Applying the Smart Constructor Pattern in C
Applying the smart constructor pattern in C# #
Excerpt #
In Domain-Driven Design (DDD), domain invariants are fundamental principles or rules that must always hold true within a specific domain. T…
Applying the smart constructor pattern in C# #
In Domain-Driven Design (DDD), domain invariants are fundamental principles or rules that must always hold true within a specific domain. These invariants define constraints or conditions that govern the behavior and state of the entities, value objects, and aggregates within the domain.
They emphasize the Always-valid rule:
Your domain model should always be in a valid state.
By using invariants, your domain model guarantees that it cannot represent illegal states. These invariants define the domain class: that class is what it is because of them. Therefore, you canât possibly violate these invariants. If you do so, the domain class would simply cease being the thing you expect it to be; itâd become something else.
As Greg Young explains it:
A unicorn without a horn is a horse, not a unicorn.
There are multiple ways to protect those invariants. Using value objects and taking advantage of the type system are a great help here. However not all invariants can be enforced by the type system.
In that situation we can use Smart Constructors which is a function to create values only if they pass a certain criteria.
Letâs look at an exampleâŠ
| public record ProductItem | |
| { | |
| public Guid ProductId { get; } | |
| public int Quantity { get; } | |
| private ProductItem(Guid productId, int quantity) | |
| { | |
| ProductId = productId; | |
| Quantity = quantity; | |
| } | |
| } |
We have a small ProductItem value object (notice that Iâm using C# record types here). As we donât want to create the value object directly, we made the constructor private.
Now we add a static method that does the necessary validation before creating the value object:
| public record ProductItem | |
| { | |
| public Guid ProductId { get; } | |
| public int Quantity { get; } | |
| private ProductItem(Guid productId, int quantity) | |
| { | |
| ProductId = productId; | |
| Quantity = quantity; | |
| } | |
| public static ProductItem From(Guid? productId, int? quantity) | |
| { | |
| if (!productId.HasValue) | |
| throw new ArgumentNullException(nameof(productId)); | |
| return quantity switch | |
| { | |
| null => throw new ArgumentNullException(nameof(quantity)), | |
| <= 0 => throw new ArgumentOutOfRangeException(nameof(quantity), "Quantity has to be a positive number"), | |
| _ => new ProductItem(productId.Value, quantity.Value) | |
| }; | |
| } | |
| } |
Remark: The example above could probably be handled using a normal constructor but you get the point.
Learn more #
Always-Valid Domain Model · Enterprise Craftsmanship
Smart constructors - HaskellWiki