对话框类调用文档类指针出错

Question

我在对话框类的OnPaint()函数中调用文档类指针 CMainFrame  *pFrame=(CMainFrame*)AfxGetApp()->m_pMainWnd;  
CDemoDoc* pDoc=(CDemoDoc  *)pFrame->GetActiveDocument();
ASSERT_VALID(pDoc);pDoc 得不到，这是怎么回事啊？？

Answer 1

CDemoDoc* pDoc=(CDemoDoc  *)pFrame->GetActiveDocument();============================================================
CDemoDoc* pDoc =  GetActiveDocument();