為什麽將內存DC復制到屏幕DC無效果？

PAINTSTRUCT ps;

g_hDC=BeginPaint(g_hWnd,&ps);

HBRUSH hbrScr=CreateSolidBrush(RGB(0,0,255));

HDC hMemDC=CreateCompatibleDC(g_hDC);

RECT rect;

GetClientRect(hwnd,&rect);

HBITMAP hBitmap=CreateCompatibleBitmap(g_hDC,rect.right,rect.bottom);//A

HBITMAP oldBmp=(HBITMAP)SelectObject(hMemDC,hBitmap);//B

FillRect(hMemDC,&g_rWindow,hbrScr);

Render(hMemDC);

BitBlt(g_hDC,0,0,1024,768,hMemDC,0,0,SRCCOPY);

SelectObject(oldBmp);

DeleteObject(hBitmap);

DeleteObject(oldBmp);

DeleteObject(hbrScr);

DeleteDC(hMemDC);

EndPaint(g_hWnd, &ps);

A行和B行：在對內存DC畫圖之前要先往內存DC裏選入壹張空白位圖，然後再在上面作圖。因為CreateCompatibleDC（）函數創建內存DC時為內存DC選入的默認位圖只有壹個像素，沒有實際用處，得用戶自己重新創建壹張才行