让组合框具有树状目录的显示功能，这个类比较有用的。 
void CTreeCombo::DisplayTree() 
{ 
CRect rect(0,0,200,200); 

CalculateDroppedRect (&rect); 

m_Tree.InitDisplay (); 

m_Tree.SetWindowPos (&wndNoTopMost, rect.left, rect.top, 
rect.Width (), rect.Height (), SWP_SHOWWINDOW ); 

m_BeginPath = m_Tree.GetCurrentTreePath (); 

SetDroppedState (TRUE); 
} 

BOOL CTreeCombo::CreateTree () 
{ 
CWnd* pParent = GetParent (); 
if (!pParent && pParent->GetSafeHwnd ()) 
{ 
return FALSE; 
} 

CRect treeRect (0,0, m_ComboWidth, m_ComboHeight); 

//can't have a control Id with WS_POPUP style 
if (! m_Tree.CreateEx ( 0, 
WC_TREEVIEW, NULL, 
WS_POPUP | WS_BORDER | TVS_SINGLEEXPAND | TVS_HASLINES | TVS_LINESATROOT | TVS_HASBUTTONS, 
treeRect, pParent, 0, NULL)) 
{ 
return FALSE; 
} 


return TRUE; 
} 

BOOL CTreeCombo::CreateEdit () 
{ 
if (!m_DropDownButton.GetSafeHwnd ()) 
{ 
return FALSE; 
} 

CRect clientRect; 
GetClientRect (&clientRect); 

CRect btnRect; 
m_DropDownButton.GetWindowRect (&btnRect); 
ScreenToClient (&btnRect); 

CRect editRect; 
editRect.left = clientRect.left; 
editRect.top = clientRect.top; 
editRect.bottom = clientRect.bottom; 
editRect.right = btnRect.left; 


BOOL bCreate = m_Edit.Create (WS_CHILD | WS_TABSTOP | ES_LEFT | WS_VISIBLE | ES_WANTRETURN, editRect, this, IDC_COMBOTREE_EDIT); 

if (bCreate) 
{ 
CWnd* pParent = GetParent (); 
CFont* pFont = pParent->GetFont (); 
m_Edit.SetFont (pFont); 
} 

return bCreate; 
}