ArrayList arl = new ArrayList();arl.Add(new Person(){ID= .......});arl.Add(new Person(){ID= .......});arl.Add(new Person(){ID= .......});Person person = arl.Where(p => p.ID == ID).FirstOrDefault(); //Geht nicht.
Person person = arl.OfType<Person>().Where(p => p.ID == ID).FirstOrDefault();
Person person = arl.OfType<Person>().Where(p => p.ID == ID).FirstOrDefault();