PaintEvent not overpainting

Question

I have a Windows Forms application in C# with drawing panel and a button - for drawing a line.

When you click the button, you can draw a line for 2 random points.

Answer 1

Whenever you want to redraw (in your code) you should call panelinvalidate drawPanel.Invalidate().

When you finish mouse moving and release the button

private void drawPanel_MouseUp(object sender, MouseEventArgs e)
{
            onMouseUpFlag = true;
            drawPanel.Invalidate();
}

And at the end I would suggest to use this code in your drawPanel_Paint instead of that of yours . You should use Graphics provided by an EventArgs, not create new one.

private void drawPanel_Paint(object sender, PaintEventArgs e)
{
            if (onMouseUpFlag)
            {
                  e.Graphics.DrawLine(p, ps, pe); 
            }
}