与两个bitblt的用法有什么不同？

Question

阅读以下代码：  为什么 m_dcMem.BitBlt（0,0，m_size.cx，m_size.cy，＆amp; dcScreen，0,0，SRCCOPY）; 有效，  和 dcScreen.BitBlt（0,0，m_size.cx，m_size.cy，＆amp; m_dcMem，0,0，SRCCOPY）; 不要？ 谢谢！ `

int CScreenDlg :: OnCreate（LPCREATESTRUCT lpCreateStruct）

if (CDialog::OnCreate(lpCreateStruct) == -1)
    return -1;
// TODO:  Add your specialized creation code here   
ShowWindow(SW_HIDE);

CDC dcScreen;
dcScreen.CreateDC("DISPLAY", NULL, NULL, NULL);
//dcScreen.Attach(GetDC(GetParent()));
m_size.cx = GetDeviceCaps(dcScreen.GetSafeHdc(), HORZRES);
m_size.cy = GetDeviceCaps(dcScreen.GetSafeHdc(), VERTRES);

m_dcMem.CreateCompatibleDC(&dcScreen);
m_bmpScreen.CreateCompatibleBitmap(&dcScreen, m_size.cx,m_size.cy);
CBitmap* pOldBmp = m_dcMem.SelectObject(&m_bmpScreen);
//m_dcMem.BitBlt(0,0,m_size.cx, m_size.cy, &dcScreen, 0,0, SRCCOPY);
dcScreen.BitBlt(0,0,m_size.cx, m_size.cy, &m_dcMem, 0,0, SRCCOPY); //why it doesn't work???
CRect rect(0,0,m_size.cx, m_size.cy);

LEFT_RECT = CRect(INFO_OFFSET, INFO_OFFSET, INFO_OFFSET + INFO_WIDTH, INFO_OFFSET + INFO_HEIGHT);
RIGHT_RECT = CRect(m_size.cx - INFO_WIDTH-INFO_OFFSET, INFO_OFFSET, m_size.cx-INFO_OFFSET, INFO_OFFSET + INFO_HEIGHT);
m_rectInfo = LEFT_RECT;

m_bmpInfoBK.LoadBitmap(IDB_BITMAP_INFOBK);
MoveWindow(rect);
ShowWindow(SW_SHOW);
m_hCursor = LoadCursor(AfxGetInstanceHandle(), MAKEINTRESOURCE(IDC_CURSOR_SELECT));
return 0;

}