Diccionario con lista, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            Dictionary<string, List<int>> dic = new Dictionary<string, List<int>>();

dic.Add("1", new List<int> { 1, 2, 3 });
            dic.Add("2", new List<int> { 4, 5, 6 });
            dic.Add("3", new List<int> { 7, 8, 9 });

dic.FirstOrDefault(t => t.Key == "1").Value.Add(10);
            dic.FirstOrDefault(t => t.Key == "1").Value.Add(11);
            dic.FirstOrDefault(t => t.Key == "1").Value.Add(11);
            
            foreach(var item in dic){
                Console.WriteLine("Llave string: " + item.Key);

foreach (var valuesList in item.Value)
                {
                    Console.WriteLine("Valores en lista: " + valuesList.ToString());
                }
            }
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,16 sec, absolute running time: 0,11 sec, cpu time: 0,11 sec, average memory usage: 13 Mb, average nr of threads: 3