Reading file that is being writen by another app

Hi,
little explanation of my problem:
there is an third party app, that i have no control of.
this app writes log file, but its not : write all, then close file, its writing when something happen,
app does not exclusively lock file
i want to be notified whenever that app writes something to file and read new content
is there any proper way of dealing with that kind of situation ?

1) i have tried FileSystemWatcher, to check if files was changed, but it notifies me only on app opening/closing file, not on every write

2) i am able to read file just the way i want using FileStream.BeginRead, but its using 100% cpu,
and spams read operations like crazy, i can successfully lower cpu usage by Thread.Sleep, but i think its not what async operations are for ?

any advice is appreciated

You could simply read the file inside of a timer loop. Read the file until you hit the end, save the index, start next time from that index. You are pegging the CPU because you are doing it inside of a tight loop which obviously won't work (and is a bad idea anyhow).

thanks for advice, i will do it as you suggested