TimerApp/Logger.cs

using System;
using System.IO;
using System.Text;

namespace TimerApp
{
    public static class Logger
    {
        private static readonly string LogFile = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "error.log");
        private static readonly object LockObj = new object();

        public static void LogError(string message, Exception? ex = null)
        {
            try
            {
                lock (LockObj)
                {
                    var sb = new StringBuilder();
                    sb.AppendLine($"[{DateTime.Now:yyyy-MM-dd HH:mm:ss}] ERROR: {message}");
                    if (ex != null)
                    {
                        sb.AppendLine($"Exception: {ex.Message}");
                        sb.AppendLine($"StackTrace: {ex.StackTrace}");
                    }
                    sb.AppendLine(new string('-', 50));

                    File.AppendAllText(LogFile, sb.ToString());
                }
            }
            catch
            {
                // Failed to log, nothing we can do
            }
        }
    }
}