Draw lines over existing image in picture box

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Hi all,
I want to draw lines over an existing image in a picturebox (VS2008, .net3.5). No issues as far as drawing it once is concerned. But I want these lines to be shifted to the mouse position as I move in the picture box. So I wrote the following code in the PictureBox_MouseMove event:-

Graphics g = pictureBox1.CreateGraphics();
            g.Clear(Color.Transparent);
            Pen p = new Pen(Color.Red, 2.0f);

            for (int n = 1; n <= 50; n++)
            {
                g.DrawLine(p, n * (Cursor.Position.X), Cursor.Position.Y - 30.0f, n * (Cursor.Position.X), Cursor.Position.Y + 30.0f);

            }

The code is working fine when there is no image in the PictureBox. Whern there is an image in the picturebox, the lines are getting drawn, but before that the image already in the picturebox is getting erased (g.Clear(Color.Transparent)).
I tried to keep the image in a panel and run the code in Panel_MouseMove event, but to no avail.
How do I draw these lines while keeping the existing image intact?
GeoNav

Posted 4-Aug-10 7:00am

GeoNav

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OriginalGriff · Answer 1 · 2010-08-04T08:38:00

Firstly, when you create a graphics item, you are responsible for destroying it. They are a (very) finite resource, and you must not create them, use them and discard them as the whole system will run out, well before the GC gets round to disposing of them for you. The best way is to put them in a using block:

using(Graphics g = pictureBox1.CreateGraphics())
   {
   g.Clear(Color.Transparent);
   Pen p = new Pen(Color.Red, 2.0f);

   for (int n = 1; n <= 50; n++)
       {
       g.DrawLine(p, n * (Cursor.Position.X), Cursor.Position.Y - 30.0f, n * (Cursor.Position.X), Cursor.Position.Y + 30.0f);

       }
   }

Secondly, don't do what you are trying to do. A Picture box is a dumb animal, and when you manually erase it's content as you do in the example you gave it doesn't know to re-draw it. You can force it to redraw it's picture, but then it will overwrite your lines.
Instead, use a Panel, and handle the Paint event. Draw the picture yourself - using the Graphics property of the PaintEventArgs "e" so you don't have to worry about destroying it - then on the same graphics context draw your lines. Simples!