public class IdRetriever {

	private IEnumerable<Person> people;

	public IdRetriever(IEnumerable<Person> people){
		this.people = people;
	}

	public IEnumerable<int> GetIds(){
		return GetTheIds()
			.Select(x=>(int)x.Id);
	}

	private IEnumerable<dynamic> GetTheIds(){
		return people.Select(x=>new {Id = x.Id});
	}

}