Is CreateCompatibleDC() necessary working with windows on one display?

Question

This example code manually reads a bitmap file, uses CreateDIBSection() to make GDI allocate memory for it, and create an hbitmap handle. Then it uses a MemoryDC to draw the bitmap to a window DC:

ftp://ftp.oreilly.com/examples/9781572319950/cd_contents/Chap15/DibSect/DibSect.c

hdc = BeginPaint (hwnd, &ps) ;
...
hdcMem = CreateCompatibleDC (hdc) ;

Why can't we use GetDC() with NULL or with hwndDesktop instead? Why can't we cache the device context instead of repeatedly creating it?

If the machine has only one display device and we are only drawing to windows why do we need to constantly harmonize bitmaps and device contexts? Once the pixeldata is copied to the buffer provided by GDI, does GDI update it when that HBITMAP is loaded into a DC and drawn on? If the user also wishes to draw on it is it necessary to synchronize access? (By calling GDIFlush() first?)

It's hard to figure this out when most all of the object properties are opaque and abstracted. I've read almost all of the related MSDN, a lot of Petzold's book, and some articles:

Display Device Contexts

CreateCompatibleDC()

CreateDIBSection()

Memory Device Contexts

Guide to Win32 Memory DC

Guide to WIN32 Paint for Intermediates

Programming Windows®, Fifth Edition

Edit:

I think my question boils down to this:

Is a device context a TYPE of display or is it an INSTANCE of graphical data that is able to be displayed. A computer typically has only a handful of displays but it could have hundreds of things to display on them.

Answer 1

GetDC(NULL) is the screen HDC and the screen is a shared resource, therefore you should only do read/query operations on this HDC. Writing to this HDC is not a good idea on Vista and higher because of the DWM.

Since a HDC can only contain one bitmap, one brush and one pen, Windows/applications obviously need more than one HDC provided by the graphics engine.

You can count on CreateCompatibleDC to be relatively cheap operation and I believe Windows has a cache of DCs it can hand out. If you are creating a game/animation type application you might want to cache some of these graphic objects on your own but a normal application should not.

You don't generally call GDIFlush unless you are sharing GDI objects across several threads. You can use SetDIBits if you want to mix raw pixel bytes access and GDI.

I don't really get the once screen argument, Windows has supported multiple monitors since Windows 98 and there is not much you can do to prevent the user from connecting another monitor.

Answer 2

I think your problem is that you are getting hung up on Microsoft's names for things, Microsoft's name "device context" and the names for calls like "CreateCompatibleDC".

"Device Context" is a bad name. The Win32 documentation will tell you that a device context is a data structure for storing the state of a particular device used for rendering graphics commands. This is only partially true. Look at the different kinds of DCs that exist: (1) screen device contexts, (2) printer device contexts, (3) the device context used by a bitmap in memory, and (4) metafile device contexts. Of these only (1) or (2) are actually doing exactly what the documentation claims they are doing. In the other cases device contexts serve as a target for drawing calls but not as containers for the state of some physical device. (This is really noticeably true in the case of metafile DC's: metafiles were an old Win32 thing that basically just cache the GDI calls going in to them to be replayed later, kind of a crude vector format.)

In a hypothetical object oriented programming version of Win32, device contexts could be instances of some class that implements an interface that exposes graphics drawing calls. A better name for such a class would be something like "Graphics" and indeed in GDI+ this is what the analogous construct is actually called. When we "Create" -- via CreateDC, CreateCompatibleDC, etc. -- we create one of these objects. When we GetDC we grab such an object that already exists.

To answer your questions:

Is a device context a TYPE of display or is it an INSTANCE of graphical data that is able to be displayed. ?

They are in so sense types of displays. You can think of them as instances of a class of objects with private implementations that expose a public interface exposing drawing commands.

Why can't we use GetDC() with NULL or with hwndDesktop instead?

You can't use GetDC(NULL) as the device context into which you are going to select an in memory bitmap because in such a situation you need to create a device context that does not already exist; GetDC(NULL) is like a singleton instance that is already in use.

So instead you usually CreateCompatibleDC(NULL) or CreateCompatibleDC(hdcScreen). Again CreateCompatibleDC(...) is a confusing name. Imagine the hypothetical object-oriented version of what is going on here. Say there is an IGraphics interface that is implemented by RasterGraphics, PrinterGraphics, and MetafileGraphics. Imagine the "RasterGraphics" class is used for both the screen and for in memory bitmaps. Then CreateCompatibleDC(...) would be would be like a factory call Graphics.CreateFrom(IGraphics g) that return a new instance of the same concrete type with perhaps some state variables initialized.

Why can't we cache the device context instead of repeatedly creating it?

You can. You do not need to delete device contexts across function calls. The only reason people often do is that they are a shared, finite resource and creating them is cheap. I think actually that they used to be very limited under old versions of Windows so old Win32 programmers tend to not cache them out of muscle memory from the old days, from Windows 95 days.

If the machine has only one display device and we are only drawing to windows why do we need to constantly harmonize bitmaps and device contexts?

Don't think of the "compatible" in CreateCompatibleDC(...) to be about "harmonizing with the screen" think of it as meaning "Okay Windows I want to create one of your graphics interface objects and I want the kind like this one, which is a normal raster graphics one and not for printers or for metafiles."