Dashed rectangles appear solid when expanding Winform window.

Question

I'm experiencing an issue with GDI+ while custom painting dashed rectangles.

The vertical portion of dashed rectangles appear as solid, continuous lines when the window size is increased or when scrolling up/down. Moving the mouse faster results in fewer and fewer solid sections. Curiously the horizontal lines do not exhibit this behaviour and appear as expected.

So far two non-optimal solutions have been to set ResizeRedraw = true or to call Invalidate() during OnResize() and OnScroll(). I'd of course like to avoid this as what I am really drawing is more complex and these slow calls destroy the fluid experience. I've also tried invalidating only the newly shown area to no avail - only a full Invalidate seems to work.

Any pointers on how to work this out?

Demo code:

using System.Drawing;
using System.Drawing.Drawing2D;
using System.Windows.Forms;

public class Form1 : Form
{
    static void Main()
    {
        Application.Run(new Form1());
    }

    public Form1()
    {
        this.ClientSize = new System.Drawing.Size(472, 349);

        DoubleBuffered = true;
        //ResizeRedraw = true;
    }

    protected override void OnPaint(PaintEventArgs e)
    {
        base.OnPaint(e);

        int dimensions = 70;

        using ( Pen pen = new Pen(Color.Gray) )
        {
            pen.DashStyle = DashStyle.Dash;

            for ( int x = 0; x < 20; ++x )
            {
                for ( int y = 0; y < 20; ++y )
                {
                    Rectangle rect = new Rectangle(x * dimensions, y * dimensions, dimensions, dimensions);

                    e.Graphics.DrawRectangle(pen, rect);
                }
            }
        }
    }
}

Answer 1

I think there are two problems: there appears to be an area at the edge of the window where rectangles aren't drawn correctly; and you're drawing the rectangles over each other, so the dashing won't work properly.

Replace your OnPaint loop with the following:

   for (int y = 0; y < Height; y += dimensions)
   {
       e.Graphics.DrawLine(pen, 0, y, Width, y);
   }
   for (int x = 0; x < Width; x += dimensions)
   {
       e.Graphics.DrawLine(pen, x, 0, x, Height);
   }