UserService.cs

using System;
using System.Collections.Generic;
using System.Text;
using System.Linq;

namespace ConsoleApp1
{
    public class UserService
    {
        public List<User> Users { get; set; }

        public UserService(List<User> users)
        {
            Users = users;
        }

        public List<User> GetUserOver18For()
        {
            var usersOver18 = new List<User>();
            foreach (var user in Users)
            {
                if (user.Age >= 18)
                {
                    usersOver18.Add(user);
                }
            }
            return usersOver18;
        }

        public List<User> GetUsersOver18Linq()
        {
            return Users.Where(User => User.Age >= 18).ToList();
        }

        public bool LoginTrue(string username, string password)
        {
            //foreach (var user in Users)
            //{
            //    if(user.Username==username && user.Password == password)
            //    {
            //        return true;
            //    }
            //}
            //return false;
            var searchuser = Users.FirstOrDefault(user => user.Username == username && user.Password == password);
            if (searchuser != null)
            {
                return true;
            }
            return false;
        }

        public bool Exist(string username)
        {
            var searchUser = Users.FirstOrDefault(user => user.Username == username);
            if (searchUser != null)
            {
                return true;
            }
            return false;
        }
    }
}