HOw to set tabstop=false of controls on certain conditions.

Question

in winforms ,, i want to set tabstop property false of some textbox,

 

if textbox is already filled with some text then its tabstop should be false

 

i did 

if(textbox1.text!="")

{

textbox1.text.tabstop=false;

}

if(textbox2.text!="")

{

textbox2.text.tabstop=false;

}  

 

but under which event should i write it ?   help me plz