MFC多线程中创建对话框（create）出错（异常）问题的分析及解决办法

在修改单线程MFC程序为多线程时，遇到了CWnd::AssertValid()函数执行出错问题。主要表现是在执行下面代码中绿色语句时出错

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 #ifdef _DEBUG
 void CWnd::AssertValid() const
 {
     if (m_hWnd == NULL)
         return; // null (unattached) windows are valid

     // check for special wnd??? values
     ASSERT(HWND_TOP == NULL); // same as desktop
     if (m_hWnd == HWND_BOTTOM)
         ASSERT(this == &CWnd::wndBottom);
     else if (m_hWnd == HWND_TOPMOST)
         ASSERT(this == &CWnd::wndTopMost);
     else if (m_hWnd == HWND_NOTOPMOST)
         ASSERT(this == &CWnd::wndNoTopMost);
     else
     {
         // should be a normal window
         ASSERT(::IsWindow(m_hWnd));

         // should also be in the permanent or temporary handle map
         CHandleMap* pMap = afxMapHWND();
         ASSERT(pMap != NULL);

         CObject* p=NULL;
         if(pMap)
         {
             ASSERT( (p = pMap->LookupPermanent(m_hWnd)) != NULL ||
                     (p = pMap->LookupTemporary(m_hWnd)) != NULL);
         }
         ASSERT((CWnd*)p == this); // must be us

         // Note: if either of the above asserts fire and you are
         // writing a multithreaded application, it is likely that
         // you have passed a C++ object from one thread to another
         // and have used that object in a way that was not intended.
         // (only simple inline wrapper functions should be used)
         //
         // In general, CWnd objects should be passed by HWND from
         // one thread to another. The receiving thread can wrap
         // the HWND with a CWnd object by using CWnd::FromHandle.
         //
         // It is dangerous to pass C++ objects from one thread to
         // another, unless the objects are designed to be used in
         // such a manner.
     }
 }
 #endif

2. 原因

    通过在网上查询资料发现问题的原因：在B线程中，直接通过A线程的窗口实例指针调用窗口关联函数。在MFC中，窗口类的事件映射表是和线程相关联，故只有在本线程中才能通过窗口实例指针调用该窗口关联函数。下面这段代码是afxMapHWND函数的实现，其中绿色部分是线程相关的变量的结构体的获取，事件映射表就是这个结构体的成员变量

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 CHandleMap* PASCAL afxMapHWND(BOOL bCreate)
 {
     AFX_MODULE_THREAD_STATE* pState = AfxGetModuleThreadState();
     if (pState->m_pmapHWND == NULL && bCreate)
     {
         BOOL bEnable = AfxEnableMemoryTracking(FALSE);
 #ifndef _AFX_PORTABLE
         _PNH pnhOldHandler = AfxSetNewHandler(&AfxCriticalNewHandler);
 #endif
         pState->m_pmapHWND = new CHandleMap(RUNTIME_CLASS(CWnd),
             ConstructDestruct<CWnd>::Construct, ConstructDestruct<CWnd>::Destruct,
             offsetof(CWnd, m_hWnd));

 #ifndef _AFX_PORTABLE
         AfxSetNewHandler(pnhOldHandler);
 #endif
         AfxEnableMemoryTracking(bEnable);
     }
     return pState->m_pmapHWND;
 }

     
    下面的一段代码是函数定义和函数调用。CMainFrame类的实例指针mainFrame是主线程(A线程)的窗口实例指针，该指针在工作线程(B线程)中被用来调用类的成员函数getHtmlView，该函数是窗口关联函数(获取窗口中的视窗)。当在工作线程中获取窗口视窗时，就会调用函数CWnd::AssertValid()，执行该函数时会调用上面的afxMapHWND函数。很显然上述代码中绿色中的结构体AFX_MODULE_THREAD_STATE 获取的是当前线程(即 工作线程)的结构体，而非主线程的结构体 ，故出现了标题1中描述的问题

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      // 在B线程中执行下述代码

 CMainFrame *mainFrame = (CMainFrame *)AfxGetApp()->GetMainWnd();
 utHtmlView *htmlView = (utHtmlView *)mainFrame->getHtmlView();

       // 函数定义

 CView *CMainFrame::getHtmlView( void )
 {
  if(!m_nMultiInterface)
   return GetActiveView();
  return (CView *)m_wndSplitter.GetPane(0, 1);
 }

3. 解决

1) 方法一

    CWnd::AssertValid只会在Debug下有效，而上述问题只会在多线程程序中出现。简单的解决方法是在Release下运行程序，就可以避免上述问题

2)方法二

    用 FromHandle 返回临时对象的指针，就可以调用各种类的方法。临时对象会在 OnIdle 中销毁。这里对 FromHandle 的实现原理从源码上进行解析
下面代码是 CWnd 的 FromHandle 方法，大致的意思为从 CHandleMap 中获取临时 CWnd 对象的指针

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 CWnd* PASCAL CWnd::FromHandle(HWND hWnd)
 {
     CHandleMap* pMap = afxMapHWND(TRUE); //create map if not exist
     ASSERT(pMap != NULL);
     CWnd* pWnd = (CWnd*)pMap->FromHandle(hWnd);
 #ifndef _AFX_NO_OCC_SUPPORT
     pWnd->AttachControlSite(pMap);
 #endif
     ASSERT(pWnd == NULL || pWnd->m_hWnd == hWnd);
     return pWnd;
 }

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 //
 // 2
 //
 CHandleMap* PASCAL afxMapHWND(BOOL bCreate)
 {
     AFX_MODULE_THREAD_STATE* pState = AfxGetModuleThreadState();
     if (pState->m_pmapHWND == NULL && bCreate)
     {
         BOOL bEnable = AfxEnableMemoryTracking(FALSE);
 #ifndef _AFX_PORTABLE
         _PNH pnhOldHandler = AfxSetNewHandler(&AfxCriticalNewHandler);
 #endif


         pState->m_pmapHWND = new CHandleMap(RUNTIME_CLASS(CTempWnd),
             offsetof(CWnd, m_hWnd));
 #ifndef _AFX_PORTABLE
         AfxSetNewHandler(pnhOldHandler);
 #endif
         AfxEnableMemoryTracking(bEnable);
     }
     return pState->m_pmapHWND;
 }

 再看

 #define offsetof(s,m) (size_t)&(((s *)0)->m)

 继续

 //
 // 3
 //
 CObject* CHandleMap::FromHandle(HANDLE h)
 {
     ASSERT(m_pClass != NULL);
     ASSERT(m_nHandles == 1 || m_nHandles == 2);

     if (h == NULL)
         return NULL;

     CObject* pObject = LookupPermanent(h);
     if (pObject != NULL)
         return pObject; // return permanent one
     else if ((pObject = LookupTemporary(h)) != NULL)
     {
         HANDLE* ph = (HANDLE*)((BYTE*)pObject + m_nOffset);
         ASSERT(ph[0] == h || ph[0] == NULL);
         ph[0] = h;
         if (m_nHandles == 2)
         {
             ASSERT(ph[1] == h || ph[1] == NULL);
             ph[1] = h;
         }
         return pObject; // return current temporary one
     }

     // This handle wasn't created by us, so we must create a temporary
     // C++ object to wrap it. We don't want the user to see this memory
     // allocation, so we turn tracing off.

     BOOL bEnable = AfxEnableMemoryTracking(FALSE);
 #ifndef _AFX_PORTABLE
     _PNH pnhOldHandler = AfxSetNewHandler(&AfxCriticalNewHandler);
 #endif

     CObject* pTemp = NULL;
     TRY
     {
         pTemp = m_pClass->CreateObject();
         if (pTemp == NULL)
             AfxThrowMemoryException();

         m_temporaryMap.SetAt((LPVOID)h, pTemp);
     }
     CATCH_ALL(e)
     {
 #ifndef _AFX_PORTABLE
         AfxSetNewHandler(pnhOldHandler);
 #endif
         AfxEnableMemoryTracking(bEnable);
         THROW_LAST();
     }
     END_CATCH_ALL

 #ifndef _AFX_PORTABLE
     AfxSetNewHandler(pnhOldHandler);
 #endif
     AfxEnableMemoryTracking(bEnable);

     // now set the handle in the object
     HANDLE* ph = (HANDLE*)((BYTE*)pTemp + m_nOffset); // after CObject
     ph[0] = h;
     if (m_nHandles == 2)
         ph[1] = h;

     return pTemp;
 }

    对标题2中的程序进行修改如下，就可以避免出现上面的问题。首先获取到主窗口实例指针，然后根据主窗口实例指针的窗口句柄获取到临时主窗口实例指针，最后就可以通过临时实例指针操作该类中的各种方法了

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 CWnd *mWnd = AfxGetApp()->GetMainWnd();
 CMainFrame *mainFrame = (CMainFrame *)(mWnd->FromHandle(mWnd->GetSafeHwnd()));
 utHtmlView *htmlView = (utHtmlView *)mainFrame->getHtmlView();

3)方法三

    方法二虽然解决了上面遇到的问题，但是不能完全解决。比如在主窗口下还有子窗口，通过上述方法主窗口可以调用类的各种方法，但是通过类方法再去调用子窗口的窗口关联函数时还是会出现问题。比如在下面的代码中，在主窗口的方法中，调用子窗口的窗口方法还是会遇到类似问题

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 CView *CMainFrame::getHtmlView( void )
 {
     if(!m_nMultiInterface)
         return GetActiveView();
     return (CView *)m_wndSplitter.GetPane(0, 1);
 }

    要彻底解决这个问题，那么就需要通过多线程的线程间消息机制解决上述问题，MFC多线程消息机制
///////////////////////////////////////////////////////////////////////////////////////////////////////////
A. 声明与定义
(1) 消息声明
#define WM_MY_MESSAGE    WM_USER + 100
(2) 类声明中添加以下声明
DECLARE_MESSAGE_MAP()

afx_msg LRESULT OnMyMessage(WPARAM wParam,LPARAM lParam);

(3) 类实现中添加下一代码
BEGIN_MESSAGE_MAP(MYCLASS, MYPARENTCLASS)
ON_MESSAGE(WM_MY_MESSAGE, OnMyMessage)
END_MESSAGE_MAP()

LRESULT MYCLASS::OnMyMessage(WPARAM wParam,LPARAM lParam)
{
    return S_OK;
}

////////////////////////////////////////////////////////////////////////////
B. 工作线程代码实现
    MYCLASS *myClass;
    同步消息传递
HRESULT hr = myClass->SendMessage(WM_MY_MESSAGE, wParam, lParam);
    异步消息传递
HRESULT hr = myClass->PostMessage(WM_MY_MESSAGE, wParam, lParam);