HOWTO: Subclass CListBox and CEdit Inside of CComboBox

• The Microsoft Foundation Classes (MFC) included with:

      - Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1, 4.2, 5.0
  

SUMMARY

While it is simple to directly subclass a combo box control, it is not simple to subclass the edit or list box inside a combo box. The problem is that it is difficult to get the HWNDs of the child controls in a portable manner.

One safe way to subclass the internal edit and list box controls is to subclass them in the WM_CTLCOLORXXX messages. Because Win32 sends separate WM_CTLCOLOREDIT and WM_CTLCOLORLISTBOX messages, these messages are safe and easy ways to get the HWNDs of the child controls of the combo box.

Below is a CSuperComboBox class, which is an MFC implementation of this method. Because MFC routes all the WM_CTLCOLOR messages to OnCtlColor, the subclassing takes place there.

MORE INFORMATION

Use ClassWizard to derive a class from CComboBox and add message handlers for WM_CTLCOLOR and WM_DESTROY. Then manually edit the header file to add the data members, m_edit and m_listbox. Finally, copy the code from the message handlers below:

Sample Code

   // SuperComboBox.h : header file
   class CSuperComboBox : public CComboBox
   {
   public:
      CEdit      m_edit;
      CListBox   m_listbox;
   protected:
      afx_msg HBRUSH OnCtlColor(CDC* pDC, CWnd* pWnd, UINT nCtlColor);
      afx_msg void OnDestroy();
   ...
   };

   // SuperComboBox.cpp : implementation file
   HBRUSH CSuperComboBox::OnCtlColor(CDC* pDC, CWnd* pWnd, UINT nCtlColor)
   {
      if (nCtlColor == CTLCOLOR_EDIT)
      {
         //[ASCII 160][ASCII 160][ASCII 160]Edit control
         if (m_edit.GetSafeHwnd() == NULL)
            m_edit.SubclassWindow(pWnd->GetSafeHwnd());
      }
      else if (nCtlColor == CTLCOLOR_LISTBOX)
      {
         //ListBox control
         if (m_listbox.GetSafeHwnd() == NULL)
            m_listbox.SubclassWindow(pWnd->GetSafeHwnd());
      }
      HBRUSH hbr = CComboBox::OnCtlColor(pDC, pWnd, nCtlColor);
      return hbr;
   }

   void CSuperComboBox::OnDestroy()
   {
      if (m_edit.GetSafeHwnd() != NULL)
         m_edit.UnsubclassWindow();
      if (m_listbox.GetSafeHwnd() != NULL)
         m_listbox.UnsubclassWindow();
      CComboBox::OnDestroy();
   }

(c) Microsoft Corporation 1997, All Rights Reserved. Contributions by Kelly Marie Ward, Microsoft Corporation