Criar coisas aleatoriamente

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace NumerosAleatorias
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            lista.Items.Clear();

            for(int m= 0; m<10; m++){
                lista.Items.Add(CriarCodigo(m));
            }





            //criar numeros aleatorios de 0 a 99
            //Random gerador = new Random();


            //for (int m = 0; m < 20; m++)
            //{
            //    int valor = gerador.Next(0,100);
            //    lista.Items.Add(valor.ToString());

            //}
        }

        private string CriarCodigo(int seed, int numeroCarateres = 30)
        {
            //gerar string com carateres aleatorios
            StringBuilder str = new StringBuilder();

            string carateres = "ABCDEFGHIJKLMNOPKRSTUVWXYZabcdefghijklmnopkrstuvwxy";
            Random r = new Random(seed);
            int n = r.Next();

            for (int vezes = 0; vezes < numeroCarateres; vezes++)
            {
                Random rnd = new Random(DateTime.Now.Millisecond + seed + n);

                int x = rnd.Next(carateres.Length);
                str.Append(carateres[x]);
                n += 33;
            }


                //Devolve str
                return str.ToString();
        }
    }
}