Shown doesn't work

Question

public void GridCreate()
    {
        Graphics g = pictureBox1.CreateGraphics();
        SolidBrush brushBlack = new SolidBrush(Color.Black);
        Rectangle[,] block = new Rectangle[16, 16];

        for (int i = 0; i <= block.GetLength(0) - 1; i++)
        {
            for (int n = 0; n <= block.GetLength(0) - 1; n++)
            {
                block[n, i] = new Rectangle(i * blockSize, n * blockSize, 20, 20);
                g.FillRectangle(brushBlack, block[n, i]);
            }
        }
        data.block = block;
    } 
private void Form1_Shown(object sender, EventArgs e)
        {
            GridCreate();
        }

I'm trying to make a grid in WindowsForms using PictureBox, but related code is not working correctly.This data.block = block; part works, but this g.FillRectangle(brushBlack, block[n, i]); doesn't work at all. I think the problem is in the Form1_Shown event, because this:

private void Form1_Click(object sender, EventArgs e)
    {
        GridCreate();
    }

executes perfectly fine.

Override protected override void OnShown(EventArgs e) gives the same result as Form1_Shown.

Answer 1

The problem is CreateGraphics(), which is a temporary surface that gets erased when the PictureBox refreshes itself.

Just create the grid once, then draw the data in the Paint() event:

    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);

        GridCreate();
        pictureBox1.Paint += pictureBox1_Paint;
    }

    private void GridCreate()
    {
        Rectangle[,] block = new Rectangle[16, 16];
        for (int i = 0; i < block.GetLength(1); i++) // this is the 2nd dimension, so GetLength(1)
        {
            for (int n = 0; n < block.GetLength(0); n++) // this is the 1st dimension, so GetLength(0)
            {
                block[n, i] = new Rectangle(i * blockSize, n * blockSize, 20, 20);
            }
        }
        data.block = block;
    }

    void pictureBox1_Paint(object sender, PaintEventArgs e)
    {
        Graphics g = e.Graphics; // use the SUPPLIED graphics, NOT CreateGraphis()!
        for (int i = 0; i < data.block.GetLength(1); i++) // this is the 2nd dimension, so GetLength(1)
        {
            for (int n = 0; n < data.block.GetLength(0); n++) // this is the 1st dimension, so GetLength(0)
            {
                g.FillRectangle(Brushes.Black, data.block[n, i]);
            }
        }
    }