Should I use the Primary Key field when comparing two DDD Entities using: .Equals

Question

Say I have a class like this:

public abstract class Product 
    {
        public Guid id {get; set;}
        public string description {get; set;}
    }

This class is populated from the database. I am writing a unit test to decide if two products are the same. How do I establish if two entities are equal:

1) Guid only - this is the primary key from the database so is unique. Not even sure that this member should be in my class

2) Description only - this is always unique

3) GUID and Description

Answer 1

When two objects are semantically the same, a comparison with .equals() should lead to a positive result. Note, that it doesn't have to be the exact same object/instance.

In a database an entity is defined by its primary key, but I think it is still possible to have two different entities, which are semantically equal. So when comparing them with .equals(), I'd expect a positive result.

Therefore, I'd go with option 2). However, it is difficult to answer this in general, because you always have to think about when two object are equal, which might vary depending on the domain you are in. So in general, this should be also part of your requirements engineering.

Answer 2

In DDD, two objects refer to the same entity if they have the same ID; so a valid question is this: how to test if two C# objects refer to the same entity?.

How to check if two entities are equal? is not really a valid question, at least not a generic one. What means for you domain that two entities are equal? Please note that this question is a perfect valid one in case of value objects but a weird one in case of entities.