FileLogger.cs

using System;
using System.IO;

namespace DelegatesAndEvents
{
    // <SnippetFileLogger>
    public class FileLogger
    {
        private readonly string logPath;
        public FileLogger(string path)
        {
            logPath = path;
            Logger.WriteMessage += LogMessage;
        }

        public void DetachLog() => Logger.WriteMessage -= LogMessage;
        // make sure this can't throw.
        private void LogMessage(string msg)
        {
            try
            {
                using (var log = File.AppendText(logPath))
                {
                    log.WriteLine(msg);
                    log.Flush();
                }
            }
            catch (Exception)
            {
                // Hmm. We caught an exception while 
                // logging. We can't really log the 
                // problem (since it's the log that's failing).
                // So, while normally, catching an exception
                // and doing nothing isn't wise, it's really the
                // only reasonable option here.
            }
        }
    }
    // </SnippetFileLogger>
}