使用TrackMouseEvent()函数，才可以让系统发送WM_MOUSE消息。请参考MSDN关于API的说明和以下这个帖子。

http://bbs.pediy.com/showthread.php?t=76795

我在msdn看到的，也是说要用TrackMouseEvent()
可是在MFC中有一个消息map的宏，不应该再去调用那个函数了吧
还是谢谢你

解决了，找到了一个代码，原来是自己定义消息宏写错了
帖出来，有相同的问题的朋友可以看下
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// TrackView.h changes
//

class CTrackView : public CView
{
// Add data members
protected:
        CRect m_rectLast;        //** Added line
        BOOL m_bMouseTracking;        //** Added line

// Add message handler prototype
        afx_msg LRESULT OnMouseLeave(WPARAM wParam, LPARAM lParam);        //** Added line

};

****************************************************************************************

// TrackView.cpp changes
//

// Add message handler
BEGIN_MESSAGE_MAP(CTrackView, CView)
        ON_MESSAGE(WM_MOUSELEAVE, OnMouseLeave)        //** Added line
END_MESSAGE_MAP()

CTrackView::CTrackView()
{
        m_bMouseTracking = FALSE;        //** Added line
}

/////////////////////////////////////////////////////////////////////////////
// CTrackView message handlers

//** Added function
LRESULT CTrackView::OnMouseLeave(WPARAM wParam, LPARAM lParam)
{
        // Draw last rect, but no new one (erase old rect)
        CClientDC dc(this);
        dc.DrawDragRect(CRect(0,0,0,0), CSize(0,0), m_rectLast, CSize(2,2));
        m_rectLast = CRect(0,0,0,0);

        m_bMouseTracking = FALSE;
        return TRUE;
}

void CTrackView::OnMouseMove(UINT nFlags, CPoint point)
{
        // Calc new rectangle
        CRect rectNew(point.x-20, point.y-20, point.x+20, point.y+20);
        CClientDC dc(this);

        // WM_MOUSEMOVE + !m_bMouseTracking becomes the equivalent of
        // WM_MOUSEENTER of which there is no such thing.
        if (!m_bMouseTracking)
        {
                TRACKMOUSEEVENT tme;
                tme.cbSize = sizeof(TRACKMOUSEEVENT);
                tme.dwFlags = TME_LEAVE;
                tme.hwndTrack = this->m_hWnd;

                if (::_TrackMouseEvent(&tme))
                {
                        m_bMouseTracking = TRUE;

                        // Draw new rect, but no last rect as we are starting anew
                        dc.DrawDragRect(rectNew, CSize(2,2), NULL, CSize(0,0));
                }
        }
        else
        {
                // Draw new rect and erase old rect
                dc.DrawDragRect(rectNew, CSize(2,2), m_rectLast, CSize(2,2));
        }

        // Remember where we drew this rectangle
        m_rectLast = rectNew;

        CView::OnMouseMove(nFlags, point);
}

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