Add IsOpen flag to SerialLink.

Co-authored-by: alexander.mushinskiy <amushinskiy@gmail.com>

Author: amushinskiy

ThreeByte.LinkLib/ThreeByte.LinkLib.SerialLink/SerialLink.cs

@@ -14,6 +14,7 @@ public class SerialLink : IDisposable
         public event EventHandler? DataReceived;
         public bool IsConnected => _isConnected;
         public bool IsEnabled => _isEnabled;
+        public bool IsOpen => _serialPort?.IsOpen ?? false;
         public bool HasData => _incomingData.Count > 0;
 
         private const int MaxDataSize = 100;

ThreeByte.LinkLib/ThreeByte.LinkLib.TcpLink/AsyncTcpLink.cs

@@ -391,7 +391,7 @@ private void ReadCallback(IAsyncResult asyncResult)
         ///     Fetches and removes (pops) the next available group of bytes as received on this link in order (FIFO)
         /// </summary>
         /// <returns>null if the link is not Enabled or there is no data currently queued to return, an array of bytes otherwise.</returns>
-        public byte[] GetMessage()
+        public byte[]? GetMessage()
         {
             if (_isDisposed)
             {
@@ -404,7 +404,7 @@ public byte[] GetMessage()
                 return null;
             }
 
-            byte[] newMessage = null;
+            byte[]? newMessage = null;
             lock (_incomingData)
             {
                 if (HasData)