请教一个双缓冲和gdiplus的有关问题

                       CBitmap m_bitmap;
            CDC memdc;
            m_bitmap.CreateBitmap(pImage->w, pImage->h, 1, 32, pImage->samples);
            BITMAP bm ;
            m_bitmap.GetBitmap(&bm);
            memdc.CreateCompatibleDC(pDC);
            CBitmap * pOld = memdc.SelectObject(&m_bitmap);
            HDC dc = memdc.GetSafeHdc();

                        //因为需要显示透明位图,我这里使用gdi+ 在memdc绘制
            Graphics grapic(dc);
            Image image(L"c:\\2.png",FALSE);
            grapic.DrawImage(&image, 0, 0, 612, 792);
            pDC->StretchBlt(0,0,pImage->w,pImage->h,&memdc,0,0, bm.bmWidth, bm.bmHeight,SRCCOPY);
            memdc.SelectObject(pOld);
            memdc.DeleteDC();
            m_bitmap.DeleteObject();
            ReleaseDC(pDC);