public class PersonOrderByAgeSpecification : OrderSpecification<Person>
{
    public PersonOrderByAgeSpecification(Sort direction= Sort.Ascending) : base(direction)
    {
    }

    public override Expression<Func<Person, IComparable>> AsExpression()
    {
        return personOrder => personOrder.Age;
    }
}

var orderSpecification = new PersonOrderByAgeSpecification(Sort.Ascending);
 var sortedPeople=  _dbContext.People.OrderBy(orderSpecification);

public override Expression<Func<Person, IComparable>> AsExpression()
{
    return personOrder => personOrder.FirstName;
}