How to determines if a hdc is useable , before I used it.

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static HDC  hdcScreen ;
 
hdcScreen = CreateDC (TEXT ("DISPLAY"), NULL, NULL, NULL) ;
 
//I saved a screen-hdc in static.
//I use it whenever I want, without creating again.

static COLORREF cr ;
 
GetCursorPos (&pt) ;
cr = GetPixel (hdcScreen, pt.x, pt.y) ;
 
//I used screen-hdc to pick color value

but when Another program called LockWindowUpdate (hwndScreen), "hdc-screen" isn't useable again in my program.

I want know How to determines if a hdc is useable , before I used it.

Posted 30-Dec-10 17:01pm

tanakahitori

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2 solutions

Solution 2

thank your answer very much Espen

I googled MSDN and watched some text

screen-DC and print-DC and so on are common-DC

DC of window you created is private-DC

and GetDC to get DC from Cache(windows memory)

when it is out of all space of Cache

the next GetDC will alloc DC-memory from heap of your program,

till all available space of memory is taken.

but DC be gotten from heap , will be fail to release.

this is why you have to Release a DC

So I tried to get 200 screen-hdc at one time, and they worked pretty well

int i ;
static HDC hdc[200] ;
 
for (i = 0 ; i < 200 ; i++)
     hdc[i] = GetDC (NULL) ;
 
//I drawed 200 Rectangles on Screen 
for (i = 0 ; i < 200 ; i++) 
     Rectangle (hdc[i], 0, 0, 500 - i * 2, 500 - i * 2) ;
   
for (i = 0 ; i < 200 ; i++)
     if (!ReleaseDC (NULL, hdc[i]))
         MessageBox (hwnd, TEXT ("failed to release"), NULL, MB_OK) ;

difference between common-DC and private-DC

attributes of common-DC you get every time are alway unchanged.

but private-DC could remember attributes modifed by last time.

arosed My problem isn't out of Cache

it is LockWindowUpdate(hwndScreen) (it will disable or enable hwndscreen)
or the screen resolution has been changed

my hdc isn't available anymore

Posted 1-Jan-11 2:29am

tanakahitori

Updated 1-Jan-11 2:31am

v2

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Espen Harlinn · Accepted Answer · 2010-12-31T00:27:00

Solution 1

The display is a common DC, so you can't expect to hold it infinitely.

See: http://msdn.microsoft.com/en-us/library/dd144871(v=vs.85).aspx[^]

So use

C#

HDC hdcScreen = GetDC(NULL);
// Do you stuff
ReleaseDC(hdcScreen);

Regards
Espen Harlinn

Posted 31-Dec-10 0:27am

Espen Harlinn

Comments

Manfred Rudolf Bihy 31-Dec-10 6:30am

Good one! 5+

Espen Harlinn 31-Dec-10 7:03am

Thanks :)