Elide repeated Action-delegate allocation by caching it (#1324)

* Elide repeated Action-delegate allocation by caching it

Here C# compiler has too little knowledge about the usage of the delegate, we know better so cache it on our own to avoid the repeated allocation.

* Use conditional compilation

Source/MQTTnet/Implementations/CrossPlatformSocket.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.IO;
 using System.Net;
 using System.Net.Sockets;
@@ -11,12 +11,14 @@ namespace MQTTnet.Implementations
     public sealed class CrossPlatformSocket : IDisposable
     {
         readonly Socket _socket;
+        readonly Action _socketDisposeAction;
 
         NetworkStream _networkStream;
 
         public CrossPlatformSocket(AddressFamily addressFamily)
         {
             _socket = new Socket(addressFamily, SocketType.Stream, ProtocolType.Tcp);
+            _socketDisposeAction = _socket.Dispose;
         }
 
         public CrossPlatformSocket()
@@ -24,12 +26,16 @@ namespace MQTTnet.Implementations
             // Having this constructor is important because avoiding the address family as parameter
             // will make use of dual mode in the .net framework.
             _socket = new Socket(SocketType.Stream, ProtocolType.Tcp);
+
+            _socketDisposeAction = _socket.Dispose;
         }
 
         public CrossPlatformSocket(Socket socket)
         {
             _socket = socket ?? throw new ArgumentNullException(nameof(socket));
             _networkStream = new NetworkStream(socket, true);
+
+            _socketDisposeAction = _socket.Dispose;
         }
 
         public bool NoDelay
@@ -112,7 +118,7 @@ namespace MQTTnet.Implementations
                 _networkStream?.Dispose();
 
                 // Workaround for: https://github.com/dotnet/corefx/issues/24430
-                using (cancellationToken.Register(() => _socket.Dispose()))
+                using (cancellationToken.Register(_socketDisposeAction))
                 {
                     cancellationToken.ThrowIfCancellationRequested();
 

Source/MQTTnet/Implementations/MqttTcpChannel.cs

@@ -18,10 +18,16 @@ namespace MQTTnet.Implementations
     {
         readonly IMqttClientOptions _clientOptions;
         readonly MqttClientTcpOptions _tcpOptions;
+        readonly Action _disposeAction;
 
         Stream _stream;
 
-        public MqttTcpChannel(IMqttClientOptions clientOptions)
+        public MqttTcpChannel()
+        {
+            _disposeAction = Dispose;
+        }
+
+        public MqttTcpChannel(IMqttClientOptions clientOptions) : this()
         {
             _clientOptions = clientOptions ?? throw new ArgumentNullException(nameof(clientOptions));
             _tcpOptions = (MqttClientTcpOptions)clientOptions.ChannelOptions;
@@ -29,7 +35,7 @@ namespace MQTTnet.Implementations
             IsSecureConnection = clientOptions.ChannelOptions?.TlsOptions?.UseTls == true;
         }
 
-        public MqttTcpChannel(Stream stream, string endpoint, X509Certificate2 clientCertificate)
+        public MqttTcpChannel(Stream stream, string endpoint, X509Certificate2 clientCertificate) : this()
         {
             _stream = stream ?? throw new ArgumentNullException(nameof(stream));
 
@@ -149,11 +155,15 @@ namespace MQTTnet.Implementations
                     return 0;
                 }
 
+#if NETCOREAPP3_0_OR_GREATER || NETSTANDARD2_1_OR_GREATER
+                return await stream.ReadAsync(buffer.AsMemory(offset, count), cancellationToken).ConfigureAwait(false);
+#else
                 // Workaround for: https://github.com/dotnet/corefx/issues/24430
-                using (cancellationToken.Register(Dispose))
+                using (cancellationToken.Register(_disposeAction))
                 {
                     return await stream.ReadAsync(buffer, offset, count, cancellationToken).ConfigureAwait(false);
                 }
+#endif
             }
             catch (ObjectDisposedException)
             {
@@ -177,18 +187,22 @@ namespace MQTTnet.Implementations
 
             try
             {
-                // Workaround for: https://github.com/dotnet/corefx/issues/24430
-                using (cancellationToken.Register(Dispose))
-                {
-                    var stream = _stream;
+                var stream = _stream;
 
-                    if (stream == null)
-                    {
-                        throw new MqttCommunicationException("The TCP connection is closed.");
-                    }
+                if (stream == null)
+                {
+                    throw new MqttCommunicationException("The TCP connection is closed.");
+                }
 
+#if NETCOREAPP3_0_OR_GREATER || NETSTANDARD2_1_OR_GREATER
+                await stream.WriteAsync(buffer.AsMemory(offset, count), cancellationToken).ConfigureAwait(false);
+#else
+                // Workaround for: https://github.com/dotnet/corefx/issues/24430
+                using (cancellationToken.Register(_disposeAction))
+                {
                     await stream.WriteAsync(buffer, offset, count, cancellationToken).ConfigureAwait(false);
                 }
+#endif
             }
             catch (ObjectDisposedException)
             {