using System;

using System.Text.RegularExpressions;

using System.Security.Cryptography;

public class Program

{

    public static void Main()

    {

        var bytes = new byte[40]; // byte size

        using (var crypto = new RNGCryptoServiceProvider())

            crypto.GetBytes(bytes);

​

         var base64 = Convert.ToBase64String(bytes);

         Console.WriteLine(base64);

​

        // In case you want to Prepend with a Guid:

        var result = Guid.NewGuid().ToString("N") + base64;

        Console.WriteLine(result);

​

        // Alphanumeric string:

        result = Regex.Replace(result,"[^A-Za-z0-9]","");

        Console.WriteLine(result);

    }

}