Value objects

Value object - is an unchangeable object that has attributes, but no distinct identity. In DDD it is very important to distinguish between Entities and Value Objects. The main difference between Entity and ValueObject is in the method Equals. Two entities are considered equal if their identifiers are equal. The ValueObject has no identifier. Two ValueObjects are considered equal if all their public properties are equal in value.

Example of value object:

public class Address : ValueObject<Address>
{
    public string City { get; set; }
    public string Street { get; set; }
    public string House { get; set; }
}

Use Address value object in User entity:

public class User : Entity<User, int>
{
    public string Name { get; private set; }
    public Address Address { get; private set; }

    private User() : base() { }

    public User(string name, Address address) : this()
    {
        Name = name;
        Address = address;
    }

    public void ChangeAddress(Address address)
    {
        Address = address;
    }
}

Example of value objects comparing:

bool isEqual;

var address1 = new Address { City = "Moscow", Street = "Prospekt Mira", House = "57" };
var address2 = new Address { City = "Moscow", Street = "Prospekt Mira", House = "57" };
var address3 = new Address { City = "Volgograd", Street = "Prospekt Mira", House = "57" };

isEqual = address1 == address2; // Returns "true"
isEqual = address1 == address3; // Returns "false" because of different cities