Question

I’m now starting out on DDD, I’ve already found a nice implementation for ValueObject but I cant seem to find any good implementation for Entities, i want a generic base entity type which will have an ID(needed by the specification) and implement currect equality operations.

Whats the most elegent solution?

Answer 1

The only characteristic of an Entity is that it has a long-lived and (semi-)permanent identity. You can encapsulate and express that by implementing IEquatable<T>. Here’s one way to do it:

public abstract class Entity<TId> : IEquatable<Entity<TId>>
{
    private readonly TId id;

    protected Entity(TId id)
    {
        if (object.Equals(id, default(TId)))
        {
            throw new ArgumentException("The ID cannot be the default value.", "id");
        }

        this.id = id;
    }

    public TId Id
    {
        get { return this.id; }
    }

    public override bool Equals(object obj)
    {
        var entity = obj as Entity<TId>;
        if (entity != null)
        {
            return this.Equals(entity);
        }
        return base.Equals(obj);
    }

    public override int GetHashCode()
    {
        return this.Id.GetHashCode();
    }

    #region IEquatable<Entity> Members

    public bool Equals(Entity<TId> other)
    {
        if (other == null)
        {
            return false;
        }
        return this.Id.Equals(other.Id);
    }

    #endregion
}