Hi ive just started using build events so im not very good at it yet, ive managed to rename an assembly once compiled like this

rename $(TargetFileName) $(TargetName).customextention

which changes the extention from .exe/.dll to .customextention

however once compiled and renamed, when i compile my solution without making changes to that project i get an error saying it cant rename a file that isnt there. so my question is how do i do something like this which doesnt work

if Exists $(TargetFileName) rename $(TargetFileName) $(TargetName).customextention