tristate check box

I have a tristate check box.The default behavour of tristate check box
in windows is that, the grayed check box will become unchecked after
click on grayed check box.
But i want , the grayed check box will become checked after click on
grayed check box in VC++ .Pls help me its urgent.
The code i have written is ........
CButton *l_CheckBox = (CButton
*)GetDlgItem(IDC_ACCESS_AUTHORITY_SELECT);
if(BST_INDETERMINATE == l_CheckBox->GetCheck())
{
l_CheckBox->SetCheck(TRUE);
}