This article discusses how to fetch User (Names/& Ids) who are registered and or logged on to Windows System including Windows Server. Please feel free to add and update as necessary, it is the wiki way!

Fetching All Users On The System

You can achieve this by using WQL to query Win32_UserAccount from your C# code as follows:

using System.Management // This namespace is required for SelectQuery Class and other management objects
SelectQuery query = new SelectQuery("Win32_UserAccount");
ManagementObjectSearcher searcher = new ManagementObjectSearcher(query);
foreach(ManagementObject enVar in searcher.Get())
    Console.WriteLine("UserName: {0}", enVar["Name"]);

Fetching All Logged On/Connected Users On Windows Server

You can achieve by using the following code:

using System.Runtime.InteropServices;       
        static extern IntPtr WTSOpenServer([MarshalAs(UnmanagedType.LPStr)] String pServerName);
        static extern void WTSCloseServer(IntPtr hServer);
        static extern Int32 WTSEnumerateSessions(
            IntPtr hServer,
            [MarshalAs(UnmanagedType.U4)] Int32 Reserved,
            [MarshalAs(UnmanagedType.U4)] Int32 Version,
            ref IntPtr ppSessionInfo,
            [MarshalAs(UnmanagedType.U4)] ref Int32 pCount);
        static extern void WTSFreeMemory(IntPtr pMemory);
        static extern bool WTSQuerySessionInformation(
            System.IntPtr hServer, int sessionId, WTS_INFO_CLASS wtsInfoClass, out System.IntPtr ppBuffer, out uint pBytesReturned);
        private struct WTS_SESSION_INFO
            public Int32 SessionID;
            public String pWinStationName;
            public WTS_CONNECTSTATE_CLASS State;
        public enum WTS_INFO_CLASS
        public enum WTS_CONNECTSTATE_CLASS
        public static IntPtr OpenServer(String Name)
            IntPtr server = WTSOpenServer(Name);
            return server;
        public static void CloseServer(IntPtr ServerHandle)
        public static void ListUsers(String ServerName)
            IntPtr serverHandle = IntPtr.Zero;
            List<String> resultList = new List<string>();
            serverHandle = OpenServer(ServerName);
                IntPtr SessionInfoPtr = IntPtr.Zero;
                IntPtr userPtr = IntPtr.Zero;
                IntPtr domainPtr = IntPtr.Zero;
                Int32 sessionCount = 0;
                Int32 retVal = WTSEnumerateSessions(serverHandle, 0, 1, ref SessionInfoPtr, ref sessionCount);
                Int32 dataSize = Marshal.SizeOf(typeof(WTS_SESSION_INFO));
                IntPtr currentSession = SessionInfoPtr;
                uint bytes = 0;
                if (retVal != 0)
                    for (int i = 0; i < sessionCount; i++)
                        WTS_SESSION_INFO si = (WTS_SESSION_INFO)Marshal.PtrToStructure((System.IntPtr)currentSession, typeof(WTS_SESSION_INFO));
                        currentSession += dataSize;
                        WTSQuerySessionInformation(serverHandle, si.SessionID, WTS_INFO_CLASS.WTSUserName, out userPtr, out bytes);
                        WTSQuerySessionInformation(serverHandle, si.SessionID, WTS_INFO_CLASS.WTSDomainName, out domainPtr, out bytes);
                                                                 // UserName and Domain is printed out here
                        Console.WriteLine("Domain and User: {0}\\{1}", Marshal.PtrToStringAnsi(domainPtr), Marshal.PtrToStringAnsi(userPtr));

By calling the method ListUsers(String ServerName) you can display all the users with their corresponding Domain as well. And in case you want to display connected/Logged On users on the system where the assembly is executed then you can use the following code:

// Environment.MachineName shows the name of this System