MFC OnEnChange handler function - infinite loop - mfc

Question

(I'm using VS++2005)
I put edit box control (with ID - ID_edit_box) on my dialog, and associate (with handler wizard) two varibles for it: control (c_editbox) and value (v_editbox) variable. Also I associate handler function OnEnChangeedit_box with that edit box control. Suppose that we may enter just one digit in edit box, and that digit can be 0 or 1. If we enter some other value - what I want is that content of that edit box is automaticaly cleared, so user can't see that he type anything (in other words user can not enter anything except 0/1 in edit box). I do that check in onEnChangeedit_box function. Here is the code:
void CSDRDlg::OnEnChangeedit_box()
{
CWnd* pWnd;
CString edit_box_temp;
pWnd = GetDlgItem(ID_edit_box);
pWnd->GetWindowText(edit_box_temp);
if ((edit_box_temp == "0" || edit_box_temp == "1")
{...do something - i.e. setfocus on some other edit box }
else
{
pWnd->SetWindowText(""); // clear the content of edit box
//... any other statement below will not be executed because the
//above line cause again call of this function
}
}
I debug and discover that line: pWnd->SetWindowText(""); cause an infinite loop because we change control content in this function which triggers again her call.
But I change above code like this:
void CSDRDlg::OnEnChangeedit_box()
{
UpdateData(TRUE);
if ((v_editbox == "0" || v_editbox== "1")
{...do something - i.e. setfocus on some other edit box }
else
{
v_editbox = "";
UpdateData(FALSE);
}
}
and that works what I want but can someone explain to me why when we call
v_editbox = "";
UpdateData(FALSE);
that doesn't cause an infinite loop.

Answer 1

Why don't you Set Min/Max value to 0/1 or Set Value Type as bool when you Add Variable for your EditBox

Answer 2

You should probably do this by subclassing CEdit and filter out invalid characters. eg:
class CSDREdit public CEdit
{
afx_msg void OnChar(UINT nChar, UINT nRepCnt, UINT nFlags);
DECLARE_MESSAGE_MAP()
};
BEGIN_MESSAGE_MAP(CSDREdit, CEdit)
ON_WM_CHAR()
END_MESSAGE_MAP()
void CSDREdit::OnChar(UINT nChar, UINT nRepCnt, UINT nFlags)
{
if (nChar != _T('0') && nChar != _T('1'))
return;
CEdit::OnChar(nChar, nRepCnt, nFlags);
}