using System;

using System.Linq;

using System.Text;

​

public class Program

{

    public static void Main()

    {

        StringBuilder builder = new StringBuilder();

        Enumerable

           .Range(65, 26)

            .Select(e => ((char)e).ToString())

            .Concat(Enumerable.Range(97, 26).Select(e => ((char)e).ToString()))

            .Concat(Enumerable.Range(0, 10).Select(e => e.ToString()))

            .OrderBy(e => Guid.NewGuid())

            .Take(6)

            .ToList().ForEach(e => builder.Append(e));

        Console.WriteLine(builder.ToString().ToUpper());

    }

}