ThenByDescending

using System;//ja  va 2s. c o m
using System.Linq;
using System.Collections;
using System.Collections.Generic;
public class MainClass {
    public static void Main() {

        string[] presidents = {"ant", "arding", "arrison", "eyes", "over", "Jackson"};

        IEnumerable<string> items =
          presidents.OrderBy(s => s.Length).ThenByDescending(s => s);

        foreach (string item in items)
            Console.WriteLine(item);
    }
}

using System;//  j a  va 2  s.co m
using System.Collections.Generic;
using System.Linq;
using System.Text;

public class CaseInsensitiveComparer : IComparer<string> {
    public int Compare(string x, string y) {
        return string.Compare(x, y, true);
    }
}

public class MainClass {
    public static void Main() {

        string[] words = { "a", "A", "b", "B", "C", "c" };

        var sortedWords =
            words.OrderBy(a => a.Length)
                    .ThenByDescending(a => a, new CaseInsensitiveComparer());

        foreach (var s in sortedWords) {
            Console.WriteLine(s);
        }
    }
}