how to write the data in already opened text file

Question

i develop one windows service in the i use filesystemWatcher if any event is raised i write that information into text file in e:\ location all work fine but

while the service is running suppose i open that text file it shows the content but after at the stop that service an un handled exception thrown what is the problem

what i understand is when the file is open we try to write the content on that file through stream that error will come what is the solution for this problem