Release S7NetPlus 0.12.0

Release highlights:
- Add synchronization to methods interacting with the PLC

S7.Net.UnitTest/Helpers/TestStruct.cs

@@ -1,4 +1,5 @@
-
+using S7.Net.Types;
+
 namespace S7.Net.UnitTest.Helpers
 {
     public struct TestStruct
@@ -7,6 +8,7 @@ namespace S7.Net.UnitTest.Helpers
         /// DB1.DBX0.0
         /// </summary>
         public bool BitVariable00;
+
         public bool BitVariable01;
         public bool BitVariable02;
         public bool BitVariable03;
@@ -19,6 +21,7 @@ namespace S7.Net.UnitTest.Helpers
         /// DB1.DBX1.0           
         /// </summary>           
         public bool BitVariable10;
+
         public bool BitVariable11;
         public bool BitVariable12;
         public bool BitVariable13;
@@ -51,5 +54,17 @@ namespace S7.Net.UnitTest.Helpers
         /// DB1.DBD16
         /// </summary>
         public ushort DWordVariable;
+
+        /// <summary>
+        /// DB1.DBX20.0
+        /// </summary>
+        [S7String(S7StringType.S7WString, 10)] 
+        public string WStringVariable;
+
+        /// <summary>
+        /// DB1.DBX44.0
+        /// </summary>
+        [S7String(S7StringType.S7String, 10)]
+        public string StringVariable;
     }
 }

S7.Net.UnitTest/S7NetTestsAsync.cs

@@ -211,7 +211,9 @@ namespace S7.Net.UnitTest
                 IntVariable = -15000,
                 LRealVariable = -154.789,
                 RealVariable = -154.789f,
-                DWordVariable = 850
+                DWordVariable = 850,
+                WStringVariable = "ÄÜÉÊéà",
+                StringVariable = "Hallo"
             };
             plc.WriteStruct(tc, DB2);
             // Values that are read from a struct are stored in a new struct, returned by the funcion ReadStruct
@@ -223,6 +225,8 @@ namespace S7.Net.UnitTest
             Assert.AreEqual(tc.LRealVariable, tc2.LRealVariable);
             Assert.AreEqual(tc.RealVariable, tc2.RealVariable);
             Assert.AreEqual(tc.DWordVariable, tc2.DWordVariable);
+            Assert.AreEqual(tc.WStringVariable, tc2.WStringVariable);
+            Assert.AreEqual(tc.StringVariable, tc2.StringVariable);
         }
 
         /// <summary>
@@ -739,7 +743,9 @@ namespace S7.Net.UnitTest
                 IntVariable = -15000,
                 LRealVariable = -154.789,
                 RealVariable = -154.789f,
-                DWordVariable = 850
+                DWordVariable = 850,
+                WStringVariable = "ÄÜÉÊéà",
+                StringVariable = "Hallo"
             };
 
             plc.WriteStruct(ts, DB2);
@@ -756,6 +762,8 @@ namespace S7.Net.UnitTest
             Assert.AreEqual(ts2.LRealVariable, ts2Generic.LRealVariable);
             Assert.AreEqual(ts2.RealVariable, ts2Generic.RealVariable);
             Assert.AreEqual(ts2.DWordVariable, ts2Generic.DWordVariable);
+            Assert.AreEqual(ts2.WStringVariable, ts2Generic.WStringVariable);
+            Assert.AreEqual(ts2.StringVariable, ts2Generic.StringVariable);
         }
 
         [TestMethod]

S7.Net.UnitTest/S7NetTestsSync.cs

@@ -212,6 +212,9 @@ namespace S7.Net.UnitTest
             tc.LRealVariable = -154.789;
             tc.RealVariable = -154.789f;
             tc.DWordVariable = 850;
+            tc.WStringVariable = "ÄÜÉÊéà";
+            tc.StringVariable = "Hallo";
+
             plc.WriteStruct(tc, DB2);
             // Values that are read from a struct are stored in a new struct, returned by the funcion ReadStruct
             TestStruct tc2 = (TestStruct)plc.ReadStruct(typeof(TestStruct), DB2);
@@ -222,6 +225,8 @@ namespace S7.Net.UnitTest
             Assert.AreEqual(tc.LRealVariable, tc2.LRealVariable);
             Assert.AreEqual(tc.RealVariable, tc2.RealVariable);
             Assert.AreEqual(tc.DWordVariable, tc2.DWordVariable);
+            Assert.AreEqual(tc.WStringVariable, tc2.WStringVariable);
+            Assert.AreEqual(tc.StringVariable, tc2.StringVariable);
         }
 
         /// <summary>
@@ -783,6 +788,8 @@ namespace S7.Net.UnitTest
             ts.LRealVariable = -154.789;
             ts.RealVariable = -154.789f;
             ts.DWordVariable = 850;
+            ts.WStringVariable = "ÄÜÉÊéà";
+            ts.StringVariable = "Hallo";
 
             plc.WriteStruct(ts, DB2);
 
@@ -797,6 +804,8 @@ namespace S7.Net.UnitTest
             Assert.AreEqual(ts2.LRealVariable, ts2Generic.LRealVariable);
             Assert.AreEqual(ts2.RealVariable, ts2Generic.RealVariable);
             Assert.AreEqual(ts2.DWordVariable, ts2Generic.DWordVariable);
+            Assert.AreEqual(ts2.WStringVariable, ts2Generic.WStringVariable);
+            Assert.AreEqual(ts2.StringVariable, ts2Generic.StringVariable);
         }
 
         [TestMethod, ExpectedException(typeof(PlcException))]
@@ -924,7 +933,14 @@ namespace S7.Net.UnitTest
             S7TestServer.Stop();
 
             var unreachablePlc = new Plc(CpuType.S7300, "255.255.255.255", 0, 2);
-            Assert.IsFalse(unreachablePlc.IsAvailable);
+            try
+            {
+                unreachablePlc.Open();
+            }
+            catch
+            {
+            }
+            Assert.IsFalse(unreachablePlc.IsConnected);
         }
 
         [TestMethod]
@@ -935,7 +951,8 @@ namespace S7.Net.UnitTest
             S7TestServer.Start(TestServerPort);
 
             var reachablePlc = CreatePlc();
-            Assert.IsTrue(reachablePlc.IsAvailable);
+            reachablePlc.Open();
+            Assert.IsTrue(reachablePlc.IsConnected);
         }
 
         [TestMethod]

S7.Net.UnitTest/TypeTests/BooleanTests.cs

@@ -0,0 +1,38 @@
+using System;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Boolean = S7.Net.Types.Boolean;
+
+namespace S7.Net.UnitTest.TypeTests
+{
+    [TestClass]
+    public class BooleanTests
+    {
+        [DataTestMethod]
+        [DataRow(0)]
+        [DataRow(1)]
+        [DataRow(2)]
+        [DataRow(3)]
+        [DataRow(4)]
+        [DataRow(5)]
+        [DataRow(6)]
+        [DataRow(7)]
+        public void TestValidSetBitValues(int index)
+        {
+            Assert.AreEqual(Math.Pow(2, index), Boolean.SetBit(0, index));
+        }
+
+        [DataTestMethod]
+        [DataRow(0)]
+        [DataRow(1)]
+        [DataRow(2)]
+        [DataRow(3)]
+        [DataRow(4)]
+        [DataRow(5)]
+        [DataRow(6)]
+        [DataRow(7)]
+        public void TestValidClearBitValues(int index)
+        {
+            Assert.AreEqual((byte) ((uint) Math.Pow(2, index) ^ uint.MaxValue), Boolean.ClearBit(byte.MaxValue, index));
+        }
+    }
+}

S7.Net.UnitTest/TypeTests/S7StringTests.cs

@@ -100,7 +100,7 @@ namespace S7.Net.UnitTest.TypeTests
         }
 
         [TestMethod]
-        public void WriteAbcWithStringLargetThanReservedLength()
+        public void WriteAbcWithStringLargerThanReservedLength()
         {
             Assert.ThrowsException<ArgumentException>(() => S7String.ToByteArray("Abc", 2));
         }
@@ -117,13 +117,24 @@ namespace S7.Net.UnitTest.TypeTests
             AssertToByteArrayAndBackEquals("Abc", 4, 4, 3, (byte) 'A', (byte) 'b', (byte) 'c', 0);
         }
 
+        [TestMethod]
+        public void OddS7StringByteLength()
+        {
+            AssertVarTypeToByteLength(VarType.S7String, 1, 4);
+        }
+
+        [TestMethod]
+        public void EvenS7StringByteLength()
+        {
+            AssertVarTypeToByteLength(VarType.S7String, 2, 4);
+        }
+
         private static void AssertFromByteArrayEquals(string expected, params byte[] bytes)
         {
             var convertedString = S7String.FromByteArray(bytes);
             Assert.AreEqual(expected, convertedString);
         }
 
-
         private static void AssertToByteArrayAndBackEquals(string value, int reservedLength, params byte[] expected)
         {
             var convertedData = S7String.ToByteArray(value, reservedLength);
@@ -131,5 +142,11 @@ namespace S7.Net.UnitTest.TypeTests
             var convertedBack = S7String.FromByteArray(convertedData);
             Assert.AreEqual(value, convertedBack);
         }
+
+        private void AssertVarTypeToByteLength(VarType varType, int count, int expectedByteLength)
+        {
+            var byteLength = Plc.VarTypeToByteLength(varType, count);
+            Assert.AreEqual(expectedByteLength, byteLength);
+        }
     }
 }

S7.Net.UnitTest/TypeTests/S7WStringTests.cs

@@ -0,0 +1,151 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using S7.Net.Types;
+using System;
+
+namespace S7.Net.UnitTest.TypeTests
+{
+    [TestClass]
+    public class S7WStringTests
+    {
+        [TestMethod]
+        public void ReadEmptyStringWithZeroLength()
+        {
+            AssertFromByteArrayEquals("", 0, 0 , 0, 0);
+        }
+
+        [TestMethod]
+        public void ReadEmptyStringWithOneCharLength()
+        {
+            AssertFromByteArrayEquals("", 0, 1, 0, 0, 0, 0);
+        }
+
+        [TestMethod]
+        public void ReadEmptyStringWithOneCharGarbage()
+        {
+
+            AssertFromByteArrayEquals("", 0, 1, 0, 0, 0x00, 0x41);
+        }
+
+        [TestMethod]
+        public void ReadMalformedStringTooShort()
+        {
+            Assert.ThrowsException<PlcException>(() => AssertFromByteArrayEquals("", 0, 1));
+        }
+
+        [TestMethod]
+        public void ReadMalformedStringSizeLargerThanCapacity()
+        {
+            Assert.ThrowsException<PlcException>(() => S7WString.FromByteArray(new byte[] { 0, 3, 0, 5, 0, 0x00, 0x41, 0x00, 0x41, 0x00, 0x41}));
+        }
+
+        [TestMethod]
+        public void ReadMalformedStringCapacityTooLarge()
+        {
+            Assert.ThrowsException<ArgumentException>(() => AssertToByteArrayAndBackEquals("", 20000, 0));
+        }
+
+        [TestMethod]
+        public void ReadA()
+        {
+            AssertFromByteArrayEquals("A", 0, 1, 0, 1, 0x00, 0x41);
+        }
+
+        [TestMethod]
+        public void ReadAbc()
+        {
+            AssertFromByteArrayEquals("Abc", 0, 3, 0, 3, 0x00, 0x41, 0x00, 0x62, 0x00, 0x63);
+        }
+
+        [TestMethod]
+        public void WriteNullWithReservedLengthZero()
+        {
+            Assert.ThrowsException<ArgumentNullException>(() => AssertToByteArrayAndBackEquals(null, 0, 0, 0, 0, 0));
+        }
+
+        [TestMethod]
+        public void WriteEmptyStringWithReservedLengthZero()
+        {
+            AssertToByteArrayAndBackEquals("", 0, 0, 0, 0, 0);
+        }
+
+        [TestMethod]
+        public void WriteAWithReservedLengthZero()
+        {
+            AssertToByteArrayAndBackEquals("", 0, 0, 0, 0, 0);
+        }
+
+        [TestMethod]
+        public void WriteNullWithReservedLengthOne()
+        {
+            Assert.ThrowsException<ArgumentNullException>(() => AssertToByteArrayAndBackEquals(null, 1, 0, 1 , 0, 0));
+        }
+
+        [TestMethod]
+        public void WriteEmptyStringWithReservedLengthOne()
+        {
+            AssertToByteArrayAndBackEquals("", 1, 0, 1, 0, 0, 0, 0);
+        }
+
+        [TestMethod]
+        public void WriteAWithReservedLengthOne()
+        {
+            AssertToByteArrayAndBackEquals("A", 1, 0, 1, 0, 1, 0x00, 0x41);
+        }
+
+        [TestMethod]
+        public void WriteAWithReservedLengthTwo()
+        {
+            AssertToByteArrayAndBackEquals("A", 2, 0, 2, 0, 1, 0x00, 0x41, 0, 0);
+        }
+
+        [TestMethod]
+        public void WriteAbcWithStringLargerThanReservedLength()
+        {
+            Assert.ThrowsException<ArgumentException>(() => S7WString.ToByteArray("Abc", 2));
+        }
+
+        [TestMethod]
+        public void WriteAbcWithReservedLengthThree()
+        {
+            AssertToByteArrayAndBackEquals("Abc", 3, 0, 3, 0, 3, 0x00, 0x41, 0x00, 0x62, 0x00, 0x63);
+        }
+
+        [TestMethod]
+        public void WriteAbcWithReservedLengthFour()
+        {
+            AssertToByteArrayAndBackEquals("Abc", 4, 0, 4, 0, 3, 0x00, 0x41, 0x00, 0x62, 0x00, 0x63, 0 , 0);
+        }
+
+        private static void AssertFromByteArrayEquals(string expected, params byte[] bytes)
+        {
+            var convertedString = S7WString.FromByteArray(bytes);
+            Assert.AreEqual(expected, convertedString);
+        }
+
+        [TestMethod]
+        public void OddS7WStringByteLength()
+        {
+            AssertVarTypeToByteLength(VarType.S7WString, 1, 6);
+        }
+
+        [TestMethod]
+        public void EvenS7WStringByteLength()
+        {
+            AssertVarTypeToByteLength(VarType.S7WString, 2, 8);
+        }
+
+        private static void AssertToByteArrayAndBackEquals(string value, int reservedLength, params byte[] expected)
+        {
+            var convertedData = S7WString.ToByteArray(value, reservedLength);
+            CollectionAssert.AreEqual(expected, convertedData);
+            var convertedBack = S7WString.FromByteArray(convertedData);
+            Assert.AreEqual(value, convertedBack);
+        }
+
+        private void AssertVarTypeToByteLength(VarType varType, int count, int expectedByteLength)
+        {
+            var byteLength = Plc.VarTypeToByteLength(varType, count);
+            Assert.AreEqual(expectedByteLength, byteLength);
+        }
+    }
+}

S7.Net/Enums.cs

@@ -178,6 +178,11 @@
         /// </summary>
         S7String,
 
+        /// <summary>
+        /// S7 WString variable type (variable)
+        /// </summary>
+        S7WString,
+
         /// <summary>
         /// Timer variable type
         /// </summary>

S7.Net/Internal/TaskQueue.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace S7.Net.Internal
+{
+    internal class TaskQueue
+    {
+        private static readonly object Sentinel = new object();
+
+        private Task prev = Task.FromResult(Sentinel);
+
+        public async Task<T> Enqueue<T>(Func<Task<T>> action)
+        {
+            var tcs = new TaskCompletionSource<object>();
+            await Interlocked.Exchange(ref prev, tcs.Task).ConfigureAwait(false);
+
+            try
+            {
+                return await action.Invoke().ConfigureAwait(false);
+            }
+            finally
+            {
+                tcs.SetResult(Sentinel);
+            }
+        }
+    }
+}

S7.Net/PLC.cs

@@ -1,7 +1,9 @@
 using System;
 using System.Collections.Generic;
+using System.IO;
 using System.Linq;
 using System.Net.Sockets;
+using S7.Net.Internal;
 using S7.Net.Protocol;
 using S7.Net.Types;
 
@@ -13,8 +15,10 @@ namespace S7.Net
     /// </summary>
     public partial class Plc : IDisposable
     {
+        private readonly TaskQueue queue = new TaskQueue();
+
         private const int CONNECTION_TIMED_OUT_ERROR_CODE = 10060;
-        
+
         //TCP connection to device
         private TcpClient? tcpClient;
         private NetworkStream? _stream;
@@ -75,47 +79,26 @@ namespace S7.Net
                 if (tcpClient != null) tcpClient.SendTimeout = writeTimeout;
             }
         }
-        
-        /// <summary>
-        /// Returns true if a connection to the PLC can be established
-        /// </summary>
-        public bool IsAvailable
-        {
-            //TODO: Fix This
-            get
-            {
-                try
-                {
-                    OpenAsync().GetAwaiter().GetResult();
-                    return true;
-                }
-                catch
-                {
-                    return false;
-                }
-            }
-        }
 
         /// <summary>
-        /// Checks if the socket is connected and polls the other peer (the PLC) to see if it's connected.
-        /// This is the variable that you should continously check to see if the communication is working
-        /// See also: http://stackoverflow.com/questions/2661764/how-to-check-if-a-socket-is-connected-disconnected-in-c
+        /// Gets a value indicating whether a connection to the PLC has been established.
         /// </summary>
-        public bool IsConnected
-        {
-            get
-            {
-                try
-                {
-                    if (tcpClient == null)
-                        return false;
-
-                    //TODO: Actually check communication by sending an empty TPDU
-                    return tcpClient.Connected;
-                }
-                catch { return false; }
-            }
-        }
+        /// <remarks>
+        /// The <see cref="IsConnected"/> property gets the connection state of the Client socket as
+        /// of the last I/O operation. When it returns <c>false</c>, the Client socket was either
+        /// never  connected, or is no longer connected.
+        ///
+        /// <para>
+        /// Because the <see cref="IsConnected"/> property only reflects the state of the connection
+        /// as of the most recent operation, you should attempt to send or receive a message to
+        /// determine the current state. After the message send fails, this property no longer
+        /// returns <c>true</c>. Note that this behavior is by design. You cannot reliably test the
+        /// state of the connection because, in the time between the test and a send/receive, the
+        /// connection could have been lost. Your code should assume the socket is connected, and
+        /// gracefully handle failed transmissions.
+        /// </para>
+        /// </remarks>
+        public bool IsConnected => tcpClient?.Connected ?? false;
 
         /// <summary>
         /// Creates a PLC object with all the parameters needed for connections.
@@ -263,6 +246,16 @@ namespace S7.Net
             }
         }
 
+        private Stream GetStreamIfAvailable()
+        {
+            if (_stream == null)
+            {
+                throw new PlcException(ErrorCode.ConnectionError, "Plc is not connected");
+            }
+
+            return _stream;
+        }
+
         #region IDisposable Support
         private bool disposedValue = false; // To detect redundant calls
 

S7.Net/PLCAddress.cs

@@ -183,7 +183,7 @@
                         case "Z":
                         case "C":
                             // Counter
-                            dataType = DataType.Timer;
+                            dataType = DataType.Counter;
                             dbNumber = 0;
                             address = int.Parse(input.Substring(1));
                             varType = VarType.Counter;
@@ -204,4 +204,4 @@
             }
         }
     }
-}
+}

S7.Net/PLCHelpers.cs

@@ -30,8 +30,8 @@ namespace S7.Net
         }
 
         /// <summary>
-        /// Create the bytes-package to request data from the PLC. You have to specify the memory type (dataType), 
-        /// the address of the memory, the address of the byte and the bytes count. 
+        /// Create the bytes-package to request data from the PLC. You have to specify the memory type (dataType),
+        /// the address of the memory, the address of the byte and the bytes count.
         /// </summary>
         /// <param name="dataType">MemoryType (DB, Timer, Counter, etc.)</param>
         /// <param name="db">Address of the memory to be read</param>
@@ -125,6 +125,8 @@ namespace S7.Net
                     return Types.String.FromByteArray(bytes);
                 case VarType.S7String:
                     return S7String.FromByteArray(bytes);
+                case VarType.S7WString:
+                    return S7WString.FromByteArray(bytes);
 
                 case VarType.Timer:
                     if (varCount == 1)
@@ -182,13 +184,15 @@ namespace S7.Net
             switch (varType)
             {
                 case VarType.Bit:
-                    return varCount + 7 / 8;
+                    return (varCount + 7) / 8;
                 case VarType.Byte:
                     return (varCount < 1) ? 1 : varCount;
                 case VarType.String:
                     return varCount;
                 case VarType.S7String:
-                    return varCount + 2;
+                    return ((varCount + 2) & 1) == 1 ? (varCount + 3) : (varCount + 2);
+                case VarType.S7WString:
+                    return (varCount * 2) + 4;
                 case VarType.Word:
                 case VarType.Timer:
                 case VarType.Int:

S7.Net/PlcAsynchronous.cs

@@ -1,11 +1,11 @@
 using S7.Net.Types;
 using System;
 using System.Collections.Generic;
+using System.IO;
 using System.Linq;
 using System.Net.Sockets;
 using System.Threading.Tasks;
 using S7.Net.Protocol;
-using System.IO;
 using System.Threading;
 using S7.Net.Protocol.S7;
 
@@ -28,9 +28,14 @@ namespace S7.Net
             var stream = await ConnectAsync().ConfigureAwait(false);
             try
             {
-                cancellationToken.ThrowIfCancellationRequested();
-                await EstablishConnection(stream, cancellationToken).ConfigureAwait(false);
-                _stream = stream;
+                await queue.Enqueue(async () =>
+                {
+                    cancellationToken.ThrowIfCancellationRequested();
+                    await EstablishConnection(stream, cancellationToken).ConfigureAwait(false);
+                    _stream = stream;
+
+                    return default(object);
+                }).ConfigureAwait(false);
             }
             catch(Exception)
             {
@@ -47,29 +52,30 @@ namespace S7.Net
             return tcpClient.GetStream();
         }
 
-        private async Task EstablishConnection(NetworkStream stream, CancellationToken cancellationToken)
+        private async Task EstablishConnection(Stream stream, CancellationToken cancellationToken)
         {
             await RequestConnection(stream, cancellationToken).ConfigureAwait(false);
             await SetupConnection(stream, cancellationToken).ConfigureAwait(false);
         }
 
-        private async Task RequestConnection(NetworkStream stream, CancellationToken cancellationToken)
+        private async Task RequestConnection(Stream stream, CancellationToken cancellationToken)
         {
             var requestData = ConnectionRequest.GetCOTPConnectionRequest(CPU, Rack, Slot);
-            await stream.WriteAsync(requestData, 0, requestData.Length).ConfigureAwait(false);
-            var response = await COTP.TPDU.ReadAsync(stream, cancellationToken).ConfigureAwait(false);
+            var response = await NoLockRequestTpduAsync(stream, requestData, cancellationToken).ConfigureAwait(false);
+
             if (response.PDUType != COTP.PduType.ConnectionConfirmed)
             {
                 throw new InvalidDataException("Connection request was denied", response.TPkt.Data, 1, 0x0d);
             }
         }
 
-        private async Task SetupConnection(NetworkStream stream, CancellationToken cancellationToken)
+        private async Task SetupConnection(Stream stream, CancellationToken cancellationToken)
         {
             var setupData = GetS7ConnectionSetup();
-            await stream.WriteAsync(setupData, 0, setupData.Length).ConfigureAwait(false);
 
-            var s7data = await COTP.TSDU.ReadAsync(stream, cancellationToken).ConfigureAwait(false);
+            var s7data = await NoLockRequestTsduAsync(stream, setupData, 0, setupData.Length, cancellationToken)
+                .ConfigureAwait(false);
+
             if (s7data.Length < 2)
                 throw new WrongNumberOfBytesException("Not enough data received in response to Communication Setup");
 
@@ -104,7 +110,7 @@ namespace S7.Net
             {
                 //This works up to MaxPDUSize-1 on SNAP7. But not MaxPDUSize-0.
                 var maxToRead = Math.Min(count, MaxPDUSize - 18);
-                await ReadBytesWithSingleRequestAsync(dataType, db, startByteAdr + index, resultBytes, index, maxToRead, cancellationToken);
+                await ReadBytesWithSingleRequestAsync(dataType, db, startByteAdr + index, resultBytes, index, maxToRead, cancellationToken).ConfigureAwait(false);
                 count -= maxToRead;
                 index += maxToRead;
             }
@@ -112,7 +118,7 @@ namespace S7.Net
         }
 
         /// <summary>
-        /// Read and decode a certain number of bytes of the "VarType" provided. 
+        /// Read and decode a certain number of bytes of the "VarType" provided.
         /// This can be used to read multiple consecutive variables of the same type (Word, DWord, Int, etc).
         /// If the read was not successful, check LastErrorCode or LastErrorString.
         /// </summary>
@@ -127,7 +133,7 @@ namespace S7.Net
         public async Task<object?> ReadAsync(DataType dataType, int db, int startByteAdr, VarType varType, int varCount, byte bitAdr = 0, CancellationToken cancellationToken = default)
         {
             int cntBytes = VarTypeToByteLength(varType, varCount);
-            byte[] bytes = await ReadBytesAsync(dataType, db, startByteAdr, cntBytes, cancellationToken);
+            byte[] bytes = await ReadBytesAsync(dataType, db, startByteAdr, cntBytes, cancellationToken).ConfigureAwait(false);
             return ParseBytes(varType, bytes, varCount, bitAdr);
         }
 
@@ -142,7 +148,7 @@ namespace S7.Net
         public async Task<object?> ReadAsync(string variable, CancellationToken cancellationToken = default)
         {
             var adr = new PLCAddress(variable);
-            return await ReadAsync(adr.DataType, adr.DbNumber, adr.StartByte, adr.VarType, 1, (byte)adr.BitNumber, cancellationToken);
+            return await ReadAsync(adr.DataType, adr.DbNumber, adr.StartByte, adr.VarType, 1, (byte)adr.BitNumber, cancellationToken).ConfigureAwait(false);
         }
 
         /// <summary>
@@ -158,7 +164,7 @@ namespace S7.Net
         {
             int numBytes = Types.Struct.GetStructSize(structType);
             // now read the package
-            var resultBytes = await ReadBytesAsync(DataType.DataBlock, db, startByteAdr, numBytes, cancellationToken);
+            var resultBytes = await ReadBytesAsync(DataType.DataBlock, db, startByteAdr, numBytes, cancellationToken).ConfigureAwait(false);
 
             // and decode it
             return Types.Struct.FromBytes(structType, resultBytes);
@@ -175,14 +181,14 @@ namespace S7.Net
         /// <returns>Returns a nulable struct. If nothing was read null will be returned.</returns>
         public async Task<T?> ReadStructAsync<T>(int db, int startByteAdr = 0, CancellationToken cancellationToken = default) where T : struct
         {
-            return await ReadStructAsync(typeof(T), db, startByteAdr, cancellationToken) as T?;
+            return await ReadStructAsync(typeof(T), db, startByteAdr, cancellationToken).ConfigureAwait(false) as T?;
         }
 
         /// <summary>
-        /// Reads all the bytes needed to fill a class in C#, starting from a certain address, and set all the properties values to the value that are read from the PLC. 
+        /// Reads all the bytes needed to fill a class in C#, starting from a certain address, and set all the properties values to the value that are read from the PLC.
         /// This reads only properties, it doesn't read private variable or public variable without {get;set;} specified.
         /// </summary>
-        /// <param name="sourceClass">Instance of the class that will store the values</param>       
+        /// <param name="sourceClass">Instance of the class that will store the values</param>
         /// <param name="db">Index of the DB; es.: 1 is for DB1</param>
         /// <param name="startByteAdr">Start byte address. If you want to read DB1.DBW200, this is 200.</param>
         /// <param name="cancellationToken">The token to monitor for cancellation requests. The default value is None.
@@ -197,7 +203,7 @@ namespace S7.Net
             }
 
             // now read the package
-            var resultBytes = await ReadBytesAsync(DataType.DataBlock, db, startByteAdr, numBytes, cancellationToken);
+            var resultBytes = await ReadBytesAsync(DataType.DataBlock, db, startByteAdr, numBytes, cancellationToken).ConfigureAwait(false);
             // and decode it
             Class.FromBytes(sourceClass, resultBytes);
 
@@ -205,7 +211,7 @@ namespace S7.Net
         }
 
         /// <summary>
-        /// Reads all the bytes needed to fill a class in C#, starting from a certain address, and set all the properties values to the value that are read from the PLC. 
+        /// Reads all the bytes needed to fill a class in C#, starting from a certain address, and set all the properties values to the value that are read from the PLC.
         /// This reads only properties, it doesn't read private variable or public variable without {get;set;} specified. To instantiate the class defined by the generic
         /// type, the class needs a default constructor.
         /// </summary>
@@ -217,11 +223,11 @@ namespace S7.Net
         /// <returns>An instance of the class with the values read from the PLC. If no data has been read, null will be returned</returns>
         public async Task<T?> ReadClassAsync<T>(int db, int startByteAdr = 0, CancellationToken cancellationToken = default) where T : class
         {
-            return await ReadClassAsync(() => Activator.CreateInstance<T>(), db, startByteAdr, cancellationToken);
+            return await ReadClassAsync(() => Activator.CreateInstance<T>(), db, startByteAdr, cancellationToken).ConfigureAwait(false);
         }
 
         /// <summary>
-        /// Reads all the bytes needed to fill a class in C#, starting from a certain address, and set all the properties values to the value that are read from the PLC. 
+        /// Reads all the bytes needed to fill a class in C#, starting from a certain address, and set all the properties values to the value that are read from the PLC.
         /// This reads only properties, it doesn't read private variable or public variable without {get;set;} specified.
         /// </summary>
         /// <typeparam name="T">The class that will be instantiated</typeparam>
@@ -234,7 +240,7 @@ namespace S7.Net
         public async Task<T?> ReadClassAsync<T>(Func<T> classFactory, int db, int startByteAdr = 0, CancellationToken cancellationToken = default) where T : class
         {
             var instance = classFactory();
-            var res = await ReadClassAsync(instance, db, startByteAdr, cancellationToken);
+            var res = await ReadClassAsync(instance, db, startByteAdr, cancellationToken).ConfigureAwait(false);
             int readBytes = res.Item1;
             if (readBytes <= 0)
             {
@@ -245,10 +251,10 @@ namespace S7.Net
         }
 
         /// <summary>
-        /// Reads multiple vars in a single request. 
+        /// Reads multiple vars in a single request.
         /// You have to create and pass a list of DataItems and you obtain in response the same list with the values.
         /// Values are stored in the property "Value" of the dataItem and are already converted.
-        /// If you don't want the conversion, just create a dataItem of bytes. 
+        /// If you don't want the conversion, just create a dataItem of bytes.
         /// The number of DataItems as well as the total size of the requested data can not exceed a certain limit (protocol restriction).
         /// </summary>
         /// <param name="dataItems">List of dataitems that contains the list of variables that must be read.</param>
@@ -256,18 +262,15 @@ namespace S7.Net
         /// Please note that cancellation is advisory/cooperative and will not lead to immediate cancellation in all cases.</param>
         public async Task<List<DataItem>> ReadMultipleVarsAsync(List<DataItem> dataItems, CancellationToken cancellationToken = default)
         {
-            //Snap7 seems to choke on PDU sizes above 256 even if snap7 
+            //Snap7 seems to choke on PDU sizes above 256 even if snap7
             //replies with bigger PDU size in connection setup.
             AssertPduSizeForRead(dataItems);
 
-            var stream = GetStreamIfAvailable();
-
             try
             {
                 var dataToSend = BuildReadRequestPackage(dataItems.Select(d => DataItem.GetDataItemAddress(d)).ToList());
-                await stream.WriteAsync(dataToSend, 0, dataToSend.Length);
+                var s7data = await RequestTsduAsync(dataToSend, cancellationToken);
 
-                var s7data = await COTP.TSDU.ReadAsync(stream, cancellationToken);
                 ValidateResponseCode((ReadWriteErrorCode)s7data[14]);
 
                 ParseDataIntoDataItems(s7data, dataItems);
@@ -306,7 +309,7 @@ namespace S7.Net
             while (count > 0)
             {
                 var maxToWrite = (int)Math.Min(count, MaxPDUSize - 35);
-                await WriteBytesWithASingleRequestAsync(dataType, db, startByteAdr + localIndex, value, localIndex, maxToWrite, cancellationToken);
+                await WriteBytesWithASingleRequestAsync(dataType, db, startByteAdr + localIndex, value, localIndex, maxToWrite, cancellationToken).ConfigureAwait(false);
                 count -= maxToWrite;
                 localIndex += maxToWrite;
             }
@@ -328,7 +331,7 @@ namespace S7.Net
             if (bitAdr < 0 || bitAdr > 7)
                 throw new InvalidAddressException(string.Format("Addressing Error: You can only reference bitwise locations 0-7. Address {0} is invalid", bitAdr));
 
-            await WriteBitWithASingleRequestAsync(dataType, db, startByteAdr, bitAdr, value, cancellationToken);
+            await WriteBitWithASingleRequestAsync(dataType, db, startByteAdr, bitAdr, value, cancellationToken).ConfigureAwait(false);
         }
 
         /// <summary>
@@ -347,7 +350,7 @@ namespace S7.Net
             if (value < 0 || value > 1)
                 throw new ArgumentException("Value must be 0 or 1", nameof(value));
 
-            await WriteBitAsync(dataType, db, startByteAdr, bitAdr, value == 1, cancellationToken);
+            await WriteBitAsync(dataType, db, startByteAdr, bitAdr, value == 1, cancellationToken).ConfigureAwait(false);
         }
 
         /// <summary>
@@ -370,7 +373,7 @@ namespace S7.Net
                 //Must be writing a bit value as bitAdr is specified
                 if (value is bool boolean)
                 {
-                    await WriteBitAsync(dataType, db, startByteAdr, bitAdr, boolean, cancellationToken);
+                    await WriteBitAsync(dataType, db, startByteAdr, bitAdr, boolean, cancellationToken).ConfigureAwait(false);
                 }
                 else if (value is int intValue)
                 {
@@ -380,11 +383,11 @@ namespace S7.Net
                                 "Addressing Error: You can only reference bitwise locations 0-7. Address {0} is invalid",
                                 bitAdr), nameof(bitAdr));
 
-                    await WriteBitAsync(dataType, db, startByteAdr, bitAdr, intValue == 1, cancellationToken);
+                    await WriteBitAsync(dataType, db, startByteAdr, bitAdr, intValue == 1, cancellationToken).ConfigureAwait(false);
                 }
                 else throw new ArgumentException("Value must be a bool or an int to write a bit", nameof(value));
             }
-            else await WriteBytesAsync(dataType, db, startByteAdr, Serialization.SerializeValue(value), cancellationToken);
+            else await WriteBytesAsync(dataType, db, startByteAdr, Serialization.SerializeValue(value), cancellationToken).ConfigureAwait(false);
         }
 
         /// <summary>
@@ -399,7 +402,7 @@ namespace S7.Net
         public async Task WriteAsync(string variable, object value, CancellationToken cancellationToken = default)
         {
             var adr = new PLCAddress(variable);
-            await WriteAsync(adr.DataType, adr.DbNumber, adr.StartByte, value, adr.BitNumber, cancellationToken);
+            await WriteAsync(adr.DataType, adr.DbNumber, adr.StartByte, value, adr.BitNumber, cancellationToken).ConfigureAwait(false);
         }
 
         /// <summary>
@@ -414,7 +417,7 @@ namespace S7.Net
         public async Task WriteStructAsync(object structValue, int db, int startByteAdr = 0, CancellationToken cancellationToken = default)
         {
             var bytes = Struct.ToBytes(structValue).ToList();
-            await WriteBytesAsync(DataType.DataBlock, db, startByteAdr, bytes.ToArray(), cancellationToken);
+            await WriteBytesAsync(DataType.DataBlock, db, startByteAdr, bytes.ToArray(), cancellationToken).ConfigureAwait(false);
         }
 
         /// <summary>
@@ -430,17 +433,14 @@ namespace S7.Net
         {
             byte[] bytes = new byte[(int)Class.GetClassSize(classValue)];
             Types.Class.ToBytes(classValue, bytes);
-            await WriteBytesAsync(DataType.DataBlock, db, startByteAdr, bytes, cancellationToken);
+            await WriteBytesAsync(DataType.DataBlock, db, startByteAdr, bytes, cancellationToken).ConfigureAwait(false);
         }
 
         private async Task ReadBytesWithSingleRequestAsync(DataType dataType, int db, int startByteAdr, byte[] buffer, int offset, int count, CancellationToken cancellationToken)
         {
-            var stream = GetStreamIfAvailable();
-
             var dataToSend = BuildReadRequestPackage(new [] { new DataItemAddress(dataType, db, startByteAdr, count)});
-            await stream.WriteAsync(dataToSend, 0, dataToSend.Length, cancellationToken);
 
-            var s7data = await COTP.TSDU.ReadAsync(stream, cancellationToken);
+            var s7data = await RequestTsduAsync(dataToSend, cancellationToken);
             AssertReadResponse(s7data, count);
 
             Array.Copy(s7data, 18, buffer, offset, count);
@@ -456,13 +456,11 @@ namespace S7.Net
         {
             AssertPduSizeForWrite(dataItems);
 
-            var stream = GetStreamIfAvailable();
-
             var message = new ByteArray();
             var length = S7WriteMultiple.CreateRequest(message, dataItems);
-            await stream.WriteAsync(message.Array, 0, length).ConfigureAwait(false);
 
-            var response = await COTP.TSDU.ReadAsync(stream, CancellationToken.None).ConfigureAwait(false);
+            var response = await RequestTsduAsync(message.Array, 0, length).ConfigureAwait(false);
+
             S7WriteMultiple.ParseResponse(response, response.Length, dataItems);
         }
 
@@ -476,15 +474,11 @@ namespace S7.Net
         /// <returns>A task that represents the asynchronous write operation.</returns>
         private async Task WriteBytesWithASingleRequestAsync(DataType dataType, int db, int startByteAdr, byte[] value, int dataOffset, int count, CancellationToken cancellationToken)
         {
-
             try
             {
-                var stream = GetStreamIfAvailable();
                 var dataToSend = BuildWriteBytesPackage(dataType, db, startByteAdr, value, dataOffset, count);
+                var s7data = await RequestTsduAsync(dataToSend, cancellationToken).ConfigureAwait(false);
 
-                await stream.WriteAsync(dataToSend, 0, dataToSend.Length, cancellationToken);
-
-                var s7data = await COTP.TSDU.ReadAsync(stream, cancellationToken);
                 ValidateResponseCode((ReadWriteErrorCode)s7data[14]);
             }
             catch (OperationCanceledException)
@@ -499,15 +493,11 @@ namespace S7.Net
 
         private async Task WriteBitWithASingleRequestAsync(DataType dataType, int db, int startByteAdr, int bitAdr, bool bitValue, CancellationToken cancellationToken)
         {
-            var stream = GetStreamIfAvailable();
-
             try
             {
                 var dataToSend = BuildWriteBitPackage(dataType, db, startByteAdr, bitValue, bitAdr);
+                var s7data = await RequestTsduAsync(dataToSend, cancellationToken).ConfigureAwait(false);
 
-                await stream.WriteAsync(dataToSend, 0, dataToSend.Length);
-
-                var s7data = await COTP.TSDU.ReadAsync(stream, cancellationToken);
                 ValidateResponseCode((ReadWriteErrorCode)s7data[14]);
             }
             catch (OperationCanceledException)
@@ -520,13 +510,33 @@ namespace S7.Net
             }
         }
 
-        private Stream GetStreamIfAvailable()
+        private Task<byte[]> RequestTsduAsync(byte[] requestData, CancellationToken cancellationToken = default) =>
+            RequestTsduAsync(requestData, 0, requestData.Length, cancellationToken);
+
+        private Task<byte[]> RequestTsduAsync(byte[] requestData, int offset, int length, CancellationToken cancellationToken = default)
         {
-            if (_stream == null)
-            {
-                throw new PlcException(ErrorCode.ConnectionError, "Plc is not connected");
-            }
-            return _stream;
+            var stream = GetStreamIfAvailable();
+
+            return queue.Enqueue(() =>
+                NoLockRequestTsduAsync(stream, requestData, offset, length, cancellationToken));
+        }
+
+        private static async Task<COTP.TPDU> NoLockRequestTpduAsync(Stream stream, byte[] requestData,
+            CancellationToken cancellationToken = default)
+        {
+            await stream.WriteAsync(requestData, 0, requestData.Length, cancellationToken).ConfigureAwait(false);
+            var response = await COTP.TPDU.ReadAsync(stream, cancellationToken).ConfigureAwait(false);
+
+            return response;
+        }
+
+        private static async Task<byte[]> NoLockRequestTsduAsync(Stream stream, byte[] requestData, int offset, int length,
+            CancellationToken cancellationToken = default)
+        {
+            await stream.WriteAsync(requestData, offset, length, cancellationToken).ConfigureAwait(false);
+            var response = await COTP.TSDU.ReadAsync(stream, cancellationToken).ConfigureAwait(false);
+
+            return response;
         }
     }
 }

S7.Net/PlcSynchronous.cs

@@ -2,8 +2,6 @@
 using System;
 using System.IO;
 using System.Collections.Generic;
-using System.Linq;
-using System.Net.Sockets;
 using S7.Net.Protocol;
 using S7.Net.Helper;
 
@@ -54,7 +52,7 @@ namespace S7.Net
         }
 
         /// <summary>
-        /// Read and decode a certain number of bytes of the "VarType" provided. 
+        /// Read and decode a certain number of bytes of the "VarType" provided.
         /// This can be used to read multiple consecutive variables of the same type (Word, DWord, Int, etc).
         /// If the read was not successful, check LastErrorCode or LastErrorString.
         /// </summary>
@@ -115,10 +113,10 @@ namespace S7.Net
 
 
         /// <summary>
-        /// Reads all the bytes needed to fill a class in C#, starting from a certain address, and set all the properties values to the value that are read from the PLC. 
+        /// Reads all the bytes needed to fill a class in C#, starting from a certain address, and set all the properties values to the value that are read from the PLC.
         /// This reads only properties, it doesn't read private variable or public variable without {get;set;} specified.
         /// </summary>
-        /// <param name="sourceClass">Instance of the class that will store the values</param>       
+        /// <param name="sourceClass">Instance of the class that will store the values</param>
         /// <param name="db">Index of the DB; es.: 1 is for DB1</param>
         /// <param name="startByteAdr">Start byte address. If you want to read DB1.DBW200, this is 200.</param>
         /// <returns>The number of read bytes</returns>
@@ -138,7 +136,7 @@ namespace S7.Net
         }
 
         /// <summary>
-        /// Reads all the bytes needed to fill a class in C#, starting from a certain address, and set all the properties values to the value that are read from the PLC. 
+        /// Reads all the bytes needed to fill a class in C#, starting from a certain address, and set all the properties values to the value that are read from the PLC.
         /// This reads only properties, it doesn't read private variable or public variable without {get;set;} specified. To instantiate the class defined by the generic
         /// type, the class needs a default constructor.
         /// </summary>
@@ -152,7 +150,7 @@ namespace S7.Net
         }
 
         /// <summary>
-        /// Reads all the bytes needed to fill a class in C#, starting from a certain address, and set all the properties values to the value that are read from the PLC. 
+        /// Reads all the bytes needed to fill a class in C#, starting from a certain address, and set all the properties values to the value that are read from the PLC.
         /// This reads only properties, it doesn't read private variable or public variable without {get;set;} specified.
         /// </summary>
         /// <typeparam name="T">The class that will be instantiated</typeparam>
@@ -186,7 +184,7 @@ namespace S7.Net
             while (count > 0)
             {
                 //TODO: Figure out how to use MaxPDUSize here
-                //Snap7 seems to choke on PDU sizes above 256 even if snap7 
+                //Snap7 seems to choke on PDU sizes above 256 even if snap7
                 //replies with bigger PDU size in connection setup.
                 var maxToWrite = Math.Min(count, MaxPDUSize - 28);//TODO tested only when the MaxPDUSize is 480
                 WriteBytesWithASingleRequest(dataType, db, startByteAdr + localIndex, value, localIndex, maxToWrite);
@@ -298,7 +296,6 @@ namespace S7.Net
 
         private void ReadBytesWithSingleRequest(DataType dataType, int db, int startByteAdr, byte[] buffer, int offset, int count)
         {
-            var stream = GetStreamIfAvailable();
             try
             {
                 // first create the header
@@ -309,9 +306,7 @@ namespace S7.Net
                 BuildReadDataRequestPackage(package, dataType, db, startByteAdr, count);
 
                 var dataToSend = package.ToArray();
-                stream.Write(dataToSend, 0, dataToSend.Length);
-
-                var s7data = COTP.TSDU.Read(stream);
+                var s7data = RequestTsdu(dataToSend);
                 AssertReadResponse(s7data, count);
 
                 Array.Copy(s7data, 18, buffer, offset, count);
@@ -331,13 +326,11 @@ namespace S7.Net
         {
             AssertPduSizeForWrite(dataItems);
 
-            var stream = GetStreamIfAvailable();
 
             var message = new ByteArray();
             var length = S7WriteMultiple.CreateRequest(message, dataItems);
-            stream.Write(message.Array, 0, length);
+            var response = RequestTsdu(message.Array, 0, length);
 
-            var response = COTP.TSDU.Read(stream);
             S7WriteMultiple.ParseResponse(response, response.Length, dataItems);
         }
 
@@ -345,12 +338,9 @@ namespace S7.Net
         {
             try
             {
-                var stream = GetStreamIfAvailable();
                 var dataToSend = BuildWriteBytesPackage(dataType, db, startByteAdr, value, dataOffset, count);
+                var s7data = RequestTsdu(dataToSend);
 
-                stream.Write(dataToSend, 0, dataToSend.Length);
-
-                var s7data = COTP.TSDU.Read(stream);
                 ValidateResponseCode((ReadWriteErrorCode)s7data[14]);
             }
             catch (Exception exc)
@@ -425,14 +415,11 @@ namespace S7.Net
 
         private void WriteBitWithASingleRequest(DataType dataType, int db, int startByteAdr, int bitAdr, bool bitValue)
         {
-            var stream = GetStreamIfAvailable();
             try
             {
                 var dataToSend = BuildWriteBitPackage(dataType, db, startByteAdr, bitValue, bitAdr);
+                var s7data = RequestTsdu(dataToSend);
 
-                stream.Write(dataToSend, 0, dataToSend.Length);
-
-                var s7data = COTP.TSDU.Read(stream);
                 ValidateResponseCode((ReadWriteErrorCode)s7data[14]);
             }
             catch (Exception exc)
@@ -442,10 +429,10 @@ namespace S7.Net
         }
 
         /// <summary>
-        /// Reads multiple vars in a single request. 
+        /// Reads multiple vars in a single request.
         /// You have to create and pass a list of DataItems and you obtain in response the same list with the values.
         /// Values are stored in the property "Value" of the dataItem and are already converted.
-        /// If you don't want the conversion, just create a dataItem of bytes. 
+        /// If you don't want the conversion, just create a dataItem of bytes.
         /// The number of DataItems as well as the total size of the requested data can not exceed a certain limit (protocol restriction).
         /// </summary>
         /// <param name="dataItems">List of dataitems that contains the list of variables that must be read.</param>
@@ -453,8 +440,6 @@ namespace S7.Net
         {
             AssertPduSizeForRead(dataItems);
 
-            var stream = GetStreamIfAvailable();
-
             try
             {
                 // first create the header
@@ -468,9 +453,7 @@ namespace S7.Net
                 }
 
                 var dataToSend = package.ToArray();
-                stream.Write(dataToSend, 0, dataToSend.Length);
-
-                var s7data = COTP.TSDU.Read(stream); //TODO use Async
+                var s7data = RequestTsdu(dataToSend);
 
                 ValidateResponseCode((ReadWriteErrorCode)s7data[14]);
 
@@ -481,5 +464,12 @@ namespace S7.Net
                 throw new PlcException(ErrorCode.ReadData, exc);
             }
         }
+
+        private byte[] RequestTsdu(byte[] requestData) => RequestTsdu(requestData, 0, requestData.Length);
+
+        private byte[] RequestTsdu(byte[] requestData, int offset, int length)
+        {
+            return RequestTsduAsync(requestData, offset, length).GetAwaiter().GetResult();
+        }
     }
 }

S7.Net/Protocol/Serialization.cs

@@ -17,10 +17,14 @@ namespace S7.Net.Protocol
             {
                 throw new Exception($"DataItem.Value is null, cannot serialize. StartAddr={dataItem.StartByteAdr} VarType={dataItem.VarType}");
             }
+
             if (dataItem.Value is string s)
-                return dataItem.VarType == VarType.S7String
-                    ? S7String.ToByteArray(s, dataItem.Count)
-                    : Types.String.ToByteArray(s, dataItem.Count);
+                return dataItem.VarType switch
+                {
+                    VarType.S7String => S7String.ToByteArray(s, dataItem.Count),
+                    VarType.S7WString => S7WString.ToByteArray(s, dataItem.Count),
+                    _ => Types.String.ToByteArray(s, dataItem.Count)
+                };
 
             return SerializeValue(dataItem.Value);
         }
@@ -46,7 +50,7 @@ namespace S7.Net.Protocol
                 case "Double":
                     return Types.LReal.ToByteArray((double)value);
                 case "DateTime":
-                    return Types.DateTime.ToByteArray((System.DateTime) value);
+                    return Types.DateTime.ToByteArray((System.DateTime)value);
                 case "Byte[]":
                     return (byte[])value;
                 case "Int16[]":
@@ -64,10 +68,10 @@ namespace S7.Net.Protocol
                 case "String":
                     // Hack: This is backwards compatible with the old code, but functionally it's broken
                     // if the consumer does not pay attention to string length.
-                    var stringVal = (string) value;
+                    var stringVal = (string)value;
                     return Types.String.ToByteArray(stringVal, stringVal.Length);
                 case "DateTime[]":
-                    return Types.DateTime.ToByteArray((System.DateTime[]) value);
+                    return Types.DateTime.ToByteArray((System.DateTime[])value);
                 case "DateTimeLong[]":
                     return Types.DateTimeLong.ToByteArray((System.DateTime[])value);
                 default:

S7.Net/Types/Boolean.cs

@@ -14,20 +14,51 @@
         }
 
         /// <summary>
-        /// Sets the value of a bit to 1 (true), given the address of the bit
+        /// Sets the value of a bit to 1 (true), given the address of the bit. Returns
+        /// a copy of the value with the bit set.
         /// </summary>
+        /// <param name="value">The input value to modify.</param>
+        /// <param name="bit">The index (zero based) of the bit to set.</param>
+        /// <returns>The modified value with the bit at index set.</returns>
         public static byte SetBit(byte value, int bit)
         {
-            return (byte)((value | (1 << bit)) & 0xFF);
+            SetBit(ref value, bit);
+
+            return value;
+        }
+
+        /// <summary>
+        /// Sets the value of a bit to 1 (true), given the address of the bit.
+        /// </summary>
+        /// <param name="value">The value to modify.</param>
+        /// <param name="bit">The index (zero based) of the bit to set.</param>
+        public static void SetBit(ref byte value, int bit)
+        {
+            value = (byte) ((value | (1 << bit)) & 0xFF);
+        }
+
+        /// <summary>
+        /// Resets the value of a bit to 0 (false), given the address of the bit. Returns
+        /// a copy of the value with the bit cleared.
+        /// </summary>
+        /// <param name="value">The input value to modify.</param>
+        /// <param name="bit">The index (zero based) of the bit to clear.</param>
+        /// <returns>The modified value with the bit at index cleared.</returns>
+        public static byte ClearBit(byte value, int bit)
+        {
+            ClearBit(ref value, bit);
+
+            return value;
         }
 
         /// <summary>
         /// Resets the value of a bit to 0 (false), given the address of the bit
         /// </summary>
-        public static byte ClearBit(byte value, int bit)
+        /// <param name="value">The input value to modify.</param>
+        /// <param name="bit">The index (zero based) of the bit to clear.</param>
+        public static void ClearBit(ref byte value, int bit)
         {
-            return (byte)((value | (~(1 << bit))) & 0xFF);
+            value = (byte) (value & ~(1 << bit) & 0xFF);
         }
-
     }
 }

S7.Net/Types/S7String.cs

@@ -45,7 +45,7 @@ namespace S7.Net.Types
         /// Converts a <see cref="T:string"/> to S7 string with 2-byte header.
         /// </summary>
         /// <param name="value">The string to convert to byte array.</param>
-        /// <param name="reservedLength">The length (in bytes) allocated in PLC for string excluding header.</param>
+        /// <param name="reservedLength">The length (in characters) allocated in PLC for the string.</param>
         /// <returns>A <see cref="T:byte[]" /> containing the string header and string value with a maximum length of <paramref name="reservedLength"/> + 2.</returns>
         public static byte[] ToByteArray(string value, int reservedLength)
         {
@@ -54,7 +54,7 @@ namespace S7.Net.Types
                 throw new ArgumentNullException(nameof(value));
             }
 
-            if (reservedLength > byte.MaxValue) throw new ArgumentException($"The maximum string length supported is {byte.MaxValue}.");
+            if (reservedLength > 254) throw new ArgumentException($"The maximum string length supported is 254.");
 
             var bytes = Encoding.ASCII.GetBytes(value);
             if (bytes.Length > reservedLength) throw new ArgumentException($"The provided string length ({bytes.Length} is larger than the specified reserved length ({reservedLength}).");

S7.Net/Types/S7StringAttribute.cs

@@ -0,0 +1,67 @@
+using System;
+
+namespace S7.Net.Types
+{
+    [AttributeUsage(AttributeTargets.Field, AllowMultiple = false)]
+    public sealed class S7StringAttribute : Attribute
+    {
+        private readonly S7StringType type;
+        private readonly int reservedLength;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="S7StringAttribute"/> class.
+        /// </summary>
+        /// <param name="type">The string type.</param>
+        /// <param name="reservedLength">Reserved length of the string in characters.</param>
+        /// <exception cref="ArgumentException">Please use a valid value for the string type</exception>
+        public S7StringAttribute(S7StringType type, int reservedLength)
+        {
+            if (!Enum.IsDefined(typeof(S7StringType), type))
+                throw new ArgumentException("Please use a valid value for the string type");
+
+            this.type = type;
+            this.reservedLength = reservedLength;
+        }
+
+        /// <summary>
+        /// Gets the type of the string.
+        /// </summary>
+        /// <value>
+        /// The string type.
+        /// </value>
+        public S7StringType Type => type;
+
+        /// <summary>
+        /// Gets the reserved length of the string in characters.
+        /// </summary>
+        /// <value>
+        /// The reserved length of the string in characters.
+        /// </value>
+        public int ReservedLength => reservedLength;
+
+        /// <summary>
+        /// Gets the reserved length in bytes.
+        /// </summary>
+        /// <value>
+        /// The reserved length in bytes.
+        /// </value>
+        public int ReservedLengthInBytes => type == S7StringType.S7String ? reservedLength + 2 : (reservedLength * 2) + 4;
+    }
+
+
+    /// <summary>
+    /// String type.
+    /// </summary>
+    public enum S7StringType
+    {
+        /// <summary>
+        /// ASCII string.
+        /// </summary>
+        S7String = VarType.S7String,
+
+        /// <summary>
+        /// Unicode string.
+        /// </summary>
+        S7WString = VarType.S7WString
+    }
+}

S7.Net/Types/S7WString.cs

@@ -0,0 +1,72 @@
+using System;
+using System.Text;
+
+namespace S7.Net.Types
+{
+    /// <summary>
+    /// Contains the methods to convert from S7 wstrings to C# strings
+    /// An S7 WString has a preceding 4 byte header containing its capacity and length
+    /// </summary>
+    public static class S7WString
+    {
+        /// <summary>
+        /// Converts S7 bytes to a string
+        /// </summary>
+        /// <param name="bytes"></param>
+        /// <returns></returns>
+        public static string FromByteArray(byte[] bytes)
+        {
+            if (bytes.Length < 4)
+            {
+                throw new PlcException(ErrorCode.ReadData, "Malformed S7 WString / too short");
+            }
+
+            int size = (bytes[0] << 8) | bytes[1];
+            int length = (bytes[2] << 8) | bytes[3];
+
+            if (length > size)
+            {
+                throw new PlcException(ErrorCode.ReadData, "Malformed S7 WString / length larger than capacity");
+            }
+
+            try
+            {
+                return Encoding.BigEndianUnicode.GetString(bytes, 4, length * 2);
+            }
+            catch (Exception e)
+            {
+                throw new PlcException(ErrorCode.ReadData,
+                    $"Failed to parse {VarType.S7WString} from data. Following fields were read: size: '{size}', actual length: '{length}', total number of bytes (including header): '{bytes.Length}'.",
+                    e);
+            }
+            
+        }
+
+        /// <summary>
+        /// Converts a <see cref="T:string"/> to S7 wstring with 4-byte header.
+        /// </summary>
+        /// <param name="value">The string to convert to byte array.</param>
+        /// <param name="reservedLength">The length (in characters) allocated in PLC for the string.</param>
+        /// <returns>A <see cref="T:byte[]" /> containing the string header and string value with a maximum length of <paramref name="reservedLength"/> + 4.</returns>
+        public static byte[] ToByteArray(string value, int reservedLength)
+        {
+            if (value is null)
+            {
+                throw new ArgumentNullException(nameof(value));
+            }
+
+            if (reservedLength > 16382) throw new ArgumentException("The maximum string length supported is 16382.");
+            
+            var buffer = new byte[4 + reservedLength * 2];
+            buffer[0] = (byte)((reservedLength >> 8) & 0xFF);
+            buffer[1] = (byte)(reservedLength & 0xFF);
+            buffer[2] = (byte)((value.Length >> 8) & 0xFF);
+            buffer[3] = (byte)(value.Length & 0xFF);
+
+            var stringLength = Encoding.BigEndianUnicode.GetBytes(value, 0, value.Length, buffer, 4) / 2;
+            if (stringLength > reservedLength) throw new ArgumentException($"The provided string length ({stringLength} is larger than the specified reserved length ({reservedLength}).");
+
+            return buffer;
+        }
+    }
+}

S7.Net/Types/Struct.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Linq;
 using System.Reflection;
 
 namespace S7.Net.Types
@@ -18,11 +19,11 @@ namespace S7.Net.Types
             double numBytes = 0.0;
 
             var infos = structType
-            #if NETSTANDARD1_3
+#if NETSTANDARD1_3
                 .GetTypeInfo().DeclaredFields;
-            #else
+#else
                 .GetFields();
-            #endif
+#endif
 
             foreach (var info in infos)
             {
@@ -61,6 +62,16 @@ namespace S7.Net.Types
                             numBytes++;
                         numBytes += 8;
                         break;
+                    case "String":
+                        S7StringAttribute? attribute = info.GetCustomAttributes<S7StringAttribute>().SingleOrDefault();
+                        if (attribute == default(S7StringAttribute))
+                            throw new ArgumentException("Please add S7StringAttribute to the string field");
+
+                        numBytes = Math.Ceiling(numBytes);
+                        if ((numBytes / 2 - Math.Floor(numBytes / 2.0)) > 0)
+                            numBytes++;
+                        numBytes += attribute.ReservedLengthInBytes;
+                        break;
                     default:
                         numBytes += GetStructSize(info.FieldType);
                         break;
@@ -91,11 +102,11 @@ namespace S7.Net.Types
 
 
             var infos = structValue.GetType()
-            #if NETSTANDARD1_3
+#if NETSTANDARD1_3
                 .GetTypeInfo().DeclaredFields;
-            #else
+#else
                 .GetFields();
-            #endif
+#endif
 
             foreach (var info in infos)
             {
@@ -120,7 +131,7 @@ namespace S7.Net.Types
                         numBytes = Math.Ceiling(numBytes);
                         if ((numBytes / 2 - Math.Floor(numBytes / 2.0)) > 0)
                             numBytes++;
-                        // hier auswerten
+                        // get the value
                         ushort source = Word.FromBytes(bytes[(int)numBytes + 1], bytes[(int)numBytes]);
                         info.SetValue(structValue, source.ConvertToShort());
                         numBytes += 2;
@@ -129,7 +140,7 @@ namespace S7.Net.Types
                         numBytes = Math.Ceiling(numBytes);
                         if ((numBytes / 2 - Math.Floor(numBytes / 2.0)) > 0)
                             numBytes++;
-                        // hier auswerten
+                        // get the value
                         info.SetValue(structValue, Word.FromBytes(bytes[(int)numBytes + 1],
                                                                           bytes[(int)numBytes]));
                         numBytes += 2;
@@ -138,7 +149,7 @@ namespace S7.Net.Types
                         numBytes = Math.Ceiling(numBytes);
                         if ((numBytes / 2 - Math.Floor(numBytes / 2.0)) > 0)
                             numBytes++;
-                        // hier auswerten
+                        // get the value
                         uint sourceUInt = DWord.FromBytes(bytes[(int)numBytes + 3],
                                                                            bytes[(int)numBytes + 2],
                                                                            bytes[(int)numBytes + 1],
@@ -150,7 +161,7 @@ namespace S7.Net.Types
                         numBytes = Math.Ceiling(numBytes);
                         if ((numBytes / 2 - Math.Floor(numBytes / 2.0)) > 0)
                             numBytes++;
-                        // hier auswerten
+                        // get the value
                         info.SetValue(structValue, DWord.FromBytes(bytes[(int)numBytes],
                                                                            bytes[(int)numBytes + 1],
                                                                            bytes[(int)numBytes + 2],
@@ -161,7 +172,7 @@ namespace S7.Net.Types
                         numBytes = Math.Ceiling(numBytes);
                         if ((numBytes / 2 - Math.Floor(numBytes / 2.0)) > 0)
                             numBytes++;
-                        // hier auswerten
+                        // get the value
                         info.SetValue(structValue, Real.FromByteArray(new byte[] { bytes[(int)numBytes],
                                                                            bytes[(int)numBytes + 1],
                                                                            bytes[(int)numBytes + 2],
@@ -172,12 +183,38 @@ namespace S7.Net.Types
                         numBytes = Math.Ceiling(numBytes);
                         if ((numBytes / 2 - Math.Floor(numBytes / 2.0)) > 0)
                             numBytes++;
-                        // hier auswerten
+                        // get the value
                         var data = new byte[8];
                         Array.Copy(bytes, (int)numBytes, data, 0, 8);
                         info.SetValue(structValue, LReal.FromByteArray(data));
                         numBytes += 8;
                         break;
+                    case "String":
+                        S7StringAttribute? attribute = info.GetCustomAttributes<S7StringAttribute>().SingleOrDefault();
+                        if (attribute == default(S7StringAttribute))
+                            throw new ArgumentException("Please add S7StringAttribute to the string field");
+
+                        numBytes = Math.Ceiling(numBytes);
+                        if ((numBytes / 2 - Math.Floor(numBytes / 2.0)) > 0)
+                            numBytes++;
+
+                        // get the value
+                        var sData = new byte[attribute.ReservedLengthInBytes];
+                        Array.Copy(bytes, (int)numBytes, sData, 0, sData.Length);
+                        switch (attribute.Type)
+                        {
+                            case S7StringType.S7String:
+                                info.SetValue(structValue, S7String.FromByteArray(sData));
+                                break;
+                            case S7StringType.S7WString:
+                                info.SetValue(structValue, S7WString.FromByteArray(sData));
+                                break;
+                            default:
+                                throw new ArgumentException("Please use a valid string type for the S7StringAttribute");
+                        }
+
+                        numBytes += sData.Length;
+                        break;
                     default:
                         var buffer = new byte[GetStructSize(info.FieldType)];
                         if (buffer.Length == 0)
@@ -209,11 +246,11 @@ namespace S7.Net.Types
             double numBytes = 0.0;
 
             var infos = type
-            #if NETSTANDARD1_3
+#if NETSTANDARD1_3
                 .GetTypeInfo().DeclaredFields;
-            #else
+#else
                 .GetFields();
-            #endif
+#endif
 
             foreach (var info in infos)
             {
@@ -254,6 +291,18 @@ namespace S7.Net.Types
                     case "Double":
                         bytes2 = LReal.ToByteArray((double)info.GetValue(structValue));
                         break;
+                    case "String":
+                        S7StringAttribute? attribute = info.GetCustomAttributes<S7StringAttribute>().SingleOrDefault();
+                        if (attribute == default(S7StringAttribute))
+                            throw new ArgumentException("Please add S7StringAttribute to the string field");
+
+                        bytes2 = attribute.Type switch
+                        {
+                            S7StringType.S7String => S7String.ToByteArray((string)info.GetValue(structValue), attribute.ReservedLength),
+                            S7StringType.S7WString => S7WString.ToByteArray((string)info.GetValue(structValue), attribute.ReservedLength),
+                            _ => throw new ArgumentException("Please use a valid string type for the S7StringAttribute")
+                        };
+                        break;
                 }
                 if (bytes2 != null)
                 {
@@ -269,7 +318,5 @@ namespace S7.Net.Types
             }
             return bytes;
         }
-
-
     }
 }