win32 桌面圖像獲取問題

代碼如下，在執(zhí)行之后還是沒有辦法獲取桌面的圖像，求大神幫忙看看

HDC                    mem_dc;
SIZE                   screen_size;
 
void 
screen_caption(){
 
    HDC desk_dc = CreateDC(L"DISPLAY", 0, 0, 0);       
 
    mem_dc = CreateCompatibleDC(desk_dc);               
 
    screen_size.cx = GetDeviceCaps(desk_dc, HORZRES);   
 
    screen_size.cy = GetDeviceCaps(desk_dc, VERTRES);   
 
    HBITMAP hBmp = CreateCompatibleBitmap(desk_dc, screen_size.cx, screen_size.cy);   
 
    SelectObject(mem_dc, hBmp);                           
 
    BitBlt(mem_dc, 0, 0, screen_size.cx, screen_size.cy, desk_dc, 0, 0, SRCCOPY);
 
}
 
LRESULT CALLBACK
ScreenProc(HWND hwnd, UINT message, WPARAM wparam, LPARAM lparam) {
 
    HDC                             hdc;
    PAINTSTRUCT                     ps;
    RECT                            rect;
 
    switch (message) {
 
    case WM_CREATE:
 
        screen_caption();
 
        return 0;
 
    case WM_PAINT:
 
        hdc = BeginPaint(hwnd, &ps);
 
        GetClientRect(hwnd, &rect);
 
        BitBlt(hdc, 0, 0, screen_size.cx, screen_size.cy, mem_dc, 0, 0, SRCCOPY);
 
        EndPaint(hwnd, &ps);
 
        return 0;
 
 
    case WM_KEYDOWN:
 
        switch (wparam) {
 
        case VK_ESCAPE:
 
            ShowWindow(hwnd, SW_HIDE);
 
            break;
 
        }
 
        return 0;
 
 
    case WM_DESTROY:
 
        PostQuitMessage(0);
 
        return 0;
 
    }
 
    return DefWindowProc(hwnd, message, wparam, lparam);
 
}

再補充一下，我是想在子窗口上顯示桌面圖像，下面是我建立子窗口的函數(shù)

void
registe_sreenshoot_window(HINSTANCE hInstance) {


    WNDCLASS            wndclass;

    wndclass.style              = CS_HREDRAW | CS_VREDRAW;

    wndclass.lpfnWndProc        = ScreenProc;

    wndclass.cbClsExtra         = 0;

    wndclass.cbWndExtra         = sizeof(HANDLE);

    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      = szScreenClassName;


    if (!RegisterClass(&wndclass)) {

        MessageBox(NULL, "注冊失敗", "錯誤", MB_ICONERROR);

        return;
    }

}


void
create_screenshoot_window(HWND *hwnd, HWND phwnd) {

    *hwnd = CreateWindow(szScreenClassName, TEXT("Child Win"),
        WS_POPUP,
        CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT,
        phwnd, NULL, hInst, NULL);

}


void
show_screenshoot_window(HWND hwnd) {

    ShowWindow(hwnd, SW_MAXIMIZE);

    UpdateWindow(hwnd);

}