>I have fixed this problem, but I want to know why it happens.
>

>Define myChkBox as ChkBox
>  Caption = 'Re\<fused'      && User can press {alt+f} to toggle value
&gt;  Procedure When()
&gt;    local lo
&gt;    lo = Thisform.ActiveControl
&gt;    This.Value = !This.Value
&gt;    lo.SetFocus()
&gt;    Return .F.
&gt;  EndProd
&gt;EndDefine
&gt;

>This works just fine when the user clicks on the chkBox with the mouse, but if the user uses {alt+f} to do it, error "ACTIVECONTROL IS NOT AN OBJECT" and debugger shows "" for ActiveControl.
>
>Why does the keyboard shortcut throw away the ActiveControl?

It seems like there's no activecontrol at that moment, which is not unusual. I have never managed to use form.ActiveControl without first checking whether it's an object. It is quite possible that no control on your form has the focus at any given moment, and that's when form.ActiveControl will be either null or undefined. So you better check for it.