using static System.Console;

using System.Collections.Generic;

​

public class Program {

    public static void Main() {

        var ints = new List<int> { 1, 2, 3 };

        var lists = new List<List<int>> { new List<int> { 1, 2, 3 }, new List<int> { 4, 5, 6 } };

        foreach (var item in ints) {

//          item = 9;

        }

        foreach (var item in lists) {

            item[0] = 9;

        }

        foreach (var item in lists) {

//          item = new List<int> { 7, 8, 9 };

        }

        foreach (var lista in lists) {

            foreach (var item in lista) {

                WriteLine(item);

            }

        }

    }

}