file or directory is corrupted and unreadable in c# using windows application

As the solution against file write operation, write caching doesn't cause file corruption. Shutting the machine off while the file is open causes the file corruption.

If you capture the exception then you should be able to delete the file.

You can probably lessen the frequency of errors if you call Flush on the log file whenever you write to it. You can almost completely eliminate the error if you close the file after every write (which, of course, would require that you open it for append before every write). That might be prohibitively expensive.

You can't completely eliminate the error as long as the only way to shut down the application is to remove power. You might consider rotating the log, though, so if it does get corrupted you only lose the last hour or so.