Is first character of MD5 hash evenly distributed?, C#

Language: Layout:

//It is!

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

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {            
            Dictionary<char, int> distribution = new Dictionary<char, int>();
            for(int i=0; i<100000; i++)
            {
                char first_letter = CalculateMD5Hash(RandomString(_rng.Next(5, 30))).ToLower()[0];
                if(distribution.ContainsKey(first_letter))
                {
                    distribution[first_letter]++;
                }
                else                    
                {
                    distribution[first_letter] = 1;
                }                
            }            
            foreach(var k in distribution.Keys.OrderBy(f => f))
            {
                Console.WriteLine("{0}: {1}", k, distribution[k]);
            }
        }
        
        private static readonly Random _rng = new Random();
        private static string _chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        
        private static string RandomString(int size)
        {
            char[] buffer = new char[size];
        
            for (int i = 0; i < size; i++)
            {
                buffer[i] = _chars[_rng.Next(_chars.Length)];
            }
            return new string(buffer);
        }
        
        private static string CalculateMD5Hash(string input)
        {
            var md5 = MD5.Create();
            byte[] inputBytes = Encoding.Unicode.GetBytes(input);
            byte[] hash = md5.ComputeHash(inputBytes);
         
            // step 2, convert byte array to hex string
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < hash.Length; i++)
            {
                sb.Append(hash[i].ToString("X2"));
            }
            return sb.ToString();
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0.33 sec, absolute running time: 2.53 sec, cpu time: 2.68 sec, average memory usage: 37 Mb, average nr of threads: 6