attac-t Skill: Craft Value Object
"Invalid objects should be impossible to create."
The Standard
- Self-Validating: Validate in constructor. Invalid state throws immediately.
- Immutable:
readonly class. No setters. Clone to modify. - Defensive Construction: Private constructor + named factories for controlled creation.
- Serializable: Must survive queue serialization. No closures, no resources.
The Anti-Patterns
| ❌ Don't | ✅ Do | Why |
|---|---|---|
| Mutable properties | readonly class |
VOs don't change |
| Skip validation | Validate in constructor | Invalid VOs shouldn't exist |
| Complex dependencies | Primitives + other VOs | Must serialize cleanly |
| Entity with no ID | Use VO intentionally | Different concepts |
Real-World Examples
See examples/.