Caracteres repetidos, C#

Language: Layout:

//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            string cadena = "HHhhOla, soy una cadena con caracteres repetidos";
        
            Console.WriteLine(cadena + "\n\n");
        
            var result = from a in cadena.ToUpper().ToCharArray().Where(Char.IsLetter)
                         group a by a into g
                         select new
                         {
                             Letra = g.Key,
                             Repeticiones = g.Count()
                         };
        
        
            foreach (var item in result)
            {
                Console.WriteLine(string.Format("El caracter {0} apareix {1} vegades", item.Letra, item.Repeticiones));
            }
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,16 sec, absolute running time: 0,09 sec, cpu time: 0,09 sec, average memory usage: 14 Mb, average nr of threads: 3