Sizeable seed based ID

ShortIds.cs

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

namespace HashTime
{
    class Program
    {
        private static readonly Dictionary<string, string> SeedBox = new Dictionary<string, string>();
        private static readonly Random Rnd = new Random();
        static string GetNum(int length) => string.Join(string.Empty, Enumerable.Range(0, length).Select(number => Rnd.Next(0, 9).ToString()));

        static void Main(string[] args)
        {
            var did = Guid.NewGuid().ToString();
            int i = 3_000_000;
            while (i-- > 0)
            {
                TestShortCode(GetNum(6) + "_" + GetNum(9) + "test" + "test" + did);
            }

            Console.WriteLine("boop");
        }
        
        /// <summary>
        /// Produces a fixed length upper case string based on the reduced byte output of the hash
        /// </summary>
        /// <param name="input"></param>
        /// <param name="maxLength"></param>
        /// <returns></returns>
        private static string SHA1HexHash(string input, int maxLength = 40)
        {
            using (SHA1 sha1 = SHA1.Create())
            {
                var hash = sha1.ComputeHash(Encoding.UTF8.GetBytes(input)).AsSpan().Slice(start: 0, length: maxLength);
                
                var sb = new StringBuilder(maxLength);
                for (var index = 0; index < maxLength; index++)
                {
                    sb.Append(hash[index].ToString("X2"));
                }

                return sb.ToString();
            }
        }
		
        /// <summary>
        /// Produces a mix case hash based on a slice of the Base64 Encoded hash
        /// </summary>
        /// <param name="input"></param>
        /// <param name="maxLength"></param>
        /// <returns></returns>
		private static string SHA1B65Hash(string input, int maxLength = 40)
        {
            using (SHA1 sha1 = SHA1.Create())
            {
                return Convert.ToBase64String(sha1.ComputeHash(Encoding.UTF8.GetBytes(input))).Substring(0,maxLength);
            }
        }

        public static void TestShortCode(string seed)
        {
            var muhSeed = SHA1B65Hash(seed, 9);
			      //var muhSeed = SHA1HexHash(seed, 9);
            
            try
            {
                if (SeedBox.ContainsKey(muhSeed))
                {
                    var matchedSeed = SeedBox.GetValueOrDefault(muhSeed);
                    if (seed != matchedSeed)
                    {
                        Console.WriteLine("Collision on " + seed + " existed for " + matchedSeed);
                    }
                }
                else
                {
                    SeedBox.Add(muhSeed, seed);
                }
            }
            catch
            {
                Console.WriteLine("Error handling MuhSeed");
            }
        }
    }
}