public abstract class Entity
{
    public abstract Guid Id { get; }

    public override bool Equals(object obj)
    {
        if (obj == null)
        {
            return false;
        }

        if (obj.GetType() != GetType())
        {
            return false;
        }

        var entity = (Entity)obj;
        return (entity.Id == Id);
    }

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

}

public override bool Equals(object obj)
{
    if (obj == null)
    {
        return false;
    }

    if (obj.GetType() != GetType())
    {
        return false;
    }

    var entity = (Entity)obj;
    if ((Id == Guid.Empty) || (entity.Id == Guid.Empty))
    {
      return false;
    }
    return (entity.Id == Id);
}