using System;

using System.Security.Cryptography;

public class Program

{

    private static readonly char[] chars =

        "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789".ToCharArray();

    public static string GenerateReservationReference(int length = 12)

    {

        String str = RandomNumberGenerator.GetString(chars, length);

        return str;

    }

​

    public static void Main()

    {

        string reservationReference = GenerateReservationReference();

        Console.WriteLine(reservationReference);

    }

​

}