using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        UserDepartmentMapper.MapUsersToDepartments(null, null);

    }

}

​

 public static class UserDepartmentMapper

    {

        public static IEnumerable<(User User, List<Department> Departments)> MapUsersToDepartments(IEnumerable<User> users, IEnumerable<Department> departments)

        {

            if (users == null || departments == null)

            {

                return new List<(User, List<Department>)>();

            }

​

            var departmentDictionary = departments.ToDictionary(d => d.DepartmentId);

​

            return users.Select(user => (

                User: user,

                Departments: user.DepartmentIds

                    .Where(deptId => departmentDictionary.ContainsKey(deptId))

                    .Select(deptId => departmentDictionary[deptId])

                    .Where(dept => dept != null)

                    .ToList()

            )).ToList();

        }

    }

​

    public class User

    {

        public string Name { get; set; }

        public List<int> DepartmentIds { get; set; } = new List<int>();

        public User(string name)

        {

            Name = name;

        }

        public User(string name, int departmentId)

        {

            Name = name;

            DepartmentIds.Add(departmentId);

        }

        public User(string name, List<int> departments)

        {

            Name = name;

            DepartmentIds = departments;

        }

    }

​

    public class Department

    {

        public int DepartmentId { get; set; }

        public string Name { get; set; }

        public Department(int departmentId, string name)

        {

            DepartmentId = departmentId;

            Name = name;

        }

    }