个人写的双缓冲 CDC绘图函数

void CDRImageStatic::ShowImg(CDC *dc, CRect rect)
{
 CBitmap bmp;
    bmp.CreateCompatibleBitmap(dc, m_nWidth, m_nHeight);
    CBitmap* pOld=NULL;

 CDC memDC;
 memDC.CreateCompatibleDC( dc );
 if (m_nColor!=NULL)
 {  
  EnterCriticalSection(&csImgCriticalSection);
  bmp.SetBitmapBits(sizeof(*m_nColor)*m_nHeight*m_nWidth, m_nColor);
  LeaveCriticalSection(&csImgCriticalSection); 
  pOld = memDC.SelectObject( &bmp );  
 }
 if((m_nDXA==0))
 {
  EnterCriticalSection(&csMyCriticalSection);

  int oldmode=memDC.SetBkMode(TRANSPARENT);
 &n