Question

I am experimenting and learning C# by coding the game of LIFE. Currently I have a pictureBox and a function drawGrid that creates a grid overlay. In order to click in each cell inside the pictureBox I have implemented the property pictureBox1_MouseClick in which there is an if decision logic to know if a cell is selected or not. The issues I am running into comes when i click the squares quickly i get an Error: System.IndexOutOfRangeException which points to fill_in[x, y] = !fill_in[x, y];.

How can I increase the clicking accuracy of the pictureBox1_MouseClick event so I dont get that error?

Specific error:

`An unhandled exception of type ‘System.IndexOutOfRangeException’ occurred in life.exe

Additional information: Index was outside the bounds of the array.`

Code

namespace life
{
    public partial class Form1 : Form
    {

        Graphics paper;
        bool[,] fill_in = new bool[450, 450];
    int cellSize = 10;


 private void drawGrid()
        {
            int numOfCells = 100;        
            Pen p = new Pen(Color.Blue);
            paper.Clear(Color.White);

            for (int i = 0; i < numOfCells; i++)
            {   
                // Vertical Lines
                paper.DrawLine(p, i * cellSize, 0, i * cellSize, numOfCells * cellSize);
                // Horizontal Lines
                paper.DrawLine(p, 0, i * cellSize, numOfCells * cellSize, i * cellSize);
            }
        }


 private void pictureBox1_MouseClick(object sender, MouseEventArgs e)
        {
            int x = cellSize * (e.X / cellSize);
            int y = cellSize * (e.Y / cellSize);

            // Reverse the value of fill_in[i, j] - if it was false, change to true,
            // and if true change to false
            fill_in[x, y] = !fill_in[x, y];

            if (fill_in[x, y])
            {
                // Fill grid square with the filled color
                paper.FillRectangle(Brushes.Red, x, y, 10, 10);
            }
            else
            {
                // Fill grid square with unfilled color 
                paper.FillRectangle(Brushes.White, x, y, 10, 10);          
            }

        }

     }
}

Answer 1

Please make sure your calculations return desired values:

int x = cellSize * (e.X / cellSize);
int y = cellSize * (e.Y / cellSize);

In this scenario x and y can be outside the boundaries of your array. Example: cellsize * (450 / cellsize) equals 450 and this causes the error since allowed indices range from 0 to 449.

EDIT:

To fix this problem (and this is a really dirty and temporary fix), please change your code as follows:

int x = cellSize * (e.X / cellSize);
int y = cellSize * (e.Y / cellSize);

x = x >= 450 ? 450 - 1 : x;
y = y >= 450 ? 450 - 1 : y;

More desirable and actually a lot smarter solution would be simply to adjust your table’s size to be exactly the same as the range of e.X and e.Y.

Hope this helps,
Piotr