I created a CMFCToolBarComboBoxButton in a CMFCToolBar as following:

afx_msg LRESULT CMainFrame::OnToolbarReset(WPARAM wp, LPARAM lp)
{
UINT uiToolBarId = (UINT) wp;
if (uiToolBarId == IDR_TOOLBAR_FILES)
{
CMFCToolBarComboBoxButton omboButton1(IDC_SRC_PRO_LIST, GetCmdMgr ()->GetCmdImage(IDC_SRC_PRO_LIST, FALSE), CBS_DROPDOWNLIST);

    comboButton1.EnableWindow(FALSE);
    comboButton1.SetCenterVert();
    comboButton1.SetDropDownHeight(200);
    comboButton1.SetFlatMode();
    m\_wndFilesBar.ReplaceButton (IDC\_SRC\_PRO\_LIST, comboButton1);
 }

}

It runs well. But when I want to add some content into it from another thread.I defined a function:

int CMainFrame::CBAddItem(LPCTSTR lpszItem, DWORD_PTR dwData)
{
int r = -1;
CMFCToolBarComboBoxButton* pSrcCombo = CMFCToolBarComboBoxButton::GetByCmd (IDC_SRC_PRO_LIST);
if(pSrcCombo)
{
r = pSrcCombo->AddItem(lpszItem, dwData);
}
return r;
}

The function CMFCToolBarComboBoxButton::GetByCmd always return NULL. And I traced this function, found：

int __stdcall CMFCToolBar::GetCommandButtons(UINT uiCmd, CObList& listButtons)
{
.....
for (POSITION posTlb = afxAllToolBars.GetHeadPosition(); posTlb != NULL;)
{
CMFCToolBar* pToolBar = (CMFCToolBar*) afxAllToolBars.GetNext(posTlb);
ENSURE(pToolBar != NULL);

    if (CWnd::FromHandlePermanent(pToolBar->m\_hWnd) != NULL)

CWnd::FromHandlePermanent(pToolBar->m_hWnd) always return NULL. So why？