Character Frequency Counter with a Dictionary

var counts = CountCharacters("hello world");

foreach (var (character, count) in counts.OrderByDescending(p => p.Value).ThenBy(p => p.Key))
    Console.WriteLine($"'{character}': {count}");

// O(n) time. GetValueOrDefault returns 0 for a character that has not been seen yet.
static Dictionary<char, int> CountCharacters(string text)
{
    var counts = new Dictionary<char, int>();
    foreach (char c in text)
    {
        if (c == ' ') continue;
        counts[c] = counts.GetValueOrDefault(c) + 1;
    }
    return counts;
}