using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<int> list = new List<int> { 2, 1 } ;

        foreach (int i in list)

        {

            //list.Sort(Comparer<int>.Default);     // InvalidOperationException

            //list.Sort();                          // InvalidOperationException

            list.Sort((a, b) => a.CompareTo(b));    // No exception

            Console.WriteLine(i);

        }

    }

}