using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var users = new List<User>

        {

            new User(1, "User1"),

            new User(2, "User2"),

            new User(3, "User3")

        };

        var userDictionary = users.ToDictionary(u => u.Id);

        var userLookup = users.ToLookup(u => u.Id, u => u);

        try

        {

            var userNotFound = userDictionary[4];

            Console.WriteLine($"Dictionary: user not found: {userNotFound}");

        }

        catch(Exception ex)

        {

            Console.WriteLine(ex.Message);

        }

        try

        {

            var userNotFound = userLookup[4];

            Console.WriteLine($"Lookup: user not found: {userNotFound}");

        }

        catch(Exception ex)

        {

            Console.WriteLine(ex.Message);

        }

    }

    public class User

    {

        public int Id { get; }

        public string Username { get; }

        public User(int id, string username)

        {

            Id = id;

            Username = username;

        }

    }

}