draw line on panel not showing up

问题:

I have a Panel called panel1 and I am trying to draw a line on my panel1 using this code:

Graphics g = panel1.CreateGraphics();
        var p = new Pen(Color.Black, 3);
        var point1 = new Point(234,118);
        var point2 = new Point(293,228);
        g.DrawLine(p, point1, point2);

But nothing is showing up. Any ideas? This is in a windows form.

回答1:

Handle the Panel's Paint event and put it in there. What's happening is that it's being drawn once in the constructor but then being drawn over in the Paint event everytime it's called.

private void panel1_Paint(object sender, PaintEventArgs e)
{
    base.OnPaint(e);
    using(Graphics g = e.Graphics)
    {
       var p = new Pen(Color.Black, 3);
       var point1 = new Point(234,118);
       var point2 = new Point(293,228);
       g.DrawLine(p, point1, point2);
    }
}

回答2:

Put it in some event after the form has been created and shown on the screen and it should work fine.

It's best to put it in the Paint event, as keyboardP stated, but it will not show up if called before the form is shown on the screen.

To test this you can add a button and add the code to the click event:

private void button1_Click(object sender, EventArgs e)
{
    using (Graphics g = panel1.CreateGraphics())
    {
        g.DrawLine(new Pen(Color.Back, 3), new Point(234,118), new Point(293,228));
    }
}

回答3:

To see your drawing - you can simply make a button with a Click Event and draw when the button is clicked. For example:

private void btnDraw_Click(object sender, EventArgs e)
{
    Graphics dc = drawingArea.CreateGraphics();
    Pen BlackPen = new Pen(Color.Black, 2);
    dc.DrawLine(BlackPen, 0, 0, 200, 200);

    BlackPen.Dispose();
    dc.Dispose();
}    

Oh, and by the way "drawingArea" is the (Name) of either a PictureBox or Panel you have added to your form (to draw in it).

回答4:

If you follow the other answers and still your drawings are not showing up try removing all controls from the control that is being drawn to. The other controls may be covering whatever you are trying to draw.