using System;
using System.Runtime.InteropServices;

namespace WindowsService1
{
     public class CurrentUser
     {
          private enum WtsInfoClass
          {
               WTSUserName = 5,
               WTSDomainName = 7,
          }

          [DllImport("Wtsapi32.dll")]
          private static extern bool WTSQuerySessionInformation(IntPtr hServer, int sessionId, WtsInfoClass wtsInfoClass, out IntPtr ppBuffer, out int pBytesReturned);

          [DllImport("Wtsapi32.dll")]
          private static extern void WTSFreeMemory(IntPtr pointer);
         
          [DllImport("Kernel32.dll", SetLastError = true)]
          [return: MarshalAs(UnmanagedType.U4)]
          public static extern int WTSGetActiveConsoleSessionId();

          public static string GetLoginedUserName(bool addDomain=false)
          {
               int sessionId = WTSGetActiveConsoleSessionId();

               IntPtr buffer;
               int strLen;
               string username = "SYSTEM";
               if (WTSQuerySessionInformation(IntPtr.Zero, sessionId, WtsInfoClass.WTSUserName, out buffer, out strLen) && strLen > 1)
               {
                    username = Marshal.PtrToStringAnsi(buffer);
                    WTSFreeMemory(buffer);

                    if (addDomain)
                    {
                         if (WTSQuerySessionInformation(IntPtr.Zero, sessionId, WtsInfoClass.WTSDomainName, out buffer, out strLen) && strLen > 1)
                         {
                              username = Marshal.PtrToStringAnsi(buffer) + "\\" + username;
                              WTSFreeMemory(buffer);
                         }
                    }
               }
               return username;
          }

     }
}

using System;
using System.IO;
using System.ServiceProcess;
using System.Timers;

namespace WindowsService1
{
     public partial class Service1 : ServiceBase
     {
          private Timer _timer = new Timer();

          public Service1()
          {
               CanHandleSessionChangeEvent = true;
               InitializeComponent();
          }
         
          protected override void OnStart(string[] args)
          {
               string userName = CurrentUser.GetLoginedUserName();
               WriteToFile($"User '{userName}', Service is started at " + DateTime.Now);
              
               _timer.Elapsed += new ElapsedEventHandler(OnElapsedTime);
               _timer.Interval = 5000; //5 seconds
               _timer.Enabled = true;
          }

          protected override void OnStop()
          {
               string userName = CurrentUser.GetLoginedUserName();
               WriteToFile($"User '{userName}', Service is stopped at " + DateTime.Now);
          }

          protected override void OnSessionChange(SessionChangeDescription changeDescription)
          {
               string userName = CurrentUser.GetLoginedUserName();
               WriteToFile($"User logof and changed to '{userName}'");
          }

          private void OnElapsedTime(object source, ElapsedEventArgs e)
          {
               string userName = CurrentUser.GetLoginedUserName();
               WriteToFile($"User '{userName}', Service is recall at " + DateTime.Now);
          }

          public void WriteToFile(string Message)
          {
               string filepath = "D:\\my_service.txt";

               // will write text in end of file
               using (StreamWriter fileStream = File.Exists(filepath) ? File.AppendText(filepath): File.CreateText(filepath))
               {
                    fileStream.WriteLine(Message);
               }
          }
     }
}