--- name: cqrs-ddd description: CQRS+DDD implementation patterns for .NET. Use when designing aggregates, implementing command\/query handlers, setting up repositories, or understanding architectural principles. Triggers on "CQRS", "DDD", "aggregate", "command handler", "query handler", "repository", "value object", "domain event", "entity configuration". --- # CQRS+DDD Implementation Patterns Comprehensive guide for implementing CQRS+DDD architecture in this .NET\/EF Core codebase. ## Quick Reference ### Layer Dependencies ``` ┌─────────────────┐ │ Api Layer │ Controllers, Middleware └────────┬────────┘ │ depends on ▼ ┌─────────────────┐ │Application Layer│ Commands, Queries, Handlers └────────┬────────┘ │ depends on ▼ ┌─────────────────┐ │ Domain Layer │◄──┐ Entities, Value Objects └─────────────────┘ │ ▲ │ │ implements │ ┌────────┴────────┐ │ │Infrastructure │───┘ Repositories, DbContext └─────────────────┘ ``` ### CQRS Separation | Command (Write) | Query (Read) | | --------------- | ------------ | | Via Aggregate Root | Direct DB query | | Transaction required | AsNoTracking() | | Business rules apply | Performance priority | ### Domain Layer Patterns ```csharp \/\/ Entity Base public abstract class Entity where TId : notnull { public TId Id { get; protected set; } } \/\/ Aggregate Root public abstract class AggregateRoot : Entity { private readonly List _domainEvents = new(); protected void AddDomainEvent(DomainEvent e) => _domainEvents.Add(e); } \/\/ Value Object (C# record) public record StudentId(Guid Value); ``` ### Application Layer Patterns ```csharp \/\/ Command Handler public class CreateCourseCommandHandler : IRequestHandler { public async Task Handle(CreateCourseCommand request, CancellationToken ct) { var course = Course.Create(new CourseCode(request.Code), request.Name); await _repository.AddAsync(course, ct); await _repository.SaveChangesAsync(ct); return course.Id.Value; } } \/\/ Query Handler public class GetCoursesQueryHandler : IRequestHandler> { public async Task> Handle(GetCoursesQuery request, CancellationToken ct) { return await _context.Courses .AsNoTracking() .Select(c => new CourseDto { ... }) .ToListAsync(ct); } } ``` ### Infrastructure Layer Patterns ```csharp \/\/ Entity Configuration public class EnrollmentConfiguration : IEntityTypeConfiguration { public void Configure(EntityTypeBuilder builder) { builder.Property(e => e.Id) .HasConversion(v => v.Value, v => new EnrollmentId(v)); builder.OwnsOne(e => e.Semester); builder.Ignore(e => e.DomainEvents); } } ``` ## Key Rules | Rule | Description | | ---- | ----------- | | 1 Aggregate = 1 Transaction | No cross-aggregate changes in same transaction | | ID References Only | Reference other aggregates by ID, not entity | | 1 Repository per Aggregate | One repository per aggregate root | | Validation in Domain | Value objects validate on construction | | No FluentValidation | Use aggregate\/value object validation | ## New Aggregate Checklist | Step | Action | File\/Location | | ---- | ------ | ------------- | | 1 | Create Flyway migration | `Migrations\/V{n}__*.sql` | | 2 | Add DbSet to DbContext | `*DbContext.cs` | | 3 | Create EntityConfiguration | `Configurations\/*Configuration.cs` | | 4 | Implement Repository | `Repositories\/*Repository.cs` | | 5 | Register DI | `Program.cs` | ## Resources | File | Content | | ---- | ------- | | [architecture.md](references\/architecture.md) | Architectural principles, bounded contexts | | [domain-layer.md](references\/domain-layer.md) | Entity, Aggregate, Value Object patterns | | [application-layer.md](references\/application-layer.md) | CQRS handlers, validation, transactions | | [infrastructure-layer.md](references\/infrastructure-layer.md) | EF Core, repositories, migrations | ## Related Skills - [testing-strategy](..\/testing-strategy\/SKILL.md) - Handler testing patterns - [slow-query-detector](..\/slow-query-detector\/SKILL.md) - N+1 and performance issues