using System;

using System.Collections.Generic;

using System.Text;

​

public class Program

{

    public static void Main()

    {

            var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890".ToCharArray();

            Random rndm = new Random();

            List<string> passcodesList = new List<string>();

​

            int numberOfInvigilators = 1000000000;

            int passcodeLength = 5;

​

​

            int iterations = 0;

​

            while(passcodesList.Count < numberOfInvigilators)

            {

                StringBuilder passcode = new StringBuilder();

​

                for (int x = 0; x < passcodeLength; x++)

                {

                    passcode.Append(chars[rndm.Next(0, chars.Length)]);

                }

​

                if (passcodesList.Contains(passcode.ToString())){

                    //passcodesList.Add(passcode.ToString());

                    Console.WriteLine("Duplicate found! this took : " + iterations + " iterations,  the code is : " + passcode.ToString());

                    Console.ReadLine();

                    //passcode.Clear();

                }

                else

                {

                    //Console.WriteLine(iterations);

                    passcodesList.Add(passcode.ToString());

                    //Console.WriteLine(passcode.ToString());

                }

​

                iterations++;

    }

    }

}

​