How to prevent changes done in class

Question

some one ask me in interview for c# developer for job one question but i cannot answer him

 

this question is :

 

if you create class and do some changes in this class and you dont need this changes

 

happen

 

what the ways that prevent this changes happen

 

tell us two way to prevent changes from doing . 

 

I work in c# developer c#

can you tell me how i can answer this question if possible