Playfair cipher. Why is the index out of bounds?

Question

Programming

Playfair encryption

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication6
{
    class Program
    {
        static int t, x1, x2, y1, y2;
        static string temp;
        const int MaxX = 6;//столбцы
        const int MaxY = 5;//строки
        static string URusa = "абвгдежзиклмнопрстуфхцчшщъыэюя";
        //string[] URusa2 = URusa.Split(',');
        static char[,] array = new char[MaxY, MaxX];

        public static void SimbolPoc(char s, out int x, out int y)
        {
            x = 0;
            y = 0;
            for (var i = 0; i < MaxY; i++)
            {
                for (var j = 0; j < MaxX; j++)
                {
                    if (s == array[i, j])
                    {
                        x = j; y = i;
                        //Environment.Exit;
                    }
                }
            }
        }
        public static void Playfair_crypt(string s, string key)
        {
            key = key.ToLower();
            s = s.ToLower();
            //Удаляем из строки все символы, Не входящие в наш алфавит
            temp = "";
            for (var i = 0; i < s.Length; i++)
            {
                if (Convert.ToInt32(URusa.Contains(s[i])) != 0)
                {
                    temp += s[i];
                }
            }
            s = temp;

            //Создание ключевой матрицы, с использованием ключевого слова "key".
            temp = "";
            for (var i = 0; i < key.Length; i++)
            {
                if (Convert.ToInt32(temp.Contains(key[i])) == 0) //!!!!!!!!!!!!!!!!!!!!!
                    temp += key[i];
            }
            for (var i = 0; i < URusa.Length; i++)
            {
                if (Convert.ToInt32(temp.Contains(URusa[i])) == 0) //!!!!!!!!!!!!!!!!!!!!!
                    temp += URusa[i];
            }
            t = 0;
            for (var i = 0; i < MaxY; i++)
            {
                for (var j = 0; j < MaxX; j++)
                {
                    array[i, j] = temp[t];
                    t++;
                    Console.Write("{0} ", array[i, j]);
                }
                Console.WriteLine();
            }
            //Console.WriteLine(s);
            //просмотр строки по парам символов и вставка разделяющего символа
            //"ъ" в случае когда в паре попались одинаковые символы.


            for (var i = 0; i < s.Length / 2; i++)
            {
                if (s[2 * i] == s[2 * i + 1])
                {
                    s = s.Insert(2 * i + 1, "ъ");
                    
                }
            }

            if (s.Length % 2 == 1)
            {
                if (s[s.Length - 1] != 'ъ')
                    s = s + 'ъ';
                else
                    s = s + 'я';
            }
            //Console.WriteLine(s);
            Console.WriteLine(s);
            temp = "";

            for (var i = 0; i < s.Length / 2; i++)
            {
                SimbolPoc(s[2 * i], out x1, out y1);
                SimbolPoc(s[2 * i + 1], out x2, out y2);
                //Console.WriteLine(x1);
                //Правило1
                if (y1 == y2)
                {
                    x1++;
                    x2++;
                    if (x1 > MaxX) x1 = x1 - MaxX;
                    if (x2 > MaxX) x2 = x2 - MaxX;
                    temp = temp + array[y1, x1] + array[y2, x2];
                }/*
                //Правило2
                if (x1 == x2)
                {
                    y1++;
                    y2++;
                    if (y1 > MaxY) y1 = y1 - MaxY;
                    if (y2 > MaxY) y2 = y2 - MaxY;
                    temp = temp + array[y1, x1] + array[y2, x2];
                }
                //Правило3
                if ((x1 != x2) && (y1 != y2)) temp = temp + array[y1, x2] + array[y2, x1];*/
            }
            Console.WriteLine(temp);
        }

        static void Main(string[] args)
        {
            Playfair_crypt("Россия", "Форум");
        }
    }
}

Gives an error on the Rule1 line - temp = temp + array[y1, x1] + array[y2, x2];- out of bounds, until this step everything seems to be doing correctly. Accordingly, other rules are also commented out. When we get to this line, and if we remove s = , we
just leave s.Insert.
indexes apparently ..
s = s.Insert(2 * i + 1, "ъ");
And when we directly reach the rearrangement of letters in the matrix, it bullies (Help!

Answer 1

Why aren't you using a debugger?