using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        Dictionary<string, string> dic = new Dictionary<string, string>();

        dic.Add("10", "a");

        dic.Add("20", "b");

        foreach (var key in dic.Keys)

            Console.WriteLine(key + " " + dic[key]);

        dic["10"] = "C";

        dic["20"] = "D";

        foreach (var key in dic.Keys)

            Console.WriteLine(key + " " + dic[key]);

        var list = new List<Tuple<string, string>>();

        list.Add(new Tuple<string, string>("10", "a"));

        list.Add(new Tuple<string, string>("20", "b"));

        foreach (var item in list.OrderBy(x => x.Item1))

            Console.WriteLine(item);

        foreach (var item in list.OrderByDescending(x => x.Item1))

            Console.WriteLine(item);

    }

}