using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var someString = "This is the string they would enter";

        var randomizer = new Random();

        var final = someString.Select(x => randomizer.Next() % 2 == 0 ? (char.IsUpper(x) ? x.ToString().ToLower().First() : x.ToString().ToUpper().First()) : x);

        var randomUpperLower = new string (final.ToArray());

        Console.WriteLine(randomUpperLower);

    }

}