c++-急！MFC強制重繪問題 ::SendMessage()

急！MFC強制重繪問題 ::SendMessage()

 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 == "書")
        m_str_BackGround = "2";
    if (m_str_BackGround == "天空")
        m_str_BackGround = "3";
    if (m_str_BackGround == "秦時明月")
        m_str_BackGround = "4";
    //刷新顯示新的背景
    CString temp = pdlg->m_backGround;  //暫存原背景圖數據
    pdlg->m_backGround = m_str_BackGround;
**  ::SendMessage(pdlg->m_hWnd, WM_PAINT, 0, 0);**
    //UpdateWindow();
    pdlg->m_backGround = temp;

    //如果設置沒變，則不需要打開數據庫(出於效率考慮)
    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); // 用於繪制的設備上下文

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

        // 使圖標在工作區矩形中居中
        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;

        // 繪制圖標
        dc.DrawIcon(x, y, m_hIcon);
    }
    else
    {
        CPaintDC dc(this); // 用於繪制的設備上下文
        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();  //這句需要去掉
    }
}

我想實現實時切換 CEmailManagementDlg的背景
CEmailSettingDlg設置背景數據，點擊確定後去刷新CEmailManagementDlg的背景
但是調試發現盡管SendMessage消息以後進入了CEmailManagementDlg的OnPaint函數，但是語句執行完以後並沒有改變背景，請問我哪裡錯了嗎。。

最佳回答：

感謝各位幫忙 問題已經找到了。。。因為以下對控件變量之間改變值之後沒有UpdateData(0),所以實際上控件變量的值並沒有改變。。所以後面判斷時出問題了。。

 if (m_str_BackGround == "蒲公英")

         m_str_BackGround = "1";
    if (m_str_BackGround == "書")
        m_str_BackGround = "2";
    if (m_str_BackGround == "天空")
        m_str_BackGround = "3";
    if (m_str_BackGround == "秦時明月")
        m_str_BackGround = "4";