using System;

using System.Text;

public static class StringExtensions

    {        

        public static string ReverseWithKey(this string source)

        {

            if (String.IsNullOrWhiteSpace(source) || source.Length < 2)

                return source;

​

           int sizeOfArr = source.Length;

            StringBuilder sb = new StringBuilder(sizeOfArr);

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

            {                

                sb.Append(source[sizeOfArr - i - 1]);

            }

            sb[1] = 'x';

            return sb.ToString();

        }

    }

public class Program

{

    public static void Main()

    {

        string source = "abcd";

        Console.WriteLine(source.ReverseWithKey());

    }

}