Yesterday I talked about the 'units of measure' feature in F#. This allows you to associate a unit of measure to a type and allows the compiler to help you avoid providing the wrong input. Unfortunately in C#, a similar feature does not exist but we can let the type system help us by creating 'Value Objects'.
What are Value Objects?
Value objects are probably best known in the context of Domain Driven Design where they are one of the tactical patterns. They allow you to encapsulate (domain) logic in a type and should always be in a valid state.
Eric Evans uses the following example to describe them:
When a child is drawing, he cares about the color of the marker he chooses, and he may care about the sharpness of the tip. But if there are two markers of the same color and shape, he probably won’t care which one he uses. If a marker is lost and replaced by another of the same color from a new pack, he can resume his work unconcerned about the switch.
Value Objects have some specific characteristics:
- They have value equality, this means that 2 value objects are considered equal if all there values are the same. (see the example of the marker above)
- They are always in a valid state. Behaviour should exist that validates the correctness of the value object at construction time.
- They are immutable; this is important to help you guarantee the 2 other characteristics. Instead of changing an existing value object, a new one is created.
They can help you avoid ‘primitive obsession’.
More information:
- https://docs.microsoft.com/en-us/dotnet/architecture/microservices/microservice-ddd-cqrs-patterns/implement-value-objects
- https://martinfowler.com/bliki/ValueObject.html
Using Value Objects for a Unit of measure
Based on the information above, it should be obvious that Value Objects are a perfect candidate to describe units of measure inside your application. Here is a small example using record types:
Using Units.NET
Of course creating Value Objects for all possible units of measure can be a lot of work. The good news is that this work is already done for you. The Units.NET library has implemented more then 1200 units of measure.
Here is a small code snippet from their Github repo: