The IsAvailable property in S7netplus v0.9.0 triggers Connect internally, which destroys any existing connection. The following snippet from the library source illustrates the issue:

public bool IsAvailable
{
    //TODO: Fix This
    get
    {
        try
        {
            OpenAsync().GetAwaiter().GetResult();
            return true;
        }
        catch
        {
            return false;
        }
    }
}

An earleir version (v0.1.9, 2018-05-14) used a fresh socket per check without disturbing the active connection:

public bool IsAvailable
{
    get
    {
#if NETFX_CORE
        return (!string.IsNullOrWhiteSpace(IP));
#else
        using (var socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp))
        {
            return Connect(socket) == ErrorCode.NoError;
        }
#endif
    }
}

You can encapsulate a similar approach in your own wrapper arround v0.9.0 to safely test connectivity.

Socket-Based Availability Check

public bool CheckAvailability
{
    get
    {
        if (_plc == null || string.IsNullOrWhiteSpace(_plc.IP))
            return false;

        using var probeSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        string msg;
        bool available = EstablishConnection(probeSocket, _plc.IP, _plc.Port, out msg) == ErrorCode.NoError;
        if (!available)
            LastErrorMessage = msg;

        return available;
    }
}

private static ErrorCode EstablishConnection(Socket sock, string host, int port, out string errorDetail)
{
    errorDetail = string.Empty;
    try
    {
        var remoteEp = new IPEndPoint(IPAddress.Parse(host), port);
        sock.Connect(remoteEp);
        return ErrorCode.NoError;
    }
    catch (SocketException se)
    {
        var code = se.SocketErrorCode == SocketError.TimedOut
            ? ErrorCode.IPAddressNotAvailable
            : ErrorCode.ConnectionError;
        errorDetail = se.Message;
        return code;
    }
    catch (Exception ex)
    {
        errorDetail = ex.Message;
        return ErrorCode.ConnectionError;
    }
}

Adding a Timeout with ICMP Ping

Socket connection timeout control can be tricky. The System.Net.NetworkInformation.Ping class provides a simpler timeout mechanism but only tests IP-level reachability, not a specific port. This is suitable after a logical sesssion is established.

public bool IsPlcReachable
{
    get
    {
        if (_plc == null || string.IsNullOrWhiteSpace(_plc.IP))
            return false;

        return SendPing(_plc.IP, 1000);
    }
}

private static bool SendPing(string targetIp, int timeoutMs)
{
    var pinger = new Ping();
    try
    {
        var reply = pinger.Send(targetIp, timeoutMs);
        return reply != null && reply.Status == IPStatus.Success;
    }
    catch (PingException)
    {
        return false;
    }
    finally
    {
        pinger.Dispose();
    }
}