using System.Collections.Generic;

using System.Linq;

using static System.Console;

using System;

using System.Text;

public class Program

{

    public static void Main()

    {

        int L = 100000;

        string testString = String.Concat(Enumerable.Repeat<string>("qwertyuiopasdfghjklzxcvbnm", 30));

        for( int i = 0;  i < L; i++)

        {   

        Transform(testString);      

        }

        Console.WriteLine("Done");

    }

    public static IEnumerable<string> Transform(string str)

    {

        var _str = new StringBuilder(str.ToLower());

        for( int i = 0; i < str.Length; i++)

        { 

            _str[i] = char.ToUpper(str[i]);

            yield return _str.ToString();

            _str[i] = char.ToLower(str[i]);

        }

    }

}