How to draw an updating line

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You need to have two drawing calls:

  • One for the non-persistent line that follows the cursor in the MouseMove using someControls.CreateGraphics

  • the other for the persisting line, triggered in the MouseUp, where

    • you store the coordinates and
    • call Invalidate on your canvas control and
    • draw in the Paint event of your canvas using its e.Graphics object.

Here is a minimal example code:

List<Point> allPoints = new List<Point>();
Point mDown = Point.Empty;

private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
{
    mDown = e.Location;
}

private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
{
    allPoints.Add(e.Location);
    pictureBox1.Invalidate();
}

private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
{
    if (e.Button.HasFlag(MouseButtons.Left))
    {
        pictureBox1.Refresh();
        using (Graphics G = pictureBox1.CreateGraphics())
            G.DrawLine(Pens.Red, mDown, e.Location);
    }
}

private void pictureBox1_Paint(object sender, PaintEventArgs e)
{
    if (allPoints.Count > 1) e.Graphics.DrawLines(Pens.Black, allPoints.ToArray());
}

Note that this uses a PictureBox as the canvas control. It is the control meant for this kind of interaction. Your code seems to draw onto the desktop, which doesn’t belong to you. Drawing onto it in a persistent manner is not anything like what you would do with the/any Paint application.

Also note that my example stores a list of points and draws them as one non-closed Polyline. To draw them closed exchange DrawLines for DrawPolygon! To draw several such polylines or polygons you need to..

  • ..decide on the user interface for it, maybe add segment points only while the control-key is pressed and otherwise finish the current polyline
  • store the points in a List<List, Point>>

Also note that this is one of the rare examples where using control.CreateGraphics is called for, as you actually do want a non-persistent drawing while the user moves the mouse.

In most other cases the Winforms graphics basic rule #1 applies:

Never use control.CreateGraphics! Never try to cache a Graphics object! Either draw into a Bitmap bmp using a Graphics g = Graphics.FromImage(bmp) or in the Paint event of a control, using the e.Graphics parameter..

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