[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(
    IntPtr hServer, int sessionId, WTS_INFO_CLASS wtsInfoClass, out
 
 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
}

public static List<string> GetLoginedUsers(string serverName)
{
    IntPtr serverHandle = IntPtr.Zero;
    List<string> resultList = new List<string>();
    serverHandle = WTSOpenServer(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));
        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);

                var domain = Marshal.PtrToStringAnsi(domainPtr);
                var username = Marshal.PtrToStringAnsi(userPtr);
                
                if(!string.IsNullOrWhiteSpace(username))
                    resultList.Add(username);

                WTSFreeMemory(userPtr);
                WTSFreeMemory(domainPtr);
            }

            WTSFreeMemory(sessionInfoPtr);
        }
    }
    finally
    {
        WTSCloseServer(serverHandle);
    }

    return resultList;
}

void Main()
{
    GetLoginedUsers(Environment.MachineName).Dump();
}

// Define other methods and classes here