Technical Story: SSW.CleanArchitecture | Issues | 111

Context and Problem Statement¶

Aggregate roots can be instantiated in a way that satisfies all the decision drivers. What is the best way to create aggregates?

Decision Drivers¶

• Must work well with EF Core
• Must be able to raise domain events on object creation
• Must keep aggregate roots in a valid state at all times

Considered Options¶

1. Factory Methods
2. Constructors
3. required init properties

Decision Outcome¶

Chosen Option 1. Factory Methods, because it is the only option that meets all the decision drivers.

See the SSW Rule Do you encapsulate domain models in Domain-Driven Design?, which lists factory methods as one of the ways to keep an aggregate encapsulated.

Consequences¶

• Private constructors can be used to ensure that aggregates are always in a valid state
• Need to use null! to remove nullable warnings

Pros and Cons of the Options¶

1. Factory Methods¶

• ✅ EF Core can still create the aggregate when hydrating data from the DB
• ✅ Domain events can be raised when aggregates are created via the application, but not when they are hydrated from the DB
• ✅ Provides a consistent way to create aggregates
• ✅ Aggregates remain encapsulated

2. Constructors¶

• ❌ EF will incorrectly raise domain events when hydrating data from the DB
• ❌ EF does not allow owned entities to be passed to constructors

3. required init properties¶

• ❌ Properties need to be passed to constructors to ensure they are in a valid state on object creation. Can't use required init properties as they then become unmodifiable

Links¶

• Do you encapsulate domain models in Domain-Driven Design?
• Do you understand the difference between anemic and rich domain models?
• Do you know when to use value objects?