1,首先在OnLButtonDown()中保存第一個點:

void CMyView::OnLButtonDown(UINT nFlags, CPoint point) 
{
 // TODO: Add your message handler code here and/or call default

 if(Start==true)                                   //開始標志
 {
  CClientDC   ClientDC(this);   
  OnPrepareDC(&ClientDC);   
  ClientDC.DPtoLP(&point); 
  
  PointA=point;                               //第一個點.
  OldPoint=point;                            //鼠標移動過程中的上一個點.
  First=true;
 }
 CView::OnLButtonDown(nFlags, point);
}

2.其次,在void OnMouseMove(UINT nFlags, CPoint point) 中加入以下畫圖分支結構

void CMyView::OnMouseMove(UINT nFlags, CPoint point) 
{
 // TODO: Add your message handler code here and/or call default 
    
 if(First&&Start){   
  CClientDC   ClientDC(this);//設備描述表   
  OnPrepareDC(&ClientDC);   
  ClientDC.DPtoLP(&point); 
  
    if(DrawStartWhat==0)
  {  
   ClientDC.SetROP2(R2_NOTXORPEN);//生成逆轉當前屏幕顏色來畫線的繪圖方式以擦去之前畫的線

   ClientDC.MoveTo(PointA);//畫反顏色線,就擦去了原來的顏色   
   ClientDC.LineTo(OldPoint);   
   ClientDC.MoveTo(PointA);   //畫新的線,以下同理.
   ClientDC.LineTo(point); 
  }
  else if(DrawStartWhat==1)
  {
   //love you!        很多人用的是R2_NOT,但是自我感這兒用R2_NOTXORPEN較好.

   ClientDC.SetROP2(R2_NOTXORPEN);//生成逆轉當前屏幕顏色來畫線的繪圖方式以擦去之前畫的圓, 
   OldPoint.y=OldPoint.x-PointA.x+PointA.y;//畫圓的坐標設定,注意它的斜率與PointA.y/PointA.x相同.
   ClientDC.Ellipse(CRect(PointA,OldPoint));
   OldPoint.x=point.x;
   OldPoint.y=point.x-PointA.x+PointA.y;

   ClientDC.Ellipse(CRect(PointA,OldPoint));
  }
  else if(DrawStartWhat==2)
  {
   ClientDC.SetROP2(R2_NOTXORPEN);//生成逆轉當前屏幕顏色來畫線的繪圖方式以擦去之前畫的橢圓
   ClientDC.Ellipse(CRect(PointA,OldPoint));
   ClientDC.Ellipse(CRect(PointA,point));
  }
  else if(DrawStartWhat==3)
  {
   ClientDC.SetROP2(R2_NOTXORPEN);//生成逆轉當前屏幕顏色來畫線的繪圖方式以擦去之前畫的矩形
   ClientDC.Rectangle(CRect(PointA,OldPoint));
   ClientDC.Rectangle(CRect(PointA,point));
  }
  OldPoint=point;  
 CView::OnMouseMove(nFlags, point);
}

3.最好在OnLButtonUP()中,回到初值,即可>

void CMyView::OnLButtonUp(UINT nFlags, CPoint point) 
{
 // TODO: Add your message handler code here and/or call default
 
 if(First)
 { 
  First=false;   
 }  
 CView::OnLButtonUp(nFlags, point);
}