C# WinForms - Paint method questions

Question

I am not sure what is the best way of using graphics - should I attach my classes to main form Paint event and then do the drawing, or it is better to call it from overidden OnPaint void like this? I mean, is it OK to do that like this:

protected override void OnPaint(PaintEventArgs e)
{
    base.OnPaint(e)  //what is this good for? My app works without it as well
    Graphics g=e.Graphics;
    DrawEnemies(g);
    UpdateHUD(g);
    DrawSelectedUnit(g);
}

`OnPain` is not going to compile, unless this is actually a subclass of a class that supports being physically tortured ;) — Thomas, Jul 07 '10 at 14:00
Nah. I'm far too witty for that. (Also, I was recently reprimanded for editing an obvious copy/paste mistake in someone's code, but that wasn't the reason I refrained from editing here.) — Thomas, Jul 07 '10 at 14:13
It just left me wondering... OnPain of what? OnPain of execution! — Lazarus, Jul 07 '10 at 14:14
overriding OnPaint you can comment base.OnPaint for performace reasons. But take care cause you will not recieve OnPaint events anymore. — serhio, Jul 07 '10 at 14:50

score 5 · Accepted Answer · answered Jul 07 '10 at 14:00

5

It is recommended that controls override the On... methods rather than subscribe to their own events.

You should call base.OnPaint to ensure the Paint method is fired properly.

From MSDN:

The OnPaint method also enables derived classes to handle the event without attaching a delegate. This is the preferred technique for handling the event in a derived class.

Notes to Inheritors
When overriding OnPaint in a derived class, be sure to call the base class's OnPaint method so that registered delegates receive the event.

answered Jul 07 '10 at 14:00

Jeff Yates

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usually, overriding OnPaint you don't need to fire Paint events anymore, so base.OnPaint is not mandatory. MSDN tells, but not everything is to follow exactly. In a similar way you override OnBackgroundPaint and comment base.OnBackgroundPaint do NOT base repaint the background - against flickering. – serhio Jul 07 '10 at 14:59
@serhio: The way to prevent background painting isn't to override OnBackgroundPaint (that's the old school brute force method carried over from Win32). Instead, you should set the Control style to include `AllPaintingInWmPaint`. – Jeff Yates Jul 07 '10 at 15:10
From MSDN: `When overriding OnPaintBackground in a derived class it is not necessary to call the base class's OnPaintBackground.` http://msdn.microsoft.com/en-us/library/system.windows.forms.control.onpaintbackground.aspx – serhio Jul 07 '10 at 15:24
@serhio: Yup, that's right, but the correct way to prevent flicker only in .NET is the control style. If you want to actually paint a new background, you would do what you describe. Of course, that is all moot for this question as the question regards OnPaint. – Jeff Yates Jul 07 '10 at 15:34

score 5 · Answer 2 · answered Jul 07 '10 at 14:02

It doesn't really matter; both work. Overriding OnPaint might be ever so slightly faster in theory, but it's not a difference that anyone will notice. Microsoft recommends overriding OnPaint but doesn't really motivate this.

You need to call base.OnPaint because this method will invoke handlers attached to the Paint event.

C# WinForms - Paint method questions

2 Answers2

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