Where should I place code ? I tried with the lostfocus event of the control but it does not work .

If you put this code in the LostFocus() you need to put a NODEFAULT after the code because the default behavior is to junp to the next control in the tab order.

Another thing :-) I build a txtbox class wich test f th user made change.
I put a property called oldvalue.
In the when event i set this property = to the actual control value.

I do this sort of thing in my framework, but I set this.OldValue = This.Value in the GotFocus and not in the When().

in the lostfocus event i test if the control value is equal to the oldvalue,

This code, in my framework, goes in the Valid()