using System;

using System.Linq;

using System.Security.Cryptography;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine(Get(8));

    }

    private const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

    public static string Get(int length)

        {

            var min = 0;

            var max = chars.Length - 1;

            var randomCode = new string(Enumerable.Repeat(chars, length).Select(s => s[GetRandomInteger(min, max)]).ToArray());

            return "REF" + randomCode;

        }

​

        public static int GetRandomInteger(int min, int max)

        {

            var scale = uint.MaxValue;

            var rand = new RNGCryptoServiceProvider();

            while (scale == uint.MaxValue)

            {

                // Get four random bytes.

                var four_bytes = new byte[4];

                rand.GetBytes(four_bytes);

​

                // Convert that into an uint.

                scale = BitConverter.ToUInt32(four_bytes, 0);

            }

​

            // Add min to the scaled difference between max and min.

            return (int)(min + ((max - min) * (scale / (double)uint.MaxValue)));

        }

}