How to retrieve the username of the user who logged onto Windows from windows service

Recently, I have been tasked to write a .NET application that will prepare the operating system environment for a user who had logged on a Windows 7 machine. As a first step, the application should grab some settings from the network by providing the username of the user to a server application. The application will then prepare the operating system environment based on the settings received.

Writing the application as a windows service is a favorable option. This is because a windows service application can be configured to run automatically and will run before any user logs into the operating system. Furthermore, a windows service can run as Local System, which mean that it can execute most operating system facilities without facing permission issues.

Detecting user login event

Detecting user login event from a windows service is easy. By overriding the OnSessionChange method from the ServiceBase class, we can detect when a user log onto windows.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Diagnostics;
using System.ServiceProcess;
using System.Text;

namespace MyWindowsService
{
    public partial class MyWindowsService : ServiceBase
    {
        public MyWindowsService()
        {
            // Generated by Visual Studios
            InitializeComponent();

            // This is needed for this window service to detect session changes
           base.CanHandleSessionChangeEvent = true;
        }

        protected override void OnStart(string[] args)
        {
            // Code to execute when windows service starts
        }

        protected override void OnStop()
        {
            // Code to execute when windows service stops
        }

        protected override void OnSessionChange(SessionChangeDescription
            changeDescription)
        {
            // Use a switch block to segment the actions to perform for
            // different session changes
            switch (changeDescription.Reason)
            {
                case SessionChangeReason.SessionLogon:
                    // Retrieve the username of the user who had just logged on
                    string username = Machine.getInstance().getUsername();
                break;

                case SessionChangeReason.SessionLogoff:
                    // Could perform any restoration of the operating system
                    // environment here.
                break;

                case SessionChangeReason.ConsoleLogon:
                    // Triggered whenever a user logs in after
                    // switching user accounts
                    // Triggered once by the windows service
                    // account whenever a user logs in
                break;
            }

            // Inform the base class of the change as well
            base.OnSessionChange(changeDescription);
        }
    }
}

As long as the windows service is started automatically, the OnSessionChange method block will be called whenever there are changes in the user session. This includes the case when a user logs on to windows, in which Machine.getInstance().getUsername() will be called. Note that besides log on and log off events, there are several other events defined in the SessionChangeReason enumeration that you can handle from your windows service.

Attempts made to retrieve the username

It makes sense for me to implement the Machine class as a Singleton. Machine.getInstance() will return me the Machine Singleton. The getUsername instance method will return the username of the user who is currently logged on to windows.

Environment.UserName and WindowsIdentity.GetCurrent().Name does not work

My first intuition to implementing the getUsername() method is to utilitise facilities from the .NET framework, namely the UserName property from the Environment class or the Name property of the WindowsIdentity class.  However, since the windows service is running as Local System , which belongs to a separate environment from the logged on user, the string "SYSTEM" was returned instead.

Looking for the right username with WMI

Windows Management Instrumentation (WMI) was my next hope. A couple of google searches brought me two methods for retrieving the right username.

Using the Win32_Process WMI class

The first method utilises the Win32_Process WMI class to inspect the owners of the explorer.exe process. Although I could get the list of usernames who are logged onto the machine, I felt that too much work is required to get the user who is currently active.

ManagementScope ms = new ManagementScope("\\\\.\\root\\cimv2");
ObjectQuery query = new ObjectQuery
    ("select * from Win32_Process where name = 'explorer.exe'");
ManagementObjectSearcher searcher = new ManagementObjectSearcher(ms, query);
ManagementObjectCollection returnCollection = searcher.Get();

foreach (ManagementObject mo in returnCollection)
{
    string[] argList = new string[] { string.Empty };
    int returnVal = Convert.ToInt32( mo.InvokeMethod("GetOwner", argList));
    if (returnVal == 0)
    {
        Console.WriteLine("User name:");
        Console.WriteLine(argList[0].ToString());
    }
}

Using the Win32_ComputerSystem WMI class

The second method utilises the Win32_ComputerSystem WMI class:

ManagementScope ms = new ManagementScope("\\\\.\\root\\cimv2");
ObjectQuery query = new ObjectQuery("SELECT * FROM Win32_ComputerSystem");
ManagementObjectSearcher searcher = new ManagementObjectSearcher(ms, query);
foreach(ManagementObject mo in searcher.Get())
{
    Console.WriteLine(mo["UserName"].ToString());
}

This method is much better than the previous one in that there is only at most one instance of the Win32_ComputerSystem WMI class. Whenever a user logged onto windows, the Username attribute will contain the username of the user. In the case when there are no users in the windows system, there will be no instances of the Win32_ComputerSystem class. These properties make Win32_ComputerSystem my ideal choice in implementing the getUsername method for my Machine class.

Implementing the Machine class

public class Machine
{
    private static Object _classLocker = new Object();
    private static Machine _machine; 

    private Machine()
    {
    } // end private Machine()

    public static Machine getInstance()
    {
        if (_machine == null)
        {
            lock (_classLocker)
            {
                if (_machine == null)
                {
                    _machine = new Machine();
                }
            }
        }
        return _machine;
    } // end public static Machine getInstance()

    public String getUsername()
    {
        string username = null;
        try
        {
            // Define WMI scope to look for the Win32_ComputerSystem object
            ManagementScope ms = new ManagementScope("\\\\.\\root\\cimv2");
            ms.Connect();

            ObjectQuery query = new ObjectQuery
                    ("SELECT * FROM Win32_ComputerSystem");
            ManagementObjectSearcher searcher =
                    new ManagementObjectSearcher(ms, query);

            // This loop will only run at most once.
            foreach (ManagementObject mo in searcher.Get())
            {
                // Extract the username
                username = mo["UserName"].ToString();
            }
            // Remove the domain part from the username
            string[] usernameParts = username.Split('\\');
            // The username is contained in the last string portion.
            username = usernameParts[usernameParts.Length - 1];
        }
        catch (Exception)
        {
            // The system currently has no users who are logged on
            // Set the username to "SYSTEM" to denote that
            username = "SYSTEM";
        }
        return username;
    } // end String getUsername()
} // end class Machine

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