如果想将一个编好的SDI应用程序转换为MDI，首先要建立一个继承于CMDIChidWnd的类，假设为CChldFrm，在CWinApp中作如下变化。
InitInstance()
{
. ...
    //instead of adding CSingleDocTemplate
    // Add CMultiDocTemplate.
    pDocTemplate = new CMultiDocTemplate(
           IDR_MAINFRAME,
           RUNTIME_CLASS(CSDIDoc),
           RUNTIME_CLASS(CChldFrm),
// For Main MDI Frame change this frame window from
// CFrameWnd derivative ( i.e. CMainFrame )
// to your CMDIChildWnd derived CChldFrm.
           RUNTIME_CLASS(CSDIView));
/// After this it is required to create the main frame window
// which will contain all the child windows. Now this window is
// what was initially frame window for SDI.
    CMainFrame* pMainFrame = new CMainFrame;
    if (!pMainFrame->LoadFrame(IDR_MAINFRAME))
            return FALSE;
     m_pMainWnd = pMainFrame;
.....
}
在从CMDIFrameWnd中继承的类CMainFrame代替CFramWnd后，所有的类都将从CMDIFrame继承，而不是CFrameWnd,编译运行后你就会发现程序已经从SDI变换到MDI。
注意：在CMainFram中必须将构造函数从private改为public.否则会出错。