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Overview

Prevent primitive obsession by enforcing strongly typed identifiers and value objects in domain models. Conversions to/from primitives are permitted only at explicit boundaries (persistence, transport, serialization), ensuring type safety and validation throughout the domain layer.

When to Use

• Designing or reviewing domain models (entities, aggregates, commands/events)
• Introducing new entity identifiers (CustomerId, OrderId, TenantId, etc.)
• Working with meaningful primitives that carry validation (EmailAddress, Money, Percentage)
• Reviewing code where primitive types (Guid, string, int) are used for domain concepts
• Implementing API boundaries, persistence layers, or serialization

Core Workflow

1. Identify primitive obsession: Locate places where Guid, string, int, or long represent domain concepts
2. Define strongly typed IDs: Create typed ID types using source-generation libraries for entity identifiers
3. Define value objects: Create value objects for meaningful primitives with validation (EmailAddress, Money)
4. Establish boundary conversions: Map to/from primitives only at explicit boundaries (API, persistence, transport)
5. Configure serialization: Add JSON converters, EF Core value converters, and type handlers as needed
6. Update service signatures: Change service methods to accept domain primitives, not raw types
7. Add boundary tests: Test that boundary conversion works and invalid primitives are rejected

Core

Defaults (non-negotiable)

• StronglyTypedIds by default for all entity identifiers in domain/application code.
• No primitive IDs (Guid, int, long, string) in the domain layer.
• Use value objects for meaningful primitives (e.g., EmailAddress, Money, Percentage, TenantId, CorrelationId).

Preferred approach

• Prefer open-source libraries that use source generation for typed IDs.
• Conversions to/from primitives are permitted only at explicit boundaries:
  • persistence adapters,
  • transport adapters (HTTP, messaging),
  • serialization/deserialization.

Review rules

• New domain types must not introduce primitive ID properties/fields.
• Mapping layers must map typed IDs explicitly; no "magic" conversions hidden in core domain types.

Load: examples

Strongly typed ID (source generator style)

• Define a CustomerId type and use it on entities/commands.
• Map to primitive Guid at the persistence boundary and transport boundary.

Value object boundaries

• Allow string in DTOs if required by external contracts.
• Convert to EmailAddress (value object) inside the application layer.

Load: advanced

Integration guidance

• EF Core: value converters for typed IDs and value objects.
• System.Text.Json: custom converters where needed for typed IDs.
• Dapper: type handlers if Dapper is used for read models.

Operational concerns

• Ensure typed ID types are stable for logging/telemetry (string representation).
• Avoid implicit conversions that obscure boundary crossings.

API Boundary Mapping & Validation

Before: Primitive Obsession at Boundaries

// API Controller - accepts primitives
[ApiController]
[Route("api/[controller]")]
public class CustomersController
{
    private readonly ICustomerService _service;

    [HttpPost]
    public async Task<IActionResult> CreateCustomer(CreateCustomerRequest request)
    {
        // No type safety - primitives passed directly to service
        var result = await _service.CreateCustomer(request.Id, request.Email);
        return Ok(result);
    }
}

// Service layer - accepts primitives, loses domain context
public class CustomerService
{
    public async Task<CustomerDto> CreateCustomer(string id, string email)
    {
        // Validation scattered across layers
        if (string.IsNullOrWhiteSpace(email))
            throw new ArgumentException("Email required");

        // No connection to domain types
        var customer = new Customer { Id = Guid.Parse(id), Email = email };
        await _repository.AddAsync(customer);
        return new CustomerDto { Id = customer.Id.ToString(), Email = email };
    }
}

// DTO - exposes internal structure
public class CreateCustomerRequest
{
    public string Id { get; set; }
    public string Email { get; set; }
}

Problems:

• No type safety between layers
• Validation scattered across concerns
• Easy to pass invalid primitives

After: Domain Primitives at Boundaries

// Domain types
public partial class CustomerId : IStronglyTypedId<Guid> { }
public partial class EmailAddress : IValueObject<string> { }

// API Controller - explicit boundary conversion
[ApiController]
[Route("api/[controller]")]
public class CustomersController
{
    private readonly ICustomerService _service;
    private readonly ICustomerMapper _mapper;

    [HttpPost]
    public async Task<IActionResult> CreateCustomer(CreateCustomerRequest request)
    {
        // Explicit conversion at boundary
        var customerId = new CustomerId(Guid.Parse(request.Id));
        var email = EmailAddress.Create(request.Email).ThrowIfFailure();

        var result = await _service.CreateCustomer(customerId, email);
        return Ok(_mapper.ToResponse(result));
    }
}

// Service layer - type-safe, domain-focused
public class CustomerService
{
    public async Task<Customer> CreateCustomer(CustomerId id, EmailAddress email)
    {
        // Domain types ensure validity before service runs
        var customer = Customer.Create(id, email).ThrowIfFailure();
        await _repository.AddAsync(customer);
        return customer;
    }
}

// Mapper - explicit conversion layer
public class CustomerMapper
{
    public CustomerResponse ToResponse(Customer customer)
    {
        return new CustomerResponse
        {
            Id = customer.Id.Value.ToString(),  // Explicit back to primitive
            Email = customer.Email.Value        // Explicit back to primitive
        };
    }
}

Benefits:

• Type safety enforced across layers
• Validation centralized in domain types
• Clear boundary crossing
• Compiler prevents invalid combinations

Validation Steps for Domain Primitive Implementation

1. API Controllers:

   • DTO properties remain primitives
   • Convert DTOs to domain types immediately upon entry
   • Use mapper/converter class for boundary crossing

2. Service Layer:

   • Accept domain primitives, not raw types
   • Never accept Guid when CustomerId exists
   • Ensure validation runs before service logic

3. Mapping & Serialization:

   • JSON serialization handles conversion via custom converters
   • EF Core value converters map domain types ↔ database columns
   • No implicit conversions in constructors

4. Testing:

   • Unit test boundary conversion in mapper
   • Integration test proves invalid primitives rejected at API
   • Verify domain type validation runs before service

Load: enforcement

Acceptance criteria for PRs

• New entities/aggregates use typed IDs.
• No domain-layer primitive IDs added.
• Boundary conversion is explicit and covered by unit tests.

Red Flags - STOP

These statements indicate primitive obsession patterns: