コレクションも配列もSortの規則を変えるにはラムダ式を使おう!

var l = new List<int>() { 100, 200 };
l.Sort((x, y) => (-x) - (-y));
Console.WriteLine(string.Join(",", l));

var ary = new[] { 100, 200 };
Array.Sort(ary, (x, y) => (-x) - (-y));
Console.WriteLine(string.Join(",", ary));