Revise paste LINQ_07

namespace LINQ_07
{
    internal class Program
    {
        static void Main(string[] args)
        {
            Barracks barracks = new Barracks();
            
            barracks.UnionDetachment();
        }
    }

public class Soldier
    {
        public Soldier(string lastname, string firstname, string rank)
        {
            LastName = lastname;
            FirstName = firstname;
            Rank = rank;
        }

public string LastName { get; }
        public string FirstName { get; }
        public string Rank { get; }
    }

public class Barracks
    {
        private List<Soldier> _firstDetachment = new List<Soldier>() 
        {
            new Soldier("Иванов", "Сергей", "Рядовой"),
            new Soldier("Кузнецов", "Дмитрий", "Сержант"),
            new Soldier("Смирнов", "Алексей", "Рядовой"),
            new Soldier("Петров", "Иван", "Ефрейтор"),
            new Soldier("Борисов", "Виктор", "Рядовой"),
            new Soldier("Баранов", "Петр", "Ефрейтор"),
            new Soldier("Соколов", "Максим", "Сержант"),
            new Soldier("Быков", "Роман", "Старшина"),
            new Soldier("Попов", "Андрей", "Лейтенант"),
            new Soldier("Богданов", "Юрий", "Полковник"),
        };

private List<Soldier> _secondDetachment = new List<Soldier>()
        {
            new Soldier("Волков", "Игорь", "Капитан"),
            new Soldier("Белов", "Анатолий", "Лейтенант"),
            new Soldier("Михайлов", "Николай", "Майор"),
            new Soldier("Беляев", "Александр", "Рядовой"),
            new Soldier("Новиков", "Артем", "Рядовой"),
            new Soldier("Бобров", "Денис", "Капитан"),
            new Soldier("Федоров", "Олег", "Старшина"),
            new Soldier("Морозов", "Владимир", "Полковник"),
            new Soldier("Лебедев", "Павел", "Сержант"),
            new Soldier("Бондарев", "Кирилл", "Сержант")
        };

public void UnionDetachment()
        {
            _secondDetachment = _secondDetachment.Union(_firstDetachment.Where(soldier => soldier.LastName.ToUpper().StartsWith("Б"))).ToList();

foreach(var soldier in _secondDetachment)
            {
                Console.WriteLine($"Фамилия: {soldier.LastName}, Имя: {soldier.FirstName}, Звание: {soldier.Rank}");
            }
        }
    }
}