How to fix error in VS2019 C# "Index was out of array bounds."?

Question

C++ / C#

public partial class Form1 : Form
    {
        const int block_size = 3;
        const int size_button = 50;
        public int[,] map = new int[block_size * block_size, block_size * block_size];
        public Form1()
        {
            InitializeComponent();
            GenerateMap();
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }
        public void GenerateMap()
        {
            for (int i = 0; i < block_size * block_size; i++)
            {
                for (int j = 0; j < block_size * block_size; j++)
                {
                    map[i, j] = (i * block_size + i / block_size + j) % (block_size * block_size) + 1;
                }
            }
            MatrixTransposition();
            SwapRowsInBlock();
            SwapColumnsInBlock();
            CreateMap();
        }



            public void  SwapRowsInBlock()
        {
            Random rand = new Random();
            var block = rand.Next(0, block_size);
            var row1 = rand.Next(0, block_size);
            var line1 = block * block_size * row1;
            var row2 = rand.Next(0, block_size);
            while (row1 == row2)
                row2 = rand.Next(0, block_size);
            var line2 = block * block_size * row2; ;
            for (int i = 0; i < block_size * block_size; i++)
            {

                    var temp = map[line1, i];        // тут пишет ошибку "Индекс находился вне границ массива."
                    map[line1, i] = map[line2, i];
                    map[line2, i] = temp;
            }
        }
        

        public void SwapColumnsInBlock()
        {
            Random rand = new Random();
            var block = rand.Next(0, block_size);
            var row1 = rand.Next(0, block_size);
            var line1 = block * block_size * row1;
            var row2 = rand.Next(0, block_size);
            while (row1 == row2)
                row2 = rand.Next(0, block_size);
            var line2 = block * block_size * row2; ;
            for (int i = 0; i < block_size * block_size; i++)
            {
                var temp = map[i, line1];
                map[i, line1] = map[ i, line2];
                map[i, line2] = temp;
            }
        }

        public void MatrixTransposition()
        {
            int[,] tmap = new int[block_size * block_size, block_size * block_size];
            for (int i = 0; i < block_size * block_size; i++)
            {
                for (int j = 0; j < block_size * block_size; j++)
                {
                    tmap[i, j] = map[j, i];
                }
            }
            map = tmap;
        }
        public void CreateMap()
        {
            for (int i = 0; i < block_size * block_size; i++)
            {
                for (int j = 0; j < block_size * block_size; j++)
                {
                    Button button = new Button();
                    button.Size = new Size(size_button, size_button);
                    button.Text = map[i, j].ToString();
                    button.Location = new Point(j * size_button, i * size_button);
                    this.Controls.Add(button);
                }
            }
        }
    }
}

Answer 1

The maximum possible value of line1 and line2 is pow(block_size,3) and the map size is pow(block_size,2). Naturally, the value overflows.