MFC 双缓冲机制，为何绘制不出来圆？

    CDC* pDC=m_ground.GetDC();
    CDC memDC;
   
    CRect Rect;
    GetClientRect(&Rect);
   
    memDC.CreateCompatibleDC(pDC);
    CPen pen(NULL,2,RGB(0,0,0));
    memDC.SelectObject(&pen);
    //创建画刷，填充圆
    CBrush brush(circle[0].brushcolor);
    memDC.SelectObject(&brush);
   
    //绘制圆
    memDC.Ellipse(circle[0].x-circle[0].r,circle[0].y-circle[0].r,circle[0].x+circle[0].r,circle[0].y+circle[0].r);
    pDC->BitBlt(0,0,Rect.right,Rect.bottom,&memDC,0,0,SRCCOPY);
    brush.DeleteObject();
    memDC.DeleteDC();