using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var source = new List<int>(){

            1,2,3,4,5

        };

source.Sort(new SortComparer());

        foreach(var i in source){

        Console.WriteLine(i);   

        }

    }

}

​

public class SortComparer : IComparer<int> {

        public int Compare(int x, int y) {

        if(x>y){

        return 1;

        }else return -1;

        }

    }