多線程創建對話框時Create出錯

太牛逼了，也可看大牛原帖：http://blog.chinaunix.net/uid-24862988-id-3793502.html

1、問題

在修改單線程MFC程序爲多線程時，遇到了CWnd::AssertValid()函數執行出錯問題。主要表現是在執行下面代碼中綠色語句時出錯

<span style="color:#666666;">#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();<span style="white-space:pre">		</span>//此處問題
        ASSERT(pMap != NULL);<span style="white-space:pre">				</span>//此處問題

        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</span>

2、原因

通過在網上查詢資料發現問題的原因：在B線程中，直接通過A線程的窗口實例指針調用窗口關聯函數。在MFC中，窗口類的事件映射表是和線程相關聯，故只有在本線程中才能通過窗口實例指針調用該窗口關聯函數。下面這段代碼是afxMapHWND函數的實現，其中綠色部分是線程相關的變量的結構體的獲取，事件映射表就是這個結構體的成員變量

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;
}

3、方法

要徹底解決這個問題，那麼就需要通過多線程的線程間消息機制解決上述問題，也就是自定義消息，在子線程中需要調用主線程create的時候，向主線程發消息，主線程消息響應函數中去create