Add support for certificate validation callback for Web Socket connections.

Build/MQTTnet.nuspec

@@ -14,12 +14,12 @@
     <releaseNotes>
 * [Core] Renamed some topic filter relevant classes (BREAKING CHANGE!).
 * [Core] Improved task management for UWP connections (thanks to @xgstation).
-* [LowLevelMqttClient] 
 * [Client] Added method to trigger PING/PONG manually (connection check etc.).
+* [Client] Added support for certificate validation callback when using Web Sockets (requires netstandard2.1+).
+* [Client] Fixed a memory leak when web socket based connections trying to reconnect with an offline server.
 * [ManagedClient] Added method to trigger PING/PONG manually (connection check etc.).
-* [Server] 
-* [MQTTnet.AspNetCore] improved compatibility with AspNetCore 3.1
-* [MQTTnet.Server] 
+* [MQTTnet.AspNetCore] improved compatibility with AspNetCore 3.1.
+* [MQTTnet.Server] Fixed wrong version output.
     </releaseNotes>
     <copyright>Copyright Christian Kratky 2016-2020</copyright>
     <tags>MQTT Message Queue Telemetry Transport MQTTClient MQTTServer Server MQTTBroker Broker NETStandard IoT InternetOfThings Messaging Hardware Arduino Sensor Actuator M2M ESP Smart Home Cities Automation Xamarin</tags> 
@@ -36,6 +36,12 @@
         <dependency id="System.Net.WebSockets" version="4.3.0" />
         <dependency id="System.Net.WebSockets.Client" version="4.3.2" />
       </group>
+      <group targetFramework="netstandard2.1">
+        <dependency id="NETStandard.Library" version="2.0.0" />
+        <dependency id="System.Net.Security" version="4.3.2" />
+        <dependency id="System.Net.WebSockets" version="4.3.0" />
+        <dependency id="System.Net.WebSockets.Client" version="4.3.2" />
+      </group>
       <group targetFramework="uap10.0">
         <dependency id="Microsoft.NETCore.UniversalWindowsPlatform" version="6.2.10" />
       </group>
@@ -55,6 +61,9 @@
     <!-- .NET Standard 2.0 -->
     <file src="..\Source\MQTTnet\bin\Release\netstandard2.0\MQTTnet.*" target="lib\netstandard2.0\"/>
 
+    <!-- .NET Standard 2.1 -->
+    <file src="..\Source\MQTTnet\bin\Release\netstandard2.1\MQTTnet.*" target="lib\netstandard2.1\"/>
+
     <!-- Universal Windows -->
     <file src="..\Source\MQTTnet\bin\Release\uap10.0\MQTTnet.*" target="lib\uap10.0\"/>
 

Build/build.ps1

@@ -28,6 +28,7 @@ vstest.console.exe ..\Tests\MQTTnet.AspNetCore.Tests\bin\Release\netcoreapp3.1\M
 &$msbuild ..\Source\MQTTnet\MQTTnet.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="net461" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
 &$msbuild ..\Source\MQTTnet\MQTTnet.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="netstandard1.3" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
 &$msbuild ..\Source\MQTTnet\MQTTnet.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="netstandard2.0" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
+&$msbuild ..\Source\MQTTnet\MQTTnet.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="netstandard2.1" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
 &$msbuild ..\Source\MQTTnet\MQTTnet.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="uap10.0" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
 
 # Build the ASP.NET Core 2.0 extension
@@ -38,6 +39,7 @@ vstest.console.exe ..\Tests\MQTTnet.AspNetCore.Tests\bin\Release\netcoreapp3.1\M
 &$msbuild ..\Source\MQTTnet.Extensions.Rpc\MQTTnet.Extensions.Rpc.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="net461" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
 &$msbuild ..\Source\MQTTnet.Extensions.Rpc\MQTTnet.Extensions.Rpc.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="netstandard1.3" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
 &$msbuild ..\Source\MQTTnet.Extensions.Rpc\MQTTnet.Extensions.Rpc.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="netstandard2.0" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
+&$msbuild ..\Source\MQTTnet.Extensions.Rpc\MQTTnet.Extensions.Rpc.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="netstandard2.1" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
 &$msbuild ..\Source\MQTTnet.Extensions.Rpc\MQTTnet.Extensions.Rpc.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="uap10.0" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
 
 # Build the Managed Client extension
@@ -45,6 +47,7 @@ vstest.console.exe ..\Tests\MQTTnet.AspNetCore.Tests\bin\Release\netcoreapp3.1\M
 &$msbuild ..\Source\MQTTnet.Extensions.ManagedClient\MQTTnet.Extensions.ManagedClient.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="net461" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
 &$msbuild ..\Source\MQTTnet.Extensions.ManagedClient\MQTTnet.Extensions.ManagedClient.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="netstandard1.3" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
 &$msbuild ..\Source\MQTTnet.Extensions.ManagedClient\MQTTnet.Extensions.ManagedClient.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="netstandard2.0" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
+&$msbuild ..\Source\MQTTnet.Extensions.ManagedClient\MQTTnet.Extensions.ManagedClient.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="netstandard2.1" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
 &$msbuild ..\Source\MQTTnet.Extensions.ManagedClient\MQTTnet.Extensions.ManagedClient.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="uap10.0" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
 
 # Build the WebSocket4Net extension
@@ -52,6 +55,7 @@ vstest.console.exe ..\Tests\MQTTnet.AspNetCore.Tests\bin\Release\netcoreapp3.1\M
 &$msbuild ..\Source\MQTTnet.Extensions.WebSocket4Net\MQTTnet.Extensions.WebSocket4Net.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="net461" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
 &$msbuild ..\Source\MQTTnet.Extensions.WebSocket4Net\MQTTnet.Extensions.WebSocket4Net.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="netstandard1.3" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
 &$msbuild ..\Source\MQTTnet.Extensions.WebSocket4Net\MQTTnet.Extensions.WebSocket4Net.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="netstandard2.0" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
+&$msbuild ..\Source\MQTTnet.Extensions.WebSocket4Net\MQTTnet.Extensions.WebSocket4Net.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="netstandard2.1" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
 &$msbuild ..\Source\MQTTnet.Extensions.WebSocket4Net\MQTTnet.Extensions.WebSocket4Net.csproj /t:Build /p:Configuration="Release" /p:TargetFramework="uap10.0" /p:FileVersion=$assemblyVersion /p:AssemblyVersion=$assemblyVersion /verbosity:m /p:SignAssembly=true /p:AssemblyOriginatorKeyFile=".\..\..\Build\codeSigningKey.pfx"
 
 # Create NuGet packages.

Source/MQTTnet/Client/MqttClient.cs

@@ -61,7 +61,7 @@ namespace MQTTnet.Client
         {
             get
             {
-                return _isConnected || Interlocked.Read(ref _isDisconnectPending) != 0;
+                return _isConnected && Interlocked.Read(ref _isDisconnectPending) == 0;
             }
         }
 

Source/MQTTnet/Client/Options/MqttClientCertificateValidationCallbackContext.cs

+using System.Net.Security;
+using System.Security.Cryptography.X509Certificates;
+
+namespace MQTTnet.Client.Options
+{
+    public class MqttClientCertificateValidationCallbackContext
+    {
+        public X509Certificate Certificate { get; set; }
+
+        public X509Chain Chain { get; set; }
+
+        public SslPolicyErrors SslPolicyErrors { get; set; }
+
+        public IMqttClientChannelOptions ClientOptions { get; set; }
+    }
+}

Source/MQTTnet/Client/Options/MqttClientOptionsBuilder.cs

@@ -262,6 +262,7 @@ namespace MQTTnet.Client.Options
                         Certificates = _tlsParameters.Certificates?.ToList(),
 #endif
                         CertificateValidationCallback = _tlsParameters.CertificateValidationCallback,
+                        CertificateValidationHandler = _tlsParameters.CertificateValidationHandler,
                         IgnoreCertificateChainErrors = _tlsParameters.IgnoreCertificateChainErrors,
                         IgnoreCertificateRevocationErrors = _tlsParameters.IgnoreCertificateRevocationErrors
                     };

Source/MQTTnet/Client/Options/MqttClientOptionsBuilderTlsParameters.cs

@@ -10,12 +10,15 @@ namespace MQTTnet.Client.Options
     {
         public bool UseTls { get; set; }
 
+        [Obsolete("This property will be removed soon. Use CertificateValidationHandler instead.")]
         public Func<X509Certificate, X509Chain, SslPolicyErrors, IMqttClientOptions, bool> CertificateValidationCallback
         {
             get;
             set;
         }
 
+        public Func<MqttClientCertificateValidationCallbackContext, bool> CertificateValidationHandler { get; set; }
+
         public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls12;
 
 #if WINDOWS_UWP
@@ -23,7 +26,6 @@ namespace MQTTnet.Client.Options
 #else
         public IEnumerable<X509Certificate> Certificates { get; set; }
 #endif
-        
 
         public bool AllowUntrustedCertificates { get; set; }
 

Source/MQTTnet/Client/Options/MqttClientTlsOptions.cs

@@ -15,6 +15,7 @@ namespace MQTTnet.Client.Options
         public bool IgnoreCertificateChainErrors { get; set; }
 
         public bool AllowUntrustedCertificates { get; set; }
+
 #if WINDOWS_UWP
         public List<byte[]> Certificates { get; set; }
 #else
@@ -23,6 +24,9 @@ namespace MQTTnet.Client.Options
 
         public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls12;
 
+        [Obsolete("This property will be removed soon. Use CertificateValidationHandler instead.")]
         public Func<X509Certificate, X509Chain, SslPolicyErrors, IMqttClientOptions, bool> CertificateValidationCallback { get; set; }
+
+        public Func<MqttClientCertificateValidationCallbackContext, bool> CertificateValidationHandler { get; set; }
     }
 }

Source/MQTTnet/Diagnostics/TargetFrameworkProvider.cs

@@ -16,6 +16,8 @@
                 return "netstandard1.3";
 #elif NETSTANDARD2_0
                 return "netstandard2.0";
+#elif NETSTANDARD2_1
+                return "netstandard2.1";
 #elif WINDOWS_UWP
                 return "uap10.0";
 #endif

Source/MQTTnet/Implementations/MqttTcpChannel.cs

@@ -73,7 +73,7 @@ namespace MQTTnet.Implementations
 
             var networkStream = socket.GetStream();
 
-            if (_options.TlsOptions.UseTls)
+            if (_options.TlsOptions?.UseTls == true)
             {
                 var sslStream = new SslStream(networkStream, false, InternalUserCertificateValidationCallback);
                 try
@@ -181,9 +181,28 @@ namespace MQTTnet.Implementations
 
         bool InternalUserCertificateValidationCallback(object sender, X509Certificate x509Certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
         {
-            if (_options.TlsOptions.CertificateValidationCallback != null)
+            #region OBSOLETE
+
+            var certificateValidationCallback = _options?.TlsOptions?.CertificateValidationCallback;
+            if (certificateValidationCallback != null)
             {
-                return _options.TlsOptions.CertificateValidationCallback(x509Certificate, chain, sslPolicyErrors, _clientOptions);
+                return certificateValidationCallback(x509Certificate, chain, sslPolicyErrors, _clientOptions);
+            }
+
+            #endregion
+
+            var certificateValidationHandler = _options?.TlsOptions?.CertificateValidationHandler;
+            if (certificateValidationHandler != null)
+            {
+                var context = new MqttClientCertificateValidationCallbackContext
+                {
+                    Certificate = x509Certificate,
+                    Chain = chain,
+                    SslPolicyErrors = sslPolicyErrors,
+                    ClientOptions = _options
+                };
+
+                return certificateValidationHandler(context);
             }
 
             if (sslPolicyErrors == SslPolicyErrors.None)

Source/MQTTnet/Implementations/MqttWebSocketChannel.cs

-using System;
+using MQTTnet.Channel;
+using MQTTnet.Client.Options;
+using MQTTnet.Internal;
+using System;
 using System.Net;
 using System.Net.WebSockets;
 using System.Security.Cryptography.X509Certificates;
 using System.Threading;
 using System.Threading.Tasks;
-using MQTTnet.Channel;
-using MQTTnet.Client.Options;
-using MQTTnet.Internal;
 
 namespace MQTTnet.Implementations
 {
-    public class MqttWebSocketChannel : Disposable, IMqttChannel
+    public sealed class MqttWebSocketChannel : IMqttChannel
     {
-        private readonly MqttClientWebSocketOptions _options;
+        readonly MqttClientWebSocketOptions _options;
 
-        private SemaphoreSlim _sendLock = new SemaphoreSlim(1, 1);
-        private WebSocket _webSocket;
+        AsyncLock _sendLock = new AsyncLock();
+        WebSocket _webSocket;
 
         public MqttWebSocketChannel(MqttClientWebSocketOptions options)
         {
@@ -53,50 +53,20 @@ namespace MQTTnet.Implementations
             }
 
             var clientWebSocket = new ClientWebSocket();
-
-            if (_options.ProxyOptions != null)
+            try
             {
-                clientWebSocket.Options.Proxy = CreateProxy();
-            }
+                SetupClientWebSocket(clientWebSocket);
 
-            if (_options.RequestHeaders != null)
-            {
-                foreach (var requestHeader in _options.RequestHeaders)
-                {
-                    clientWebSocket.Options.SetRequestHeader(requestHeader.Key, requestHeader.Value);
-                }
+                await clientWebSocket.ConnectAsync(new Uri(uri), cancellationToken).ConfigureAwait(false);
             }
-
-            if (_options.SubProtocols != null)
+            catch (Exception)
             {
-                foreach (var subProtocol in _options.SubProtocols)
-                {
-                    clientWebSocket.Options.AddSubProtocol(subProtocol);
-                }
+                // Prevent a memory leak when always creating new instance which will fail while connecting.
+                clientWebSocket.Dispose();
+                throw;
             }
 
-            if (_options.CookieContainer != null)
-            {
-                clientWebSocket.Options.Cookies = _options.CookieContainer;
-            }
-
-            if (_options.TlsOptions?.UseTls == true && _options.TlsOptions?.Certificates != null)
-            {
-                clientWebSocket.Options.ClientCertificates = new X509CertificateCollection();
-                foreach (var certificate in _options.TlsOptions.Certificates)
-                {
-#if WINDOWS_UWP
-                    clientWebSocket.Options.ClientCertificates.Add(new X509Certificate(certificate));
-#else
-                    clientWebSocket.Options.ClientCertificates.Add(certificate);
-#endif
-                    
-                }
-            }
-
-            await clientWebSocket.ConnectAsync(new Uri(uri), cancellationToken).ConfigureAwait(false);
             _webSocket = clientWebSocket;
-
             IsSecureConnection = uri.StartsWith("wss://", StringComparison.OrdinalIgnoreCase);
         }
 
@@ -131,27 +101,87 @@ namespace MQTTnet.Implementations
                 return;
             }
 
-            await _sendLock.WaitAsync(cancellationToken).ConfigureAwait(false);
-            try
+            using (await _sendLock.WaitAsync(cancellationToken).ConfigureAwait(false))
             {
                 await _webSocket.SendAsync(new ArraySegment<byte>(buffer, offset, count), WebSocketMessageType.Binary, true, cancellationToken).ConfigureAwait(false);
             }
-            finally
-            {
-                _sendLock?.Release();
-            }
         }
 
-        protected override void Dispose(bool disposing)
+        public void Dispose()
+        {
+            Cleanup();
+        }
+
+        void SetupClientWebSocket(ClientWebSocket clientWebSocket)
         {
-            if (disposing)
+
+            if (_options.ProxyOptions != null)
+            {
+                clientWebSocket.Options.Proxy = CreateProxy();
+            }
+
+            if (_options.RequestHeaders != null)
+            {
+                foreach (var requestHeader in _options.RequestHeaders)
+                {
+                    clientWebSocket.Options.SetRequestHeader(requestHeader.Key, requestHeader.Value);
+                }
+            }
+
+            if (_options.SubProtocols != null)
             {
-                Cleanup();
+                foreach (var subProtocol in _options.SubProtocols)
+                {
+                    clientWebSocket.Options.AddSubProtocol(subProtocol);
+                }
             }
-            base.Dispose(disposing);
+
+            if (_options.CookieContainer != null)
+            {
+                clientWebSocket.Options.Cookies = _options.CookieContainer;
+            }
+
+            if (_options.TlsOptions?.UseTls == true && _options.TlsOptions?.Certificates != null)
+            {
+                clientWebSocket.Options.ClientCertificates = new X509CertificateCollection();
+                foreach (var certificate in _options.TlsOptions.Certificates)
+                {
+#if WINDOWS_UWP
+                    clientWebSocket.Options.ClientCertificates.Add(new X509Certificate(certificate));
+#else
+                    clientWebSocket.Options.ClientCertificates.Add(certificate);
+#endif
+
+                }
+            }
+
+            var certificateValidationHandler = _options.TlsOptions?.CertificateValidationHandler;
+#if NETSTANDARD2_1
+            if (certificateValidationHandler != null)
+            {
+                clientWebSocket.Options.RemoteCertificateValidationCallback = new System.Net.Security.RemoteCertificateValidationCallback((sender, certificate, chain, sslPolicyErrors) =>
+                {
+                    // TODO: Find a way to add client options to same callback. Problem is that they have a different type.
+                    var context = new MqttClientCertificateValidationCallbackContext
+                    {
+                        Certificate = certificate,
+                        Chain = chain,
+                        SslPolicyErrors = sslPolicyErrors,
+                        ClientOptions = _options
+                    };
+
+                    return certificateValidationHandler(context);
+                });
+            }
+#else
+            if (certificateValidationHandler != null)
+            {
+                throw new NotSupportedException("The remote certificate validation callback for Web Sockets is only supported for netstandard 2.1+");
+            }
+#endif
         }
 
-        private void Cleanup()
+        void Cleanup()
         {
             _sendLock?.Dispose();
             _sendLock = null;
@@ -169,7 +199,7 @@ namespace MQTTnet.Implementations
             }
         }
 
-        private IWebProxy CreateProxy()
+        IWebProxy CreateProxy()
         {
             if (string.IsNullOrEmpty(_options.ProxyOptions?.Address))
             {

Source/MQTTnet/Internal/BlockingQueue.cs

@@ -4,11 +4,12 @@ using System.Threading;
 
 namespace MQTTnet.Internal
 {
-    public class BlockingQueue<TItem> : Disposable
+    public sealed class BlockingQueue<TItem> : IDisposable
     {
-        private readonly object _syncRoot = new object();
-        private readonly LinkedList<TItem> _items = new LinkedList<TItem>();
-        private readonly ManualResetEventSlim _gate = new ManualResetEventSlim(false);
+        readonly object _syncRoot = new object();
+        readonly LinkedList<TItem> _items = new LinkedList<TItem>();
+
+        ManualResetEventSlim _gate = new ManualResetEventSlim(false);
 
         public int Count
         {
@@ -28,13 +29,13 @@ namespace MQTTnet.Internal
             lock (_syncRoot)
             {
                 _items.AddLast(item);
-                _gate.Set();
+                _gate?.Set();
             }
         }
 
-        public TItem Dequeue(CancellationToken cancellationToken = default(CancellationToken))
+        public TItem Dequeue(CancellationToken cancellationToken = default)
         {
-            while (true)
+            while (!cancellationToken.IsCancellationRequested)
             {
                 lock (_syncRoot)
                 {
@@ -48,17 +49,19 @@ namespace MQTTnet.Internal
 
                     if (_items.Count == 0)
                     {
-                        _gate.Reset();
+                        _gate?.Reset();
                     }
                 }
 
-                _gate.Wait(cancellationToken);
+                _gate?.Wait(cancellationToken);
             }
+
+            throw new OperationCanceledException();
         }
-        
-        public TItem PeekAndWait(CancellationToken cancellationToken = default(CancellationToken))
+
+        public TItem PeekAndWait(CancellationToken cancellationToken = default)
         {
-            while (true)
+            while (!cancellationToken.IsCancellationRequested)
             {
                 lock (_syncRoot)
                 {
@@ -69,12 +72,14 @@ namespace MQTTnet.Internal
 
                     if (_items.Count == 0)
                     {
-                        _gate.Reset();
+                        _gate?.Reset();
                     }
                 }
 
-                _gate.Wait(cancellationToken);
+                _gate?.Wait(cancellationToken);
             }
+
+            throw new OperationCanceledException();
         }
 
         public void RemoveFirst(Predicate<TItem> match)
@@ -109,13 +114,10 @@ namespace MQTTnet.Internal
             }
         }
 
-        protected override void Dispose(bool disposing)
+        public void Dispose()
         {
-            if (disposing)
-            {
-                _gate.Dispose();
-            }
-            base.Dispose(disposing);
+            _gate?.Dispose();
+            _gate = null;
         }
     }
 }

Source/MQTTnet/MQTTnet.csproj

 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>netstandard1.3;netstandard2.0</TargetFrameworks>
+    <TargetFrameworks>netstandard1.3;netstandard2.0;netstandard2.1</TargetFrameworks>
     <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">$(TargetFrameworks);net452;net461</TargetFrameworks>
     <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' AND '$(MSBuildRuntimeType)' != 'Core' AND '$(SolutionName)' != 'MQTTnet.noUWP' ">$(TargetFrameworks);uap10.0</TargetFrameworks>
     <AssemblyName>MQTTnet</AssemblyName>

Tests/MQTTnet.Core.Tests/BlockingQueue_Tests.cs

-using System.Threading;
-using System.Threading.Tasks;
-using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Internal;
+using System;
+using System.Threading;
+using System.Threading.Tasks;
 
 namespace MQTTnet.Tests
 {
@@ -33,7 +34,7 @@ namespace MQTTnet.Tests
 
             Assert.AreEqual("a", queue.RemoveFirst());
             Assert.AreEqual("b", queue.RemoveFirst());
-            
+
             Assert.AreEqual(1, queue.Count);
 
             Assert.AreEqual("c", queue.Dequeue());
@@ -81,7 +82,7 @@ namespace MQTTnet.Tests
         }
 
         [TestMethod]
-        public void Wait_For_Times()
+        public void Wait_For_Items()
         {
             var number = 0;
 
@@ -104,5 +105,21 @@ namespace MQTTnet.Tests
                 Interlocked.Increment(ref number);
             }
         }
+
+        [TestMethod]
+        [ExpectedException(typeof(OperationCanceledException))]
+        public void Use_Disposed_Queue()
+        {
+            var queue = new BlockingQueue<int>();
+#pragma warning disable CS4014 // Because this call is not awaited, execution of the current method continues before the call is completed
+            Task.Run(() =>
+            {
+                Thread.Sleep(1000);
+                queue.Dispose();
+            });
+#pragma warning restore CS4014 // Because this call is not awaited, execution of the current method continues before the call is completed
+
+            queue.Dequeue(new CancellationTokenSource(TimeSpan.FromSeconds(2)).Token);
+        }
     }
-}
+}
\ No newline at end of file

Tests/MQTTnet.Core.Tests/MqttClient_Tests.cs

@@ -23,6 +23,18 @@ namespace MQTTnet.Tests
     {
         public TestContext TestContext { get; set; }
 
+        [TestMethod]
+        public async Task Send_Manual_Ping()
+        {
+            using (var testEnvironment = new TestEnvironment(TestContext))
+            {
+                await testEnvironment.StartServerAsync();
+                var client = await testEnvironment.ConnectClientAsync();
+
+                await client.PingAsync(CancellationToken.None);
+            }
+        }
+
         [TestMethod]
         public async Task Send_Reply_In_Message_Handler_For_Same_Client()
         {

Tests/MQTTnet.TestApp.NetCore/Program.cs

-using System;
+using MQTTnet.Client.Options;
+using MQTTnet.Diagnostics;
+using MQTTnet.Server;
+using Newtonsoft.Json;
+using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Net.Security;
-using System.Security.Cryptography.X509Certificates;
 using System.Threading;
 using System.Threading.Tasks;
-using MQTTnet.Client;
-using MQTTnet.Client.Options;
-using MQTTnet.Diagnostics;
-using MQTTnet.Server;
-using Newtonsoft.Json;
 
 namespace MQTTnet.TestApp.NetCore
 {
@@ -129,10 +127,15 @@ namespace MQTTnet.TestApp.NetCore
             var options = new MqttClientOptionsBuilder()
                 .WithTls(new MqttClientOptionsBuilderTlsParameters
                 {
-                    CertificateValidationCallback = (X509Certificate x, X509Chain y, SslPolicyErrors z, IMqttClientOptions o) =>
+                    CertificateValidationHandler = context =>
                         {
-                            // TODO: Check conditions of certificate by using above parameters.
-                            return true;
+                            // TODO: Check conditions of certificate by using above context.
+                            if (context.SslPolicyErrors == SslPolicyErrors.None)
+                            {
+                                return true;
+                            }
+
+                            return false;
                         }
                 })
                 .Build();