string code = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
Random rd = new Random();
List<string> list = new List<string>();
for (int i = 0; i < 6; i++)
{
list.Add(code[rd.Next(code.Length)].ToString());
}
list.Sort();
Console.WriteLine(String.Join("", list.ToArray()));