class Program 
   { 
       [DllImport("wtsapi32.dll")] 
       static extern IntPtr WTSOpenServer([MarshalAs(UnmanagedType.LPStr)] String pServerName);

       [DllImport("wtsapi32.dll")] 
       static extern void WTSCloseServer(IntPtr hServer);

       [DllImport("wtsapi32.dll")] 
       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);

       [DllImport("wtsapi32.dll")] 
       static extern void WTSFreeMemory(IntPtr pMemory);

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

       [StructLayout(LayoutKind.Sequential)] 
       private struct WTS_SESSION_INFO 
       { 
           public Int32 SessionID;

           [MarshalAs(UnmanagedType.LPStr)] 
           public String pWinStationName;

           public WTS_CONNECTSTATE_CLASS State; 
       }

       public enum WTS_INFO_CLASS 
       { 
           WTSInitialProgram, 
           WTSApplicationName, 
           WTSWorkingDirectory, 
           WTSOEMId, 
           WTSSessionId, 
           WTSUserName, 
           WTSWinStationName, 
           WTSDomainName, 
           WTSConnectState, 
           WTSClientBuildNumber, 
           WTSClientName, 
           WTSClientDirectory, 
           WTSClientProductId, 
           WTSClientHardwareId, 
           WTSClientAddress, 
           WTSClientDisplay, 
           WTSClientProtocolType 
       } 
       public enum WTS_CONNECTSTATE_CLASS 
       { 
           WTSActive, 
           WTSConnected, 
           WTSConnectQuery, 
           WTSShadow, 
           WTSDisconnected, 
           WTSIdle, 
           WTSListen, 
           WTSReset, 
           WTSDown, 
           WTSInit 
       }

       static void Main(string[] args) 
       { 
           ListUsers(Environment.MachineName); 
           Console.ReadKey(); 
       }

       public static IntPtr OpenServer(String Name) 
       { 
           IntPtr server = WTSOpenServer(Name); 
           return server; 
       } 
       public static void CloseServer(IntPtr ServerHandle) 
       { 
           WTSCloseServer(ServerHandle); 
       } 
       public static void ListUsers(String ServerName) 
       { 
           IntPtr serverHandle = IntPtr.Zero; 
           List<String> resultList = new List<string>(); 
           serverHandle = OpenServer(ServerName);

           try 
           { 
               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)); 
               Int32 currentSession = (int)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);

                       Console.WriteLine(string.Format("SessionID : {0} Domain : {1} User : {2} StationName : {3} State : {4}", 
                           si.SessionID, Marshal.PtrToStringAnsi(domainPtr), Marshal.PtrToStringAnsi(userPtr), si.pWinStationName, si.State));

                       WTSFreeMemory(userPtr); 
                       WTSFreeMemory(domainPtr); 
                   }

                   WTSFreeMemory(SessionInfoPtr); 
               } 
           } 
           finally 
           { 
               CloseServer(serverHandle); 
           }

       }

   }