C# For loop 几秒后恒速立即完成

Question

I have this codo for draw rectangles into ImageBox one by one.

        g = Graphics.FromImage(pictureBox1.Image);
        int size = 40;
        int squares = 0;

        for (int H = 0; H < 1490; H += size)
        {
            for (int W = 0; W < 1490; W += size)
            {
                var rectangle = new Rectangle(W, H, size, size);
                g.DrawRectangle(new Pen(Color.Green, 3), rectangle);
                this.Refresh();
                squares++;
                label1.Text = squares.ToString();
            }
        }

In Visual Studio everything works as it should do, but when I build it works for first 300 ~ rectangles and then lag and instantly complete.

I figure that Refresh() stop working.

Answer 1

Using Refresh(); is slow and not effective. Rather use something pictureBox1.Image = pictureBox1.Image;. I used StopWatch with while loop to wait few ms for slowing loop and setting fixed interval even if loop takes more time that others.

Thanks @jdweng (good point) and @TaW (lots of good informations).