面板控件绘制事件不适用于用户控件

Question

I created one Windows Forms User Control, I drag, dropped a panel inside it and over the panel I drew the graph in the Paint event of Panel.

private void pnlViewer_Paint(object sender, PaintEventArgs e)
{
    e.Graphics.TranslateTransform(pnlViewer.AutoScrollPosition.X, pnlViewer.AutoScrollPosition.Y);
    e.Graphics.FillRectangle(Brushes.Black, Screen.PrimaryScreen.Bounds);
    //**draw Y Axis**
    int y;
    for (int i = 0; i <= 50; i++)
    {
        y = (i * cellHeight) + cellHeight;
        e.Graphics.DrawLine(new Pen(Color.FromArgb(50, 50, 50)),
                            new Point(0, y), new Point(pageWidth, y));
    }
    //**draw X Axis**
    int x;
    for (int i = 0; i < 50; i++)
    {
        x = (i * cellWidth) + ribbonWidth;
        e.Graphics.DrawLine(new Pen(Color.FromArgb(50, 50, 50)),
                            new Point(x, 0), new Point(x, pageHeight));
    }
    DrawWaveForm(e.Graphics); **// Here the actual data of graph will draw**
}

When I drag this user control onto a WinForm, it calls this paint event of User Control from windows form, but while calling to this event the graph is shown but after some time the graph becomes blank.

I tried Invalidate(true), Update(), Refresh() all such methods but they were of no use.

Actually it shows half of the graph over form and after next the same paint event is fired then it shows the full graph that I require, but actually I want on first Paint event instead of half graphs showing the full graph.

Answer 1

    e.Graphics.DrawLine(new Pen(Color.FromArgb(50, 50, 50)),
                        new Point(0, y), new Point(pageWidth, y));

You are not disposing the System.Drawing object in this code. Possibly in other code as well. This can go unnoticed for a long time, the garbage collector tends to hide the problem. But if it doesn't run often enough then the operating system can get sulky about you using so many GDI handles and it won't allow your program to create any more of them. The quota is 10,000 handles, a very large number but easily consumed if you repaint often. Typical when you draw a constantly updating graph for example. What happens next varies, somewhere between an exception and noticing that your program doesn't paint correctly anymore.

Always use the using statement in painting code to avoid this failure mode:

using (var pen = new Pen(Color.FromArgb(50, 50, 50))) {
    e.Graphics.DrawLine(pen, new Point(0, y), new Point(pageWidth, y));
}