question

Hi folks  Esterday   With my bosses We discuss one new web  project and  one of them told me not to use string becouse   if two  users  are in the same asp page they'll  not get currect value of the string  varible  in other words if we have 
 
strin a;
 a = txt_box.text;
 
 int b  = int.parse(a) +1;
txt_box.text =b.tostring();
 
they'll get wrong answerts, I tested this and i get the currect  answert but i wanna know is this true that my bosses told me about the varibles in code  that is using by more than one user and in whch situation this may not work corectly ?  
 
 
best regards Peter
this is becouse thqy wanna use session[]     to avoid the  stirng error  
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