count-letter-frequencies, C#

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//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;
using System.IO;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            //Your code goes here
            int[] c = new int[(int)char.MaxValue];

// 2.
        // Read entire text file.
        string s = "43565456555465489hjgfhgjghjghjghsdllllffssafg5567435438704545";

// 3.
        // Iterate over each character.
        foreach (char t in s)
        {
            // Increment table.
            c[(int)t]++;
        }

// 4.
        // Write all letters found.
        for (int i = 0; i < (int)char.MaxValue; i++)
        {
            if (c[i] > 0 &&
                char.IsLetterOrDigit((char)i))
            {
                Console.WriteLine("Letter: {0}  Frequency: {1}",
                    (char)i,
                    c[i]);
            }
        }
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,12 sec, absolute running time: 0,09 sec, cpu time: 0,06 sec, average memory usage: 14 Mb, average nr of threads: 3