axel's dev corner

One particularly unnerving thing in the MFC Document / View implementation is the fact that CMDIFrameWnd::GetActiveDocument() function does return NULL, since it is not the window actually holding the document. You should use a function like the following one to get the document in the MDIFrameWnd:

CDocument* CMyFrame::GetActiveDocumentForSure()
{
    // Get the active MDI child window. 
    CMDIChildWnd *child = MDIGetActive();
    if (!child) return 0;
    // Get the active view 
    CView *view = (CView *) child->GetActiveView();
    if (!view) return 0;
    // get the document from the view
    return view->GetDocument();
}