removed duplicated code by using csproj with targetframeworks

Frameworks/MQTTnet.NetStandard/Implementations/MqttServerAdapter.cs

@@ -88,7 +88,11 @@ namespace MQTTnet.Implementations
             {
                 try
                 {
+#if NET45
+                    var clientSocket = await Task.Factory.FromAsync(_defaultEndpointSocket.BeginAccept, _defaultEndpointSocket.EndAccept, null).ConfigureAwait(false);
+#else
                     var clientSocket = await _defaultEndpointSocket.AcceptAsync().ConfigureAwait(false);
+#endif
                     var clientAdapter = new MqttChannelCommunicationAdapter(new MqttTcpChannel(clientSocket, null), new MqttPacketSerializer());
                     ClientAccepted?.Invoke(clientAdapter);
                 }
@@ -108,7 +112,11 @@ namespace MQTTnet.Implementations
             {
                 try
                 {
+#if NET45
+                    var clientSocket = await Task.Factory.FromAsync(_tlsEndpointSocket.BeginAccept, _tlsEndpointSocket.EndAccept, null).ConfigureAwait(false);
+#else
                     var clientSocket = await _tlsEndpointSocket.AcceptAsync().ConfigureAwait(false);
+#endif
 
                     var sslStream = new SslStream(new NetworkStream(clientSocket));
                     await sslStream.AuthenticateAsServerAsync(_tlsCertificate, false, SslProtocols.Tls12, false).ConfigureAwait(false);

Frameworks/MQTTnet.NetStandard/Implementations/MqttTcpChannel.cs

@@ -50,7 +50,11 @@ namespace MQTTnet.Implementations
                 _socket = new Socket(SocketType.Stream, ProtocolType.Tcp);
             }
 
+#if NET45
+            await Task.Factory.FromAsync(_socket.BeginConnect, _socket.EndConnect, _options.Server, _options.GetPort(), null).ConfigureAwait(false);
+#else
             await _socket.ConnectAsync(_options.Server, _options.GetPort()).ConfigureAwait(false);
+#endif
 
             if (_options.TlsOptions.UseTls)
             {

Frameworks/MQTTnet.NetStandard/MQTTnet.Netstandard.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard1.3</TargetFramework>
+    <TargetFrameworks>netstandard1.3;net45</TargetFrameworks>
     <AssemblyName>MQTTnet</AssemblyName>
     <RootNamespace>MQTTnet</RootNamespace>
     <AssemblyVersion>2.4.0.0</AssemblyVersion>
@@ -22,7 +22,7 @@
     <ProjectReference Include="..\..\MQTTnet.Core\MQTTnet.Core.csproj" />
   </ItemGroup>
 
-  <ItemGroup>
+  <ItemGroup Condition="'$(TargetFramework)'=='netstandard1.3'">
     <PackageReference Include="System.Net.Security" Version="4.3.2" />
     <PackageReference Include="System.Net.WebSockets" Version="4.3.0" />
     <PackageReference Include="System.Net.WebSockets.Client" Version="4.3.1" />