how to autofill textboxes when switching from one tab to another in c#

how to autofill textboxes when switching from one tab to another in c#

I am developing an application (in c# using Visual studio 2012) where i
have to read a config file which is in .txt format. I am also using tab
control to switch between different tab. what i want to do is that, when i
click on config tab, my application should read the config file and place
the data that i require into the text boxes that are present under that
tab. Any help would be appreciated.
i have tried placing the code to autofill my textboxes in the form
constructor and it works, but i don't want to do it through constructor
because it will only read the config file when the form is created and not
when i switch from one tab to another.