class Restaurant
    {
        public string Name { get; set; }
        public List<RestaurantItem> RestaurantItems { get; set; }
    }

class RestaurantItem
    {
        public string Name { get; set; }
    }

public partial class Form1 : Form
    {
        private List<Restaurant> restaurants;

        public Form1()
        {
            InitializeComponent();

            restaurants = new List<Restaurant>
            {
                new Restaurant
                {
                    Name = "Pierwsza",
                    RestaurantItems = new List<RestaurantItem>
                    {
                        new RestaurantItem {Name = "R1_Item1"},
                        new RestaurantItem {Name = "R1_Item2"},
                        new RestaurantItem {Name = "R1_Item3"}
                    }
                },
                new Restaurant
                {
                    Name = "Druga",
                    RestaurantItems = new List<RestaurantItem>
                    {
                        new RestaurantItem {Name = "R2_Item1"},
                        new RestaurantItem {Name = "R2_Item2"},
                        new RestaurantItem {Name = "R2_Item3"}
                    }
                },
                new Restaurant
                {
                    Name = "Trzecia",
                    RestaurantItems = new List<RestaurantItem>
                    {
                        new RestaurantItem {Name = "R3_Item1"},
                        new RestaurantItem {Name = "R3_Item2"},
                        new RestaurantItem {Name = "R3_Item3"}
                    }
                }
            };

            listBox1.DataSource = restaurants;
            listBox1.DisplayMember = "Name";

        }

        private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            var restaurantID = listBox1.SelectedIndex;

            listBox2.DataSource = restaurants[restaurantID].RestaurantItems;
            listBox2.DisplayMember = "Name";
        }
    }