Dynamically re-creating a combo box

Introduction

Something which I frequently find myself needing to do is to switch a combo box to or from sorted. This is one reason (though not the main reason) I came up with my Dynamically Switchable Control. However, changing these styles after creation is something that combo-box does not support. This function presented here will recreate the combo box so that style changes take effect. The function will retain all list items, with item data, and the currently selected, or entered, text.

When the style is modified on a combobox to include or remove the CBS_SORT for example, it can be seen (by using GetStyle(), or using a tool such as Spy++) that the control does in fact have the new style, and yet the control's behaviour has not changed. The function here simply gets the current info from the control, i.e. styles, font etc, and recreates the control using these values.

How to use

I originally had a CComboBox-derived class which one member function to recreate the controls, but decided that this was more trouble than it was worth, as most of my combos derived from other things. Therefore I have presented this as a single function, which should be included in a global library file, so that you can invoke it on any combobox at any time.

In order to change the style of a combo, you should perform the usual method of setting the new style e.g.

combo.ModifyStyle(0, CBS_SORT);

and then after that call you should call:

RecreateComboBox(&combo)

The function takes an optional void pointer, lpParam, which is passed to CreateEx on recreating the control. If you have special requirements when creating your controls, and you normally pass in some data for the creation parameters, then you should pass the same info in here. Most developers can simply ignore this parameter.

The Function

The function to recreate the combo is below:

// recreate the combo box by copying styles etc, and list items
// and applying them to a newly created control
BOOL RecreateComboBox(CComboBox* pCombo, LPVOID lpParam/*=0*/)
{
	if (pCombo == NULL)
		return FALSE;
	if (pCombo->GetSafeHwnd() == NULL)
		return FALSE;

	CWnd* pParent = pCombo->GetParent();
	if (pParent == NULL)
		return FALSE;

	// get current attributes
	DWORD dwStyle = pCombo->GetStyle();
	DWORD dwStyleEx = pCombo->GetExStyle();
	CRect rc;
	pCombo->GetDroppedControlRect(&rc);
	pParent->ScreenToClient(&rc);	// map to client co-ords
	UINT nID = pCombo->GetDlgCtrlID();
	CFont* pFont = pCombo->GetFont();
	CWnd* pWndAfter = pCombo->GetNextWindow(GW_HWNDPREV);

	// get the currently selected text (and whether it is a valid list selection)
	CString sCurText;
	int nCurSel = pCombo->GetCurSel();
	BOOL bItemSelValid = nCurSel != -1;
	if (bItemSelValid)
		pCombo->GetLBText(nCurSel, sCurText);
	else
		pCombo->GetWindowText(sCurText);

	// copy the combo box items into a temp combobox (not sorted)
	// along with each item's data
	CComboBox comboNew;
	if (! comboNew.CreateEx(dwStyleEx, _T("COMBOBOX"), _T(""), 
                                 dwStyle, rc, pParent, nID, lpParam))
	  return FALSE;
	comboNew.SetFont(pFont);
	int nNumItems = pCombo->GetCount();
	for (int n = 0; n < nNumItems; n++)
	{
		CString sText;
		pCombo->GetLBText(n, sText);
		int nNewIndex = comboNew.AddString(sText);
		comboNew.SetItemData(nNewIndex, pCombo->GetItemData(n));
	}
	// re-set selected text
	if (bItemSelValid)
		comboNew.SetCurSel(comboNew.FindStringExact(-1, sCurText));
	else
		comboNew.SetWindowText(sCurText);

	// destroy the existing window, then attach the new one
	pCombo->DestroyWindow();
	HWND hwnd = comboNew.Detach();
	pCombo->Attach(hwnd);

	// position correctly in z-order
	pCombo->SetWindowPos(pWndAfter == NULL ? 
                                &CWnd::wndBottom : 
                               pWndAfter, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE);

	return TRUE;
}

See Also

• Dynamically re-creating a list box : Function to recreate a listbox at run-time to allow new styles, preserving its data, and selections
• Dynamically switchable multi-purpose control : Control which allows run-time switching between a number of control types, e.g. combo, edit etc

History

• Version 1 - 26 Jul 2002 - First version
• Version 2 - 30 Jul 2002 - modified to include changes suggested by Jean-Michel LE FOL and Davide Zaccanti