修改后的CMemDC類：(增加了紅色的那行，非常重要喔。另外加了些注釋)
/////////////////////////////////////////////////////////////////////////////
// CMemDC

class CMemDC : public CDC
{
public:
?? CDCHandle???? m_dc;????????? // 屏幕的DC Owner DC
?? CBitmap?????? m_bitmap;????? // Offscreen bitmap
?? CBitmapHandle m_hOldBitmap;? // Originally selected bitmap
?? RECT????????? m_rc;????????? // Rectangle of drawing area

?? CMemDC(HDC hDC, LPRECT pRect, COLORREF clrBackGround = 0)
?? {
????? ATLASSERT(hDC!=NULL);
?? //保存屏幕的DC到m_dc中
?? m_dc = hDC;

????? if( pRect!=NULL )
?? {
??? m_rc = *pRect;
?? }
?? else
?? {
??? m_dc.GetClipBox(&m_rc);
?? }

?? //將被本窗口覆蓋了的父窗口的區域填充至本窗口的背景

??? //調用CDC的CreateCompatibleDC()，它會給父類CDC的成員變量m_hDC賦值
?? CreateCompatibleDC(hDC);
??
????? ::LPtoDP(hDC, (LPPOINT) &m_rc, sizeof(RECT)/sizeof(POINT));
?? //讀取屏幕位圖到m_bitmap
????? m_bitmap.CreateCompatibleBitmap(hDC, m_rc.right-m_rc.left, m_rc.bottom-m_rc.top);
??
?? //CDC選擇新位圖，并保存舊的位圖
????? m_hOldBitmap = SelectBitmap(m_bitmap);
?? //在CDC中畫圖，將原先的屏幕復制到內存中
?? BitBlt(0, 0, m_rc.right-m_rc.left, m_rc.bottom-m_rc.top, hDC, m_rc.left, m_rc.top, SRCCOPY);

?? //設置CDC的背景色為#FF00FF
?? SetBkColor(RGB(0,255,0));
????? ::DPtoLP(m_dc, (LPPOINT) &m_rc, sizeof(RECT)/sizeof(POINT));
????? SetWindowOrg(m_rc.left, m_rc.top);

?? //add by Robert
?? if(clrBackGround != 0)
?? {
??CBrush brush;
??brush.CreateSolidBrush(clrBackGround);
??FillRect(pRect, (HBRUSH)brush);
?? }
?? //end add
?? }
?? ~CMemDC()
?? {
????? // Copy the offscreen bitmap onto the screen.
??? //注意，是從父類的m_hDC中復制位圖到屏幕
????? m_dc.BitBlt(m_rc.left, m_rc.top, m_rc.right-m_rc.left, m_rc.bottom-m_rc.top,
????????????????? m_hDC, m_rc.left, m_rc.top, SRCCOPY);
????? // CDC恢復以前的位圖 Swap back the original bitmap.
????? SelectBitmap(m_hOldBitmap);
?? }
};

修改后的CMemDC類