一个小问题

昨天写一个简单的画图程序，遇到一个奇怪的问题，目的是想点鼠标左键，画个以此点为中心画个椭圆，最初的代码如下：

<!--<br><br>Code highlighting produced by Actipro CodeHighlighter (freeware)<br>http://www.CodeHighlighter.com/<br><br>-->voidCE02View::OnLButtonDown(UINTnFlags,CPointpoint)
{
CPaintDCdc(this);
OnPrepareDC(&dc);
if(this->m_nColor==GRAY_BRUSH)
{
this->m_nColor=WHITE_BRUSH;
}
else
{
this->m_nColor=GRAY_BRUSH;
}
this->m_rectEllipse.left=point.x/2;
this->m_rectEllipse.right=point.x+point.x/2;
this->m_rectEllipse.top=point.y+point.y/2;
this->m_rectEllipse.bottom=point.y/2;
this->OnDraw(&dc);
CView::OnLButtonDown(nFlags,point);
}

voidCE02View::OnDraw(CDC*pDC)
{
CE02Doc*pDoc=GetDocument();
ASSERT_VALID(pDoc);
pDC->SelectStockObject(this->m_nColor);
pDC->Ellipse(this->m_rectEllipse);
}

并且也考虑了第一次运行时会发出WM_PAINT事件，而在OnPaint方法里会去调用OnDraw()，所以我把OnPaint重写了，让其不调用OnDraw,这样第一次运行就不会去画了。

<!--<br><br>Code highlighting produced by Actipro CodeHighlighter (freeware)<br>http://www.CodeHighlighter.com/<br><br>-->voidCE02View::OnPaint()
{
CPaintDCdc(this);//devicecontextforpainting
}

可尽管这样，点击鼠标左键还是无法画出东西来，问了下师兄，他也不熟悉VC,弄了一会儿也无功而返，今天想了下可能是DC有问题，换了个CClientDC，唉，好了，真是怪了，用CPaintDC我是想模仿这OnPaint()的代码来写，按道理说应该可以的，可为什么在OnLButtonDown里就不行呢？最后可以运行的代码如下：

<!--<br><br>Code highlighting produced by Actipro CodeHighlighter (freeware)<br>http://www.CodeHighlighter.com/<br><br>-->voidCE02View::OnLButtonDown(UINTnFlags,CPointpoint)
{
CClientDCdc(this);
OnPrepareDC(&dc);
if(this->m_nColor==GRAY_BRUSH)
{
this->m_nColor=WHITE_BRUSH;
}
else
{
this->m_nColor=GRAY_BRUSH;
}
this->m_rectEllipse.left=point.x/2;
this->m_rectEllipse.right=point.x+point.x/2;
this->m_rectEllipse.top=point.y+point.y/2;
this->m_rectEllipse.bottom=point.y/2;
this->OnDraw(&dc);
CView::OnLButtonDown(nFlags,point);
}

这样看来，CPaintDC是专门用来在响应WM_PAINT消息时使用的，其他消息处理程序中不能使用