純 Win32 SDK程式為什麼每次要擷取裝置描述表句柄

做過Win32 API程式的就會發現，要對視窗輸出時，每次都要擷取裝置描述表句柄；都是對同一個視窗輸出，裝置描述表是一個；為什麼每次都要擷取裝置描述表句柄呢？擷取一次行嗎？

做一個Win32 API程式，擷取多次裝置描述表句柄；遞增Y坐标，輸出裝置描述表句柄的值；

/*-------------------------------------------------
   hdcDemo.C -- bobo, 2019.12.27
  -------------------------------------------------*/

#include <windows.h>
#include <windowsx.h>

LRESULT CALLBACK WndProc (HWND, UINT, WPARAM, LPARAM) ;
int         x=200, y;

int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance,
                    PSTR  szCmdLine, int iCmdShow)
{
     static TCHAR szAppName[] = TEXT ("hdcDemo1") ;
     HWND         hwnd ;
     MSG          msg ;
     WNDCLASS     wndclass ;
     
     wndclass.style         = CS_HREDRAW | CS_VREDRAW ;
     wndclass.lpfnWndProc   = WndProc ;
     wndclass.cbClsExtra    = 0 ;
     wndclass.cbWndExtra    = 0 ;
     wndclass.hInstance     = hInstance ;
     wndclass.hIcon         = LoadIcon (NULL, IDI_APPLICATION) ;
     wndclass.hCursor       = LoadCursor (NULL, IDC_ARROW) ;
     wndclass.hbrBackground = (HBRUSH) GetStockObject (WHITE_BRUSH) ;
     wndclass.lpszMenuName  = NULL ;
     wndclass.lpszClassName = szAppName ;
     
     if (!RegisterClass (&wndclass))
     {
          MessageBox (NULL, TEXT ("Program requires Windows NT!"), 
                      szAppName, MB_ICONERROR) ;
          return 0 ;
     }
     
     hwnd = CreateWindow (szAppName, TEXT ("hdc Demo"),
                          WS_OVERLAPPEDWINDOW,
                          CW_USEDEFAULT, CW_USEDEFAULT,
                          CW_USEDEFAULT, CW_USEDEFAULT,
                          NULL, NULL, hInstance, NULL) ;
     
     ShowWindow (hwnd, iCmdShow) ;
     UpdateWindow (hwnd) ;
     
     while (GetMessage (&msg, NULL, 0, 0))
     {
          TranslateMessage (&msg) ;
          DispatchMessage (&msg) ;
     }
     return msg.wParam ;
}

LRESULT CALLBACK WndProc (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
     HDC         hdc ;     
     PAINTSTRUCT ps ;
     RECT        rect ;
	 char buffer[65];
     
     switch (message)
     {
	 case WM_CREATE:
		 y=130;
		 return 0;
     case WM_SIZE :
          return 0 ;
	
	 case WM_RBUTTONDOWN :
		 hdc=GetDC(hwnd);
		 itoa(hdc,buffer,10);
		 TextOut(hdc,100,y,buffer,9);
		 y=y+30;
		 return 0;
          
     case WM_LBUTTONDOWN :
          hdc=GetDC(hwnd);
		  itoa(hdc,buffer,10);
		  TextOut(hdc,100,100,buffer,9);
          return 0 ;
          
     case WM_PAINT :
          hdc = BeginPaint (hwnd, &ps) ;
          hdc=GetDC(hwnd);
		  itoa(hdc,buffer,10);
		  TextOut(hdc,x,y,buffer,9);          
          EndPaint (hwnd, &ps) ;
		  y=y+30;
          return 0 ;
               
     case WM_DESTROY :
          PostQuitMessage (0) ;
          return 0 ;
     }
     return DefWindowProc (hwnd, message, wParam, lParam) ;
}
           

根據目前我看到的情況；在同一次進入視窗過程，擷取2次裝置描述表句柄，第二次擷取的值是負值，無效的；如果每次進入視窗過程，擷取裝置描述表句柄，傳回的句柄值是不同的；

是以需要每次擷取；

目前我認為每次變換裝置描述表句柄值，是Windows的保護機制之一；防止人通過裝置描述表句柄，跟蹤進裝置描述表内部，了解到相關設計機制；