我是否必须继续创建Graphics对象

Question

I am an old delphi programmer, I am used to creating objects and using them entire time for efficient memory usage. But in c# (maybe all the tutorials I've ever seen), you are creating stuffs with new every time (thanks to garbage collector!!, let me do the coding)..

Anyway, I am trying to create a designing software which has lots of drawing. My question is: do I have to create a graphics object, or use the protected override void OnPaint(PaintEventArgs e) e.Graphics every painting event.. because when I create a graphic object and then resize the control that I draw on, the graphic object that I created, has that clipping problem and only draws old rectangle region..

thanks

Answer 1

Caching objects makes sense when the object is expensive to create, cheap to store and relatively simple to keep updated. A Graphics object is unique in that none of these conditions are true:

• It is very cheap to create, takes well less than a microsecond.
• It is very expensive to store, the underlying device context is stored in the desktop heap of a session. The number of objects that can be stored is small, no more than 65535. All programs that run in the session share that heap.
• It is very hard to keep updated, things happen behind your back that invalidates the device context. Like the user or your program changing the window size, invalidating the Graphics.ClipBounds property. You are wasting the opportunity to use the correct Graphics object, the one passed to you in a Paint event handler. Particularly a bug factory when you use double-buffering.

Caching a Graphics object is a bug.

Answer 2

If you want to draw on the surface always use the Graphics object from the Paint event!

If you want to draw into a Bitmap you create a Graphics object and use it as long as you want.

For the Paint event to work you need to collect all drawing in a List of graphic actions; so you will want to make a nice class to store all parameters needed.

In your case you may want to consider a mixed approach: Old graphic actions draw into a bitmap, which is the eg BackgroundImage or Image of your control

Current/ongoing drawing are done on the surface. This amounts to using the bitmap as a cache , so you don't have to redraw lots of actions on every little change etc

This is closely related to your undo/redo implementation. You could set a limit and draw those before into a Btimap and those after onto the surface..

PS: You also should rethink your GC attitude. It is simple, efficient and a blessing to have around. (And, yes, I have done my share of TP&Delphi, way back when they were affordable..) - Yes, we do the coding, but GC is not about coding but about house keeping. Boring at best.. (And you can always design to avoid it, but not with a Graphics object in a windows system.)

Answer 3

A general rule for every class that implements IDisposable is to Dispose() it, as soon as possible. Make sure you know about the using(...){} statement.

For drawing in WinForms (GDI+) the best practice is indeed to use the Graphics object from PaintEventArgs. And because you didn't create that one, do not Dispose() it. Don't stash it either.

Answer 4

I have to completely disagree with other more experienced members here who say it's no big deal or in fact better to recreate the Graphics object over and over.

The HDC is a pointer to a HDC__ struct, which is a struct with one member, "int unused". It's an absolute waste and stupidity to create another instance/object every time drawing needs to be done. The HDC is NOT large, it's either 4 or 8 bytes, and the struct it points to is in nearly all cases 4 bytes. Furthermore, on the point that one person made, it doesn't help that the graphics object be made with the "static" keyword at the beginning of the WndProc() before the switch, because the only way to give the Graphics object a device context or handle to paint on is by calling its constructor, so "static" does nothing to save you from creating it over and over again.

On top of that Microsoft recommends that you create a HDC pointer and assign it to the same value PAINTSTRUCT already has, every, single WM_PAINT message it sends.

I'm sorry but the WinAPI in my opinion is very bad. Just as an example, I spent all day researching how to make a child WS_EX_LAYERED window, to find out that in order to enable Win 8 features one has to add code in XML with the OS's ID number to the manifest. Just ridiculous.