GDI繪圖中經(jīng)常會創(chuàng)建一些畫刷、畫筆和位圖等資源，通常這些資源通過調(diào)用函數(shù)CreateSolidBrush、CreatePen、CreateCompatibleBitmap等方法創(chuàng)建。這些資源在使用完成以后必須通過調(diào)用DeleteObject()函數(shù)釋放資源，否則會發(fā)生資源泄漏，最終導(dǎo)致系統(tǒng)資源被耗光，不能再次成功創(chuàng)建資源。

考慮下面一段代碼：

HBITMAP hBmpLayer1,hBmpOldLayer1;
HBITMAP hBmpLayer0,hBmpOldLayer0;
HDC hDcLayer1 = 0;
HDC hDcLayer0 = 0;
int mouseX =0,mouseY =0;

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
 int wmId, wmEvent;
 PAINTSTRUCT ps;
 HDC hdc;
 TCHAR szHello[MAX_LOADSTRING];
 LoadString(hInst, IDS_HELLO, szHello, MAX_LOADSTRING);

 switch (message) 
 {
  case WM_COMMAND:
   wmId    = LOWORD(wParam); 
   wmEvent = HIWORD(wParam); 
   // Parse the menu selections:
   switch (wmId)
   {
    case IDM_ABOUT:
       DialogBox(hInst, (LPCTSTR)IDD_ABOUTBOX, hWnd, (DLGPROC)About);
       break;
    case IDM_EXIT:
       DestroyWindow(hWnd);
       break;
    default:
       return DefWindowProc(hWnd, message, wParam, lParam);
   }
   break;
  case WM_CREATE:
   {
    RECT rt;
    rt.left = 0;
    rt.right = 20;
    rt.top = 0;
    rt.bottom = 20;
    hDcLayer1 = CreateCompatibleDC(GetDC(NULL));
    hBmpLayer1 = CreateCompatibleBitmap(GetDC(NULL),20,20);
    hBmpOldLayer1 = (HBITMAP)SelectObject(hDcLayer1,hBmpLayer1);
    FillRect(hDcLayer1,&rt,CreateSolidBrush(0));
    rt.left =2;
    rt.top = 2;
    rt.bottom =17;
    rt.right =17;
    FillRect(hDcLayer1,&rt,CreateSolidBrush(0xff00ff));

    return DefWindowProc(hWnd, message, wParam, lParam);
   }
  case WM_PAINT:
   RECT rt;
   int i,k,x,y,error;
   HPEN hPen,hOldPen;
   HBRUSH hBrshBlack,hBrsh;
   hdc = BeginPaint(hWnd, &ps);
   if(!hdc)
   {
    error = GetLastError();
   }
   // TODO: Add any drawing code here...

   GetClientRect(hWnd, &rt);
   if(hDcLayer0 != NULL)
   {
    SelectObject(hDcLayer0,hBmpOldLayer0);
    DeleteDC(hDcLayer0);//釋放資源
    DeleteObject(hBmpLayer0);//釋放資源
    hDcLayer0 = NULL;
    hBmpLayer0 = NULL;
    hBmpOldLayer0 = NULL;
   }
   hDcLayer0 = CreateCompatibleDC(hdc);
   if(!hDcLayer0)
   {
    error = GetLastError();
   }

   hBmpLayer0 = CreateCompatibleBitmap(hdc,rt.right-rt.left,rt.bottom-rt.top);
   if(!hBmpLayer0)
   {
    error = GetLastError();
   }
   hBmpOldLayer0 = (HBITMAP)SelectObject(hDcLayer0,hBmpLayer0);
   if(!hBmpOldLayer0)
   {
    error = GetLastError();
   }
   hBrshBlack = CreateSolidBrush(0);
   FillRect(hDcLayer0,&rt,hBrshBlack);
   x=rt.left;
   y=rt.top+(rt.bottom-rt.top)/2;

   hPen = CreatePen(PS_SOLID, 1, RGB(0xff,0x0,0x0));
   hOldPen = (HPEN)SelectObject(hDcLayer0 ,hPen);//選入畫筆
   MoveToEx(hDcLayer0 ,x,y,NULL);
   LineTo(hDcLayer0 ,rt.right,y+5);//繪圖

   SelectObject(hDcLayer0 ,hOldPen);//選出畫筆
   DeleteObject(hPen);//釋放資源

   hPen = CreatePen(PS_SOLID, 1, RGB(0x0,0xff,0x0));
   hOldPen = (HPEN)SelectObject(hDcLayer0 ,hPen);
   MoveToEx(hDcLayer0 ,x,y+10,NULL);
   LineTo(hDcLayer0 ,rt.right,y+10);//繪圖

   SelectObject(hDcLayer0 ,hOldPen);//選出畫筆
   DeleteObject(hPen);

   hPen = CreatePen(PS_SOLID, 1, RGB(0x0,0x0,0xff));
   hOldPen = (HPEN)SelectObject(hDcLayer0 ,hPen);//選入畫筆
   MoveToEx(hDcLayer0 ,x,y-10,NULL);
   LineTo(hDcLayer0 ,rt.right,y-10);//繪圖

   SelectObject(hDcLayer0 ,hOldPen);//選出畫筆
   DeleteObject(hPen);//釋放畫筆資源

   DeleteObject(hBrshBlack);//釋放畫刷資源

   error = BitBlt(hdc,0,0,rt.right,rt.bottom,hDcLayer0,0,0,SRCCOPY);
   if(!error)
    error =GetLastError();
   TransparentBlt(hdc,mouseX,mouseY,20,20,hDcLayer1,0,0,20,20,0);
   EndPaint(hWnd, &ps);
   break;
  case WM_MOUSEMOVE:
   {
    mouseX = lParam&0xffff;
    mouseY = lParam>>16;
    InvalidateRect(hWnd,NULL,FALSE);
    break;
   }
  case WM_DESTROY:
   PostQuitMessage(0);
   break;
  default:
   return DefWindowProc(hWnd, message, wParam, lParam);
   }
   return 0;
}

如果將程序中釋放資源的部分去掉，那么在鼠標(biāo)移動一段時間以后，畫面將變成黑白圖像，其原因就是多次創(chuàng)建資源而沒有釋放，導(dǎo)致資源耗盡，無法再次分配資源。

對于DC資源，如果使用CreateCompitableDC()方法創(chuàng)建，則在使用完成后需要使用DeleteDC()釋放資源。

如果是通過GetDC()方法獲取的資源，則需要調(diào)用ReleaseDC()釋放資源。

對于已選入了DC的畫筆、畫刷、位圖等資源，必須將其選出后才能進(jìn)行資源釋放，否則不成功。