system.diagnostic trace not creating file in asp.net

If your System.Diagnostic trace is not creating a file in asp.net make sure the the user that the website/webapp is running under has write permissions to the log file folder(default is web root ie where the web.config is).

You can find the user that the website/webapp is running under by going into IIS. Go to the application pool that the website/webapp is running under and the Identity column will show the user that needs write permissions to the log file folder.