急！MFC强制重绘问题 ::SendMessage()

Question

 void CEmailSettingDlg::OnBnClickedOk()
{
    UpdateData(1);
    CEmailManagementDlg *pdlg = (CEmailManagementDlg*)AfxGetApp()->GetMainWnd();
    if (m_str_BackGround == "蒲公英")
        m_str_BackGround = "1";
    if (m_str_BackGround == "书")
        m_str_BackGround = "2";
    if (m_str_BackGround == "天空")
        m_str_BackGround = "3";
    if (m_str_BackGround == "秦时明月")
        m_str_BackGround = "4";
    //刷新显示新的背景
    CString temp = pdlg->m_backGround;  //暂存原背景图数据
    pdlg->m_backGround = m_str_BackGround;
**  ::SendMessage(pdlg->m_hWnd, WM_PAINT, 0, 0);**
    //UpdateWindow();
    pdlg->m_backGround = temp;

    //如果设置没变，则不需要打开数据库(出于效率考虑)
    if (!(m_str_AutoReply == pdlg->m_auto_reply && m_str_BackGround == pdlg->m_backGround))
    {
        CString temp;
        temp.Format("update user_psw_tab set auto_reply = '%s', back_ground = %d where user_name = '%s'", m_str_AutoReply, atoi(m_str_BackGround), pdlg->m_str_UserName);
        TRACE(temp);
        pdlg->m_db.ExecuteSQL(temp);
        AfxMessageBox("更新成功!");
    }

    CDialogEx::OnOK();
}

 void CEmailManagementDlg::OnPaint()
{
    if (IsIconic())
    {
        CPaintDC dc(this); // 用于绘制的设备上下文

        SendMessage(WM_ICONERASEBKGND, reinterpret_cast<WPARAM>(dc.GetSafeHdc()), 0);

        // 使图标在工作区矩形中居中
        int cxIcon = GetSystemMetrics(SM_CXICON);
        int cyIcon = GetSystemMetrics(SM_CYICON);
        CRect rect;
        GetClientRect(&rect);
        int x = (rect.Width() - cxIcon + 1) / 2;
        int y = (rect.Height() - cyIcon + 1) / 2;

        // 绘制图标
        dc.DrawIcon(x, y, m_hIcon);
    }
    else
    {
        CPaintDC dc(this); // 用于绘制的设备上下文
        CRect rc;
        GetClientRect(&rc);
        CDC dcMem;
        dcMem.CreateCompatibleDC(&dc);
        CBitmap bmpBackground;

        switch (atoi(m_backGround))
        {
        case 1:
            bmpBackground.LoadBitmap(IDB_BITMAP_FLOWER);
            break;
        case 2:
            bmpBackground.LoadBitmap(IDB_BITMAP_BOOK);
            break;
        case 3:
            bmpBackground.LoadBitmap(IDB_BITMAP_SKY);
            break;
        case 4:
            bmpBackground.LoadBitmap(IDB_BITMAP_QIN);
            break;
        }

        BITMAP bitmap;
        bmpBackground.GetBitmap(&bitmap);
        CBitmap* pbmpPri = dcMem.SelectObject(&bmpBackground);
        dc.StretchBlt(0, 0, rc.Width(), rc.Height(), &dcMem, 0, 0, bitmap.bmWidth, bitmap.bmHeight, SRCCOPY);

        //CDialogEx::OnPaint();  //这句需要去掉
    }
}

我想实现实时切换 CEmailManagementDlg的背景
CEmailSettingDlg设置背景数据，点击确定后去刷新CEmailManagementDlg的背景
但是调试发现尽管SendMessage消息以后进入了CEmailManagementDlg的OnPaint函数，但是语句执行完以后并没有改变背景，请问我哪里错了吗。。

Answer 1

感谢各位帮忙 问题已经找到了。。。因为以下对控件变量之间改变值之后没有UpdateData(0),所以实际上控件变量的值并没有改变。。所以后面判断时出问题了。。

 if (m_str_BackGround == "蒲公英")

         m_str_BackGround = "1";
    if (m_str_BackGround == "书")
        m_str_BackGround = "2";
    if (m_str_BackGround == "天空")
        m_str_BackGround = "3";
    if (m_str_BackGround == "秦时明月")
        m_str_BackGround = "4";