I am new to DDD and I am working on a project where they implement DDD concepts.
I know that in DDD we have Entities and Value objects, and I know in case of value objects if we need to ensure their equality we should compare the properties' values of each value object.
We are using C# as programming language and I am wondering why we need to override GetHashCode() and Equals() methods ?
I mean why don't we only compare properties' values to compare value objects ?
In part it is because you want to design your classes in such a way that it's easy to use.
For example It's much easier to write/Lattitude
coordinate1.Equals(coordinate2)
Than if you write it like this
cooridate1.Latitude == coordinate2.Latitude && cooridnate2.Longitude == cooridnate2.Longitude
Also in a bit more complex cases it's just too easy to make mistakes. For example if you write
price.Amount == price2.Amount
now you forgot to include the currency. And for some value objects you might evan have non-obvious equalities
public override bool Equals(Currency c) {
//2 Kind of currency, dollars and cents.
var amount = this.Amount;
if (this.Currency != "Cents") amount = amount * 100;
var amount2 = c.Amount;
if (c.Currency != "Cents") amount2 = amount2 * 100;
return amount == amount2;
}
GetHashcode is used by things like HashSet and other algorithms that expect Equatable objects.
Tip of the day: Also provide the == and != methods for your value objects. By default they will check for referential equality and that's not what you case about in case of value objects. And it's just 1 source of errors that you just don't want to have happening.
Value objects don't have identity. They are defined by there attributes. Their attributes determine their identity. So we compare based on their attributes.
That is why when comparing value objects, that would require overriding the Equals and GetHashCode methods.
