TicTac.h:

class CMyApp : public CWinApp
{
public:
    virtual BOOL InitInstance ();
};

class CMainWindow : public CWnd
{

public:
    CMainWindow ();

protected:     
    afx_msg void OnLButtonDown (UINT nFlags, CPoint point);    

    DECLARE_MESSAGE_MAP ()
};

TicTac.cpp:

#include <afxwin.h>
#include "TicTac.h"

CMyApp myApp;

/////////////////////////////////////////////////////////////////////////
// CMyApp member functions

BOOL CMyApp::InitInstance ()
{
    m_pMainWnd = new CMainWindow;
    m_pMainWnd->ShowWindow (m_nCmdShow);
    m_pMainWnd->UpdateWindow ();
    return TRUE;
}

/////////////////////////////////////////////////////////////////////////
// CMainWindow message map and member functions

BEGIN_MESSAGE_MAP (CMainWindow, CWnd)
    ON_WM_LBUTTONDOWN ()  
END_MESSAGE_MAP ()


CMainWindow::CMainWindow ()
{ 
    //
    // Register a WNDCLASS.
    //
    CString strWndClass = AfxRegisterWndClass (
        CS_DBLCLKS,                                        // Class style
        AfxGetApp ()->LoadStandardCursor (IDC_ARROW),   // Class cursor
        (HBRUSH) (COLOR_3DFACE + 1),                    // Background brush
        AfxGetApp ()->LoadStandardIcon (IDI_WINLOGO)    // Class icon
    );

    //
    // Create a window.
    //
    CreateEx (0, strWndClass, _T ("Tic-Tac-Toe"),
        WS_OVERLAPPED | WS_SYSMENU | WS_CAPTION | WS_MINIMIZEBOX,
        CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT,
        NULL, NULL);
    
}


void CMainWindow::OnLButtonDown (UINT nFlags, CPoint point)
{    
    CClientDC dc (this);
    dc.MoveTo(0, 0);    
    dc.LineTo(100, 100);
}

目的就是点击鼠标左键,画一条线. 在xp上点击后,画的线立马显示, 在win7上点击后,画的线不出现,除非我将鼠标移到其它窗口上点击一下,再回来才显示,到底怎么回事呢?

估计是刷新问题，你代码中刷新下窗口试试

[ 本帖最后由 yuccn 于 2012-11-3 15:19 编辑 ]

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怎么刷新呢?

UpdataWindow()