Can you explain me the effect of the lines:
Textbox::Gotfocus
and then
NODEFAULT

In addition to what Alex said, the reason that you must execute the base class behavior first, then set SelStart and SelLength and suppress the native base class behavior afterward is that the when the native base class behavior runs, one of the effects is to reset SelStart and SelLength. That is why you use TextBox::GotFocus and NODEFAULT: to control when the base class behavior runs relative to your custom code. The default behavior is for VFP to run any custom code in you method and then run the native behavior (if any) afterward.