggregate root's public methods
• Define repository interfaces in Core (return aggregates, not EF entities)
• Use domain events for cross-aggregate or cross-context communication
• Keep domain services stateless; prefer entity methods when behavior fits one entity

Hard Rules

1. No anemic models. If a class has only getters/setters and all logic lives in a separate service, that is a violation. Move behavior to the entity. (Fowler, "Anemic Domain Model")
2. Aggregate root is the only external entry point. No code outside the aggregate may hold a reference to an internal entity or mutate it directly. (Vernon, ch. 10)
3. Value objects are immutable. Use readonly record struct or sealed class with init-only properties. No setters. (Evans, ch. 5)
4. Repositories return aggregates. Never return EF Core entities, IQueryable, or raw DTOs from a domain repository interface. (Vernon, ch. 12)
5. Bounded-context boundaries are hard. No shared internal types across contexts. Use ACL or integration events. (Evans, ch. 14)
6. Domain layer has zero infrastructure dependencies. No using Microsoft.EntityFrameworkCore, no HttpClient, no framework attributes in domain types.