Sort characters descending

Given a string, write a method that returns array of chars (ASCII characters) sorted in descending order.
Expected input and output
SortCharactersDescending("onomatopoeia") → tpoooonmieaa SortCharactersDescending("fohjwf42os") → wsoojhff42
using System;

namespace CSharpExercises.Exercises.Strings
{
    class SortCharactersDescendingTask
    {
        static char[] SortCharactersDescending(string str)
        {
            char[] charArray = str.ToCharArray();
            char ch;

            for (int i = 1; i < str.Length; i++)
            {
                for (int j = 0; j < str.Length - 1; j++)
                {
                    if (charArray[j] < charArray[j + 1])
                    {
                        ch = charArray[j];
                        charArray[j] = charArray[j + 1];
                        charArray[j + 1] = ch;
                    }
                }
            }

            return charArray;
        }

        static void Main(string[] args)
        {
            Console.WriteLine(SortCharactersDescending("Aliquam pulvinar aliquam libero, in fringilla erat.")); // vuuutrrrrqqponnnmmlllllliiiiiiigfeebaaaaaaA.,            
            Console.WriteLine(SortCharactersDescending("Thi2 12  5@mpl3 Str!nG~"));                             // ~trpnmlihTSG@53221!
        }
    }
}