public class Database
    {
        public Database()
        {
            this.Users = new HashSet<IUser>();
        }

        public ICollection<IUser> Users { get; set; }

        public IUser RegisterUser(IUser user)
        {
            bool IsUserNameTaken = this.Users.Any(x => x.Username == user.Username);

            if (!IsUserNameTaken)
            {
                this.Users.Add(user);
            }
            return user;
        }

        public int GetGetRegisteredUsersCount()
        {
            return this.Users.Where(x => x.IsDeleted == false).Count();
        }

        public bool DeleteUser(string username)
        {
            IUser user = this.GetUser(username);

            if (user == null)
            {
                return false;
            }
            user.IsDeleted = true;
            return true;
        }

        public IUser GetUser(string username)
        {
            IUser user = this.Users.Where(x => x.IsDeleted == false).FirstOrDefault(x => x.Username == username);
            return user;
        }
    }