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 = 50000;

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

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

        {   

        Transform(testString);

        }          

        Console.WriteLine("Done");

    }

    public static string[] Transform(string str)

    {

    var strs = new List<string>();

    var sb = new StringBuilder();

​

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

    {

        sb.Clear();

        sb.Append(str);

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

        strs.Add(sb.ToString());

    }

    return strs.ToArray();

    }

}