testRandomString

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

namespace testRandomString
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        string GenRandomString(string Alphabet, int Length)
        {
            //string Ret = "";
            Random rnd = new Random();
            StringBuilder sb = new StringBuilder(Length-1);
            int Position = 0;

            for (int i = 0; i < Length; i++)
            {
                Position = rnd.Next(0, Alphabet.Length-1);
                sb.Append(Alphabet[Position]);
                //Ret = Ret + Alphabet[Position]; //- так делать не стоит, в данном случае StringBuilder дает явный выигрыш в скорости
            }

            //return Ret;

            return sb.ToString();
        }
        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            DateTime start = DateTime.Now;

            txtReturn.Text = GenRandomString("QWERTYUIOPASDFGHJKLZXCVBNMqwertyuiopasdfghjklzxcvbnm.", 500000);

            DateTime end = DateTime.Now;


            MessageBox.Show((end  - start).ToString());

        }
    }
}