鼠标的划线问题

目标，鼠标按下，开始划线，鼠标抬起，划线结束，再次划线的时候可以进行擦除。

void CTest1View::OnLButtonDown(UINT nFlags, CPoint point)
{
    // TODO: Add your message handler code here and/or call default
    Cstartpoint=point;
    b_Draw=true;
    CView::OnLButtonDown(nFlags, point);
}

void CTest1View::OnMouseMove(UINT nFlags, CPoint point)
{
    // TODO: Add your message handler code here and/or call default
       if(isDrawLine)
    {
    if(Cstartpoint.x==0)
    {//如果是第一次绘图，则绘图，并给Cstartpoint赋值
      dc.MoveTo(radius,radius);
         dc.LineTo(point);
      Cstartpoint.x=point.x;Cstartpoint.y=point.y;
    }
        //如果不是第一次绘图，则先擦除上次绘制的线
     dc.SetROP2(R2_NOT);
        dc.MoveTo(radius,radius);
        dc.LineTo(Cstartpoint);
        Cstartpoint.x=point.x;Cstartpoint.y=point.y;
  //绘制本次的线
     dc.MoveTo(radius,radius);
        dc.LineTo(point);
     }    
    CView::OnMouseMove(nFlags, point);
}
void CTest1View::OnLButtonUp(UINT nFlags, CPoint point)
{
    // TODO: Add your message handler code here and/or call default
    b_Draw=false;

          //擦除最后留下的那根线

        CTest1View dc(this);
        dc.SetROP2(R2_NOT);
        dc.MoveTo(radius,radius);
        dc.LineTo(Cstartpoint);
    CView::OnLButtonUp(nFlags, point);
}