Introduction

"An object that represents a descriptive aspect of the domain with no conceptual identity is called a VALUE OBJECT." (Eric Evans).

As opposite to Entities, which have their identities (Id), a Value Object has not it's identity. If identities of two Entities are different, they are considered as different objects/entities even if all other properties of those entities are same. Think two different persons have same Name, Surname and Age but they are different people if their identity numbers are different. But, for an Address (which is a classic Value Object) class, if two addresses has same Country, City, Street number... etc. they are considered as the same address.

In Domain Driven Design (DDD), Value Object is another type of domain object which can include business logic and is an essential part of the domain.

Value Object Base Class

ABP has a ValueObject<T> base class which can be inherited in order to easily create Value Object types. Example Address Value Object type:

public class Address : ValueObject<Address>
{
    public Guid CityId { get; private set; } //A reference to a City entity.

    public string Street { get; private set; }

    public int Number { get; private set; }

    public Address(Guid cityId, string street, int number)
    {
        CityId = cityId;
        Street = street;
        Number = number;
    }
}

ValueObject base class overrides equality operator (and other related operator and methods) to compare two value object and assumes that they are identical if all properties are identical. So, all of these tests pass:

var address1 = new Address(new Guid("21C67A65-ED5A-4512-AA29-66308FAAB5AF"), "Baris Manco Street", 42);
var address2 = new Address(new Guid("21C67A65-ED5A-4512-AA29-66308FAAB5AF"), "Baris Manco Street", 42);

Assert.Equal(address1, address2);
Assert.Equal(address1.GetHashCode(), address2.GetHashCode());
Assert.True(address1 == address2);
Assert.False(address1 != address2);

Even they are different objects in memory, they are identical for our domain.

Best Practices

Here, some best practices for Value Objects:

• Design a value object as immutable (as like the Address above) if there is not a very good reason for designing it as mutable.
• The properties that make up a Value Object should form a conceptual whole. For example, CityId, Street and Number shouldn't be serarate properties of a Person entity. Also, this makes Person entity simpler.