using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        Dictionary<string, List<int>> x = new Dictionary<string, List<int>>();

        x.Add("key", new List<int>());

        x["key"].Add(1);

        x.TryGetValue("key", out List<int> y);

        y.Add(2);

        x["key"].ForEach((L) => {

            Console.WriteLine(L);

        });

    }

}

​