Enable nullable

Samples/MQTTnet.Samples.csproj

@@ -15,6 +15,7 @@
         <NuGetAudit>true</NuGetAudit>
         <NuGetAuditLevel>low</NuGetAuditLevel>
         <AnalysisLevel>latest-Recommended</AnalysisLevel>
+        <LangVersion>default</LangVersion>
     </PropertyGroup>
 
     <ItemGroup>

Source/MQTTnet.AspnetCore/MQTTnet.AspNetCore.csproj

@@ -39,6 +39,8 @@
         <NuGetAuditLevel>low</NuGetAuditLevel>
         <AnalysisLevel>latest-Recommended</AnalysisLevel>
         <IsAotCompatible>true</IsAotCompatible>
+        <LangVersion>default</LangVersion>
+        <Nullable>enable</Nullable>
     </PropertyGroup>
 
     <ItemGroup>

Source/MQTTnet.Extensions.Rpc/IMqttRpcClient.cs

@@ -12,7 +12,12 @@ namespace MQTTnet.Extensions.Rpc;
 
 public interface IMqttRpcClient : IDisposable
 {
-    Task<byte[]> ExecuteAsync(TimeSpan timeout, string methodName, byte[] payload, MqttQualityOfServiceLevel qualityOfServiceLevel, IDictionary<string,object> parameters = null);
+    Task<byte[]> ExecuteAsync(TimeSpan timeout, string methodName, byte[] payload, MqttQualityOfServiceLevel qualityOfServiceLevel, IDictionary<string, object>? parameters = null);
 
-    Task<byte[]> ExecuteAsync(string methodName, byte[] payload, MqttQualityOfServiceLevel qualityOfServiceLevel, IDictionary<string, object> parameters = null, CancellationToken cancellationToken = default);
+    Task<byte[]> ExecuteAsync(
+        string methodName,
+        byte[] payload,
+        MqttQualityOfServiceLevel qualityOfServiceLevel,
+        IDictionary<string, object>? parameters = null,
+        CancellationToken cancellationToken = default);
 }

Source/MQTTnet.Extensions.Rpc/MQTTnet.Extensions.Rpc.csproj

@@ -37,6 +37,8 @@
         <NuGetAuditLevel>low</NuGetAuditLevel>
         <AnalysisLevel>latest-Recommended</AnalysisLevel>
         <IsAotCompatible>true</IsAotCompatible>
+        <LangVersion>default</LangVersion>
+        <Nullable>enable</Nullable>
     </PropertyGroup>
 
     <ItemGroup>

Source/MQTTnet.Extensions.Rpc/MqttRpcClient.cs

@@ -20,7 +20,7 @@ public sealed class MqttRpcClient : IMqttRpcClient
     readonly IMqttClient _mqttClient;
     readonly MqttRpcClientOptions _options;
 
-    readonly ConcurrentDictionary<string, AsyncTaskCompletionSource<byte[]>> _waitingCalls = new ConcurrentDictionary<string, AsyncTaskCompletionSource<byte[]>>();
+    readonly ConcurrentDictionary<string, AsyncTaskCompletionSource<byte[]>> _waitingCalls = new();
 
     public MqttRpcClient(IMqttClient mqttClient, MqttRpcClientOptions options)
     {
@@ -42,7 +42,12 @@ public sealed class MqttRpcClient : IMqttRpcClient
         _waitingCalls.Clear();
     }
 
-    public async Task<byte[]> ExecuteAsync(TimeSpan timeout, string methodName, byte[] payload, MqttQualityOfServiceLevel qualityOfServiceLevel, IDictionary<string, object> parameters = null)
+    public async Task<byte[]> ExecuteAsync(
+        TimeSpan timeout,
+        string methodName,
+        byte[] payload,
+        MqttQualityOfServiceLevel qualityOfServiceLevel,
+        IDictionary<string, object>? parameters = null)
     {
         using var timeoutToken = new CancellationTokenSource(timeout);
         try
@@ -60,7 +65,12 @@ public sealed class MqttRpcClient : IMqttRpcClient
         }
     }
 
-    public async Task<byte[]> ExecuteAsync(string methodName, byte[] payload, MqttQualityOfServiceLevel qualityOfServiceLevel, IDictionary<string, object> parameters = null, CancellationToken cancellationToken = default)
+    public async Task<byte[]> ExecuteAsync(
+        string methodName,
+        byte[] payload,
+        MqttQualityOfServiceLevel qualityOfServiceLevel,
+        IDictionary<string, object>? parameters = null,
+        CancellationToken cancellationToken = default)
     {
         ArgumentNullException.ThrowIfNull(methodName);
 
@@ -82,7 +92,7 @@ public sealed class MqttRpcClient : IMqttRpcClient
 
         var requestMessageBuilder = new MqttApplicationMessageBuilder().WithTopic(requestTopic).WithPayload(payload).WithQualityOfServiceLevel(qualityOfServiceLevel);
 
-        if (_mqttClient.Options.ProtocolVersion == MqttProtocolVersion.V500)
+        if (_mqttClient.Options!.ProtocolVersion == MqttProtocolVersion.V500)
         {
             requestMessageBuilder.WithResponseTopic(responseTopic);
         }
@@ -103,8 +113,7 @@ public sealed class MqttRpcClient : IMqttRpcClient
             await _mqttClient.SubscribeAsync(subscribeOptions, cancellationToken).ConfigureAwait(false);
             await _mqttClient.PublishAsync(requestMessage, cancellationToken).ConfigureAwait(false);
 
-            using (cancellationToken.Register(
+            using (cancellationToken.Register(() =>
-                       () =>
                    {
                        awaitable.TrySetCanceled();
                    }))

Source/MQTTnet.Extensions.Rpc/MqttRpcClientExtensions.cs

@@ -12,7 +12,7 @@ namespace MQTTnet.Extensions.Rpc;
 
 public static class MqttRpcClientExtensions
 {
-    public static Task<byte[]> ExecuteAsync(this IMqttRpcClient client, TimeSpan timeout, string methodName, string payload, MqttQualityOfServiceLevel qualityOfServiceLevel, IDictionary<string,object> parameters = null)
+    public static Task<byte[]> ExecuteAsync(this IMqttRpcClient client, TimeSpan timeout, string methodName, string payload, MqttQualityOfServiceLevel qualityOfServiceLevel, IDictionary<string,object>? parameters = null)
     {
         if (client == null) throw new ArgumentNullException(nameof(client));
 

Source/MQTTnet.Extensions.Rpc/MqttRpcTopicPair.cs

@@ -6,7 +6,7 @@ namespace MQTTnet.Extensions.Rpc;
 
 public sealed class MqttRpcTopicPair
 {
-    public string RequestTopic { get; set; }
+    public required string RequestTopic { get; init; }
 
-    public string ResponseTopic { get; set; }
+    public required string ResponseTopic { get; init; }
 }

Source/MQTTnet.Extensions.Rpc/TopicGenerationContext.cs

@@ -10,8 +10,12 @@ namespace MQTTnet.Extensions.Rpc;
 
 public sealed class TopicGenerationContext
 {
-    public TopicGenerationContext(IMqttClient mqttClient, MqttRpcClientOptions options, string methodName,
+    public TopicGenerationContext(
-        IDictionary<string, object> parameters, MqttQualityOfServiceLevel qualityOfServiceLevel)
+        IMqttClient mqttClient,
+        MqttRpcClientOptions options,
+        string methodName,
+        IDictionary<string, object>? parameters,
+        MqttQualityOfServiceLevel qualityOfServiceLevel)
     {
         MethodName = methodName ?? throw new ArgumentNullException(nameof(methodName));
         Parameters = parameters;
@@ -22,12 +26,12 @@ public sealed class TopicGenerationContext
 
     public string MethodName { get; }
 
-    public IDictionary<string, object> Parameters { get; }
-
     public IMqttClient MqttClient { get; }
 
     public MqttRpcClientOptions Options { get; }
 
+    public IDictionary<string, object>? Parameters { get; }
+
     /// <summary>
     ///     Gets or sets the quality of service level.
     ///     The Quality of Service (QoS) level is an agreement between the sender of a message and the receiver of a message

Source/MQTTnet.Extensions.TopicTemplate/MQTTnet.Extensions.TopicTemplate.csproj

@@ -33,6 +33,8 @@
         <GenerateDocumentationFile>true</GenerateDocumentationFile>
         <NoWarn>1591;NETSDK1138</NoWarn>
         <IsAotCompatible>true</IsAotCompatible>
+        <LangVersion>default</LangVersion>
+        <Nullable>enable</Nullable>
     </PropertyGroup>
 
     <ItemGroup>

Source/MQTTnet.Server/EnhancedAuthentication/ExchangeEnhancedAuthenticationOptions.cs

@@ -8,9 +8,9 @@ namespace MQTTnet.Server.EnhancedAuthentication;
 
 public sealed class ExchangeEnhancedAuthenticationOptions
 {
-    public byte[] AuthenticationData { get; set; }
+    public byte[]? AuthenticationData { get; set; }
 
-    public string ReasonString { get; set; }
+    public string? ReasonString { get; set; }
 
-    public List<MqttUserProperty> UserProperties { get; set; }
+    public List<MqttUserProperty>? UserProperties { get; set; }
 }

Source/MQTTnet.Server/EnhancedAuthentication/ExchangeEnhancedAuthenticationResult.cs

@@ -4,9 +4,9 @@ namespace MQTTnet.Server.EnhancedAuthentication;
 
 public sealed class ExchangeEnhancedAuthenticationResult
 {
-    public string ReasonString { get; set; }
+    public string? ReasonString { get; init; }
 
-    public List<MqttUserProperty> UserProperties { get; set; }
+    public List<MqttUserProperty>? UserProperties { get; init; }
 
-    public byte[] AuthenticationData { get; set; }
+    public byte[]? AuthenticationData { get; init; }
 }

Source/MQTTnet.Server/Events/ClientConnectedEventArgs.cs

@@ -22,15 +22,15 @@ public sealed class ClientConnectedEventArgs : EventArgs
         SessionItems = sessionItems ?? throw new ArgumentNullException(nameof(sessionItems));
     }
 
-    public byte[] AuthenticationData => _connectPacket.AuthenticationData;
+    public byte[]? AuthenticationData => _connectPacket.AuthenticationData;
 
-    public string AuthenticationMethod => _connectPacket.AuthenticationMethod;
+    public string? AuthenticationMethod => _connectPacket.AuthenticationMethod;
 
     /// <summary>
     ///     Gets the client identifier of the connected client.
     ///     Hint: This identifier needs to be unique over all used clients / devices on the broker to avoid connection issues.
     /// </summary>
-    public string ClientId => _connectPacket.ClientId;
+    public string ClientId => _connectPacket.ClientId!;
 
     /// <summary>
     ///     Gets the endpoint of the connected client.
@@ -38,7 +38,7 @@ public sealed class ClientConnectedEventArgs : EventArgs
     public EndPoint RemoteEndPoint { get; }
 
     [Obsolete("Use RemoteEndPoint instead.")]
-    public string Endpoint => RemoteEndPoint?.ToString();
+    public string? Endpoint => RemoteEndPoint?.ToString();
 
     /// <summary>
     ///     Gets the protocol version which is used by the connected client.
@@ -53,16 +53,16 @@ public sealed class ClientConnectedEventArgs : EventArgs
     /// <summary>
     ///     Gets the user name of the connected client.
     /// </summary>
-    public string UserName => _connectPacket.Username;
+    public string? UserName => _connectPacket.Username;
 
     /// <summary>
     ///     Gets the password of the connected client.
     /// </summary>
-    public string Password => Encoding.UTF8.GetString(_connectPacket.Password.AsSpan());
+    public string? Password => Encoding.UTF8.GetString(_connectPacket.Password.AsSpan());
 
     /// <summary>
     ///     Gets the user properties sent by the client.
     ///     <remarks>MQTT 5.0.0+ feature.</remarks>
     /// </summary>
-    public List<MqttUserProperty> UserProperties => _connectPacket?.UserProperties;
+    public List<MqttUserProperty>? UserProperties => _connectPacket?.UserProperties;
 }

Source/MQTTnet.Server/Events/ClientDisconnectedEventArgs.cs

@@ -35,24 +35,24 @@ public sealed class ClientDisconnectedEventArgs : EventArgs
     ///     Gets the client identifier.
     ///     Hint: This identifier needs to be unique over all used clients / devices on the broker to avoid connection issues.
     /// </summary>
-    public string ClientId => _connectPacket.ClientId;
+    public string ClientId => _connectPacket.ClientId!;
 
     /// <summary>
     ///     Gets the user name of the client.
     /// </summary>
-    public string UserName => _connectPacket.Username;
+    public string? UserName => _connectPacket.Username;
 
     /// <summary>
     ///     Gets the password of the client.
     /// </summary>
-    public string Password => Encoding.UTF8.GetString(_connectPacket.Password.AsSpan());
+    public string? Password => Encoding.UTF8.GetString(_connectPacket.Password.AsSpan());
 
     public MqttClientDisconnectType DisconnectType { get; }
 
     public EndPoint RemoteEndPoint { get; }
 
     [Obsolete("Use RemoteEndPoint instead.")]
-    public string Endpoint => RemoteEndPoint?.ToString();
+    public string? Endpoint => RemoteEndPoint?.ToString();
 
     /// <summary>
     ///     Gets the reason code sent by the client.
@@ -66,7 +66,7 @@ public sealed class ClientDisconnectedEventArgs : EventArgs
     ///     Only available for clean disconnects.
     ///     <remarks>MQTT 5.0.0+ feature.</remarks>
     /// </summary>
-    public string ReasonString => _disconnectPacket?.ReasonString;
+    public string? ReasonString => _disconnectPacket?.ReasonString;
 
     /// <summary>
     ///     Gets the session expiry interval sent by the client.
@@ -85,5 +85,5 @@ public sealed class ClientDisconnectedEventArgs : EventArgs
     ///     Only available for clean disconnects.
     ///     <remarks>MQTT 5.0.0+ feature.</remarks>
     /// </summary>
-    public List<MqttUserProperty> UserProperties => _disconnectPacket?.UserProperties;
+    public List<MqttUserProperty>? UserProperties => _disconnectPacket?.UserProperties;
 }

Source/MQTTnet.Server/Internal/Adapter/MqttTcpServerAdapter.cs

@@ -12,7 +12,7 @@ namespace MQTTnet.Server.Internal.Adapter;
 
 public sealed class MqttTcpServerAdapter : IMqttServerAdapter
 {
-    readonly List<MqttTcpServerListener> _listeners = new List<MqttTcpServerListener>();
+    readonly List<MqttTcpServerListener> _listeners = new();
 
     CancellationTokenSource _cancellationTokenSource;
 

Source/MQTTnet.Server/Internal/Adapter/MqttTcpServerListener.cs

@@ -165,8 +165,8 @@ public sealed class MqttTcpServerListener : IDisposable
 
     async Task TryHandleClientConnectionAsync(CrossPlatformSocket clientSocket)
     {
-        Stream stream = null;
+        Stream? stream = null;
-        EndPoint remoteEndPoint = null;
+        EndPoint? remoteEndPoint = null;
 
         try
         {

Source/MQTTnet.Server/Internal/MqttClientSessionsManager.cs

@@ -667,7 +667,7 @@ public sealed class MqttClientSessionsManager : ISubscriptionChangedNotification
         }
     }
 
-    async Task<MqttConnectPacket> ReceiveConnectPacket(IMqttChannelAdapter channelAdapter, CancellationToken cancellationToken)
+    async Task<MqttConnectPacket?> ReceiveConnectPacket(IMqttChannelAdapter channelAdapter, CancellationToken cancellationToken)
     {
         try
         {

Source/MQTTnet.Server/MQTTnet.Server.csproj

@@ -35,9 +35,10 @@
         <NuGetAudit>true</NuGetAudit>
         <NuGetAuditLevel>low</NuGetAuditLevel>
         <ImplicitUsings>enable</ImplicitUsings>
-        <Nullable>disable</Nullable>
+        <Nullable>enable</Nullable>
         <AnalysisLevel>latest-Recommended</AnalysisLevel>
         <IsAotCompatible>true</IsAotCompatible>
+        <LangVersion>default</LangVersion>
     </PropertyGroup>
 
     <ItemGroup>

Source/MQTTnet.Server/MqttServerExtensions.cs

@@ -21,7 +21,7 @@ public static class MqttServerExtensions
     public static Task InjectApplicationMessage(
         this MqttServer server,
         string topic,
-        string payload = null,
+        string? payload = null,
         MqttQualityOfServiceLevel qualityOfServiceLevel = MqttQualityOfServiceLevel.AtMostOnce,
         bool retain = false)
     {

Source/MQTTnet.Server/Options/MqttServerOptionsBuilder.cs

@@ -20,7 +20,7 @@ public class MqttServerOptionsBuilder
         return _options;
     }
 
-    public MqttServerOptionsBuilder WithClientCertificate(RemoteCertificateValidationCallback validationCallback = null, bool checkCertificateRevocation = false)
+    public MqttServerOptionsBuilder WithClientCertificate(RemoteCertificateValidationCallback? validationCallback = null, bool checkCertificateRevocation = false)
     {
         _options.TlsEndpointOptions.ClientCertificateRequired = true;
         _options.TlsEndpointOptions.CheckCertificateRevocation = checkCertificateRevocation;
@@ -183,7 +183,7 @@ public class MqttServerOptionsBuilder
         return this;
     }
 
-    public MqttServerOptionsBuilder WithEncryptionCertificate(byte[] value, IMqttServerCertificateCredentials credentials = null)
+    public MqttServerOptionsBuilder WithEncryptionCertificate(byte[] value, IMqttServerCertificateCredentials? credentials = null)
     {
         ArgumentNullException.ThrowIfNull(value);
 

Source/MQTTnet.Server/SubscribeResponse.cs

@@ -16,7 +16,7 @@ public sealed class SubscribeResponse
     /// </summary>
     public MqttSubscribeReasonCode ReasonCode { get; set; }
 
-    public string ReasonString { get; set; }
+    public string? ReasonString { get; set; }
 
-    public List<MqttUserProperty> UserProperties { get; } = [];
+    public List<MqttUserProperty>? UserProperties { get; set; } = [];
 }

Source/MQTTnet.Server/UnsubscribeResponse.cs

@@ -15,7 +15,7 @@ public sealed class UnsubscribeResponse
     /// </summary>
     public MqttUnsubscribeReasonCode ReasonCode { get; set; }
 
-    public string ReasonString { get; set; }
+    public string? ReasonString { get; set; }
 
-    public List<MqttUserProperty> UserProperties { get; } = [];
+    public List<MqttUserProperty>? UserProperties { get; } = [];
 }

Source/MQTTnet.TestApp/MQTTnet.TestApp.csproj

@@ -14,6 +14,8 @@
         <NuGetAudit>true</NuGetAudit>
         <NuGetAuditLevel>low</NuGetAuditLevel>
         <AnalysisLevel>latest-Recommended</AnalysisLevel>
+        <LangVersion>default</LangVersion>
+        <Nullable>enable</Nullable>
     </PropertyGroup>
 
     <ItemGroup>

Source/MQTTnet.Tests/MQTTnet.Tests.csproj

@@ -14,6 +14,8 @@
         <NuGetAudit>true</NuGetAudit>
         <NuGetAuditLevel>low</NuGetAuditLevel>
         <AnalysisLevel>latest-Recommended</AnalysisLevel>
+        <LangVersion>default</LangVersion>
+        <Nullable>enable</Nullable>
     </PropertyGroup>
 
     <ItemGroup>

Source/MQTTnet/Adapter/IMqttChannelAdapter.cs

@@ -18,9 +18,9 @@ public interface IMqttChannelAdapter : IDisposable
 
     long BytesSent { get; }
 
-    X509Certificate2 ClientCertificate { get; }
+    X509Certificate2? ClientCertificate { get; }
 
-    EndPoint RemoteEndPoint { get; }
+    EndPoint? RemoteEndPoint { get; }
 
     bool IsSecureConnection { get; }
 
@@ -30,7 +30,7 @@ public interface IMqttChannelAdapter : IDisposable
 
     Task DisconnectAsync(CancellationToken cancellationToken);
 
-    Task<MqttPacket> ReceivePacketAsync(CancellationToken cancellationToken);
+    Task<MqttPacket?> ReceivePacketAsync(CancellationToken cancellationToken);
 
     void ResetStatistics();
 

Source/MQTTnet/Adapter/IMqttClientAdapterFactory.cs

@@ -8,5 +8,5 @@ namespace MQTTnet.Adapter;
 
 public interface IMqttClientAdapterFactory
 {
-    IMqttChannelAdapter CreateClientAdapter(MqttClientOptions options, MqttPacketInspector packetInspector, IMqttNetLogger logger);
+    IMqttChannelAdapter CreateClientAdapter(MqttClientOptions options, MqttPacketInspector? packetInspector, IMqttNetLogger logger);
 }

Source/MQTTnet/Adapter/MqttChannelAdapter.cs

@@ -50,15 +50,15 @@ public sealed class MqttChannelAdapter : Disposable, IMqttChannelAdapter
 
     public long BytesSent => Volatile.Read(ref _statistics._bytesSent);
 
-    public X509Certificate2 ClientCertificate => _channel.ClientCertificate;
+    public X509Certificate2? ClientCertificate => _channel.ClientCertificate;
 
-    public EndPoint RemoteEndPoint => _channel.RemoteEndPoint;
+    public EndPoint? RemoteEndPoint => _channel.RemoteEndPoint;
 
     public bool IsSecureConnection => _channel.IsSecureConnection;
 
     public MqttPacketFormatterAdapter PacketFormatterAdapter { get; }
 
-    public MqttPacketInspector PacketInspector { get; set; }
+    public MqttPacketInspector? PacketInspector { get; set; }
 
     public async Task ConnectAsync(CancellationToken cancellationToken)
     {
@@ -73,7 +73,7 @@ public sealed class MqttChannelAdapter : Disposable, IMqttChannelAdapter
              * block forever. Even a cancellation token is not supported properly.
              */
 
-            var timeout = new TaskCompletionSource<object>();
+            var timeout = new TaskCompletionSource<object?>();
             await using (cancellationToken.Register(() => timeout.TrySetResult(null)))
             {
                 var connectTask = Task.Run(
@@ -133,7 +133,7 @@ public sealed class MqttChannelAdapter : Disposable, IMqttChannelAdapter
         }
     }
 
-    public async Task<MqttPacket> ReceivePacketAsync(CancellationToken cancellationToken)
+    public async Task<MqttPacket?> ReceivePacketAsync(CancellationToken cancellationToken)
     {
         cancellationToken.ThrowIfCancellationRequested();
         ThrowIfDisposed();

Source/MQTTnet/Adapter/MqttPacketInspector.cs

@@ -17,7 +17,7 @@ public sealed class MqttPacketInspector
     readonly AsyncEvent<InspectMqttPacketEventArgs> _asyncEvent;
     readonly MqttNetSourceLogger _logger;
 
-    MemoryStream _receivedPacketBuffer;
+    MemoryStream? _receivedPacketBuffer;
 
     public MqttPacketInspector(AsyncEvent<InspectMqttPacketEventArgs> asyncEvent, IMqttNetLogger logger)
     {
@@ -61,7 +61,7 @@ public sealed class MqttPacketInspector
             return CompletedTask.Instance;
         }
 
-        var buffer = _receivedPacketBuffer.ToArray();
+        var buffer = _receivedPacketBuffer!.ToArray();
         _receivedPacketBuffer.SetLength(0);
 
         return InspectPacket(buffer, MqttPacketFlowDirection.Inbound);

Source/MQTTnet/Certificates/BlobCertificateProvider.cs

@@ -11,7 +11,7 @@ public class BlobCertificateProvider(byte[] blob) : ICertificateProvider
 {
     public byte[] Blob { get; } = blob ?? throw new ArgumentNullException(nameof(blob));
 
-    public string Password { get; set; }
+    public string? Password { get; set; }
 
     public X509Certificate2 GetCertificate()
     {

Source/MQTTnet/Channel/IMqttChannel.cs

@@ -13,8 +13,9 @@ namespace MQTTnet.Channel;
 
 public interface IMqttChannel : IDisposable
 {
-    X509Certificate2 ClientCertificate { get; }
+    X509Certificate2? ClientCertificate { get; }
-    EndPoint RemoteEndPoint { get; }
+
+    EndPoint? RemoteEndPoint { get; }
 
     bool IsSecureConnection { get; }
 

Source/MQTTnet/Connecting/MqttClientConnectResult.cs

@@ -14,19 +14,19 @@ public sealed class MqttClientConnectResult
     ///     Gets the client identifier which was chosen by the server.
     ///     MQTTv5 only.
     /// </summary>
-    public string AssignedClientIdentifier { get; init; }
+    public string? AssignedClientIdentifier { get; init; }
 
     /// <summary>
     ///     Gets the authentication data.
     ///     MQTTv5 only.
     /// </summary>
-    public byte[] AuthenticationData { get; init; }
+    public byte[]? AuthenticationData { get; init; }
 
     /// <summary>
     ///     Gets the authentication method.
     ///     MQTTv5 only.
     /// </summary>
-    public string AuthenticationMethod { get; init; }
+    public string? AuthenticationMethod { get; init; }
 
     /// <summary>
     ///     Gets a value indicating whether a session was already available or not.
@@ -45,7 +45,7 @@ public sealed class MqttClientConnectResult
     ///     Gets the reason string.
     ///     MQTTv5 only.
     /// </summary>
-    public string ReasonString { get; init; }
+    public string? ReasonString { get; init; }
 
     public ushort? ReceiveMaximum { get; init; }
 
@@ -53,7 +53,7 @@ public sealed class MqttClientConnectResult
     ///     Gets the response information.
     ///     MQTTv5 only.
     /// </summary>
-    public string ResponseInformation { get; init; }
+    public string? ResponseInformation { get; init; }
 
     /// <summary>
     ///     Gets the result code.
@@ -78,7 +78,7 @@ public sealed class MqttClientConnectResult
     ///     Gets an alternate server which should be used instead of the current one.
     ///     MQTTv5 only.
     /// </summary>
-    public string ServerReference { get; init; }
+    public string? ServerReference { get; init; }
 
     public uint? SessionExpiryInterval { get; init; }
 
@@ -109,7 +109,7 @@ public sealed class MqttClientConnectResult
     ///     The feature is very similar to the HTTP header concept.
     ///     MQTTv5 only.
     /// </summary>
-    public List<MqttUserProperty> UserProperties { get; init; }
+    public List<MqttUserProperty>? UserProperties { get; init; }
 
     /// <summary>
     ///     Gets a value indicating whether wildcards can be used in subscriptions at the current server.

Source/MQTTnet/Diagnostics/Logger/IMqttNetLogger.cs

@@ -10,5 +10,5 @@ public interface IMqttNetLogger
 {
     bool IsEnabled { get; }
 
-    void Publish(MqttNetLogLevel level, string source, string message, object[] parameters, Exception exception);
+    void Publish(MqttNetLogLevel level, string source, string message, object?[]? parameters, Exception? exception);
 }

Source/MQTTnet/Diagnostics/Logger/MqttNetEventLogger.cs

@@ -9,17 +9,17 @@ namespace MQTTnet.Diagnostics.Logger;
 /// <summary>
 ///     This logger fires an event when a new message was published.
 /// </summary>
-public sealed class MqttNetEventLogger(string logId = null) : IMqttNetLogger
+public sealed class MqttNetEventLogger(string? logId = null) : IMqttNetLogger
 {
-    public event EventHandler<MqttNetLogMessagePublishedEventArgs> LogMessagePublished;
+    public event EventHandler<MqttNetLogMessagePublishedEventArgs>? LogMessagePublished;
 
     public bool IsEnabled => LogMessagePublished != null;
 
-    public string LogId { get; } = logId;
+    public string? LogId { get; } = logId;
 
-    public IFormatProvider FormatProvider { get; set; }
+    public IFormatProvider? FormatProvider { get; set; }
 
-    public void Publish(MqttNetLogLevel level, string source, string message, object[] parameters, Exception exception)
+    public void Publish(MqttNetLogLevel level, string source, string message, object?[]? parameters, Exception? exception)
     {
         var eventHandler = LogMessagePublished;
         if (eventHandler == null)
@@ -30,7 +30,7 @@ public sealed class MqttNetEventLogger(string logId = null) : IMqttNetLogger
             return;
         }
 
-        if (parameters?.Length > 0 && message?.Length > 0)
+        if (parameters?.Length > 0 && message.Length > 0)
         {
             try
             {

Source/MQTTnet/Diagnostics/Logger/MqttNetLogMessage.cs

@@ -8,19 +8,19 @@ namespace MQTTnet.Diagnostics.Logger;
 
 public sealed class MqttNetLogMessage
 {
-    public string LogId { get; set; }
+    public string? LogId { get; set; }
 
     public DateTime Timestamp { get; set; }
 
     public int ThreadId { get; set; }
 
-    public string Source { get; set; }
+    public string? Source { get; set; }
 
     public MqttNetLogLevel Level { get; set; }
 
-    public string Message { get; set; }
+    public string? Message { get; set; }
 
-    public Exception Exception { get; set; }
+    public Exception? Exception { get; set; }
 
     public override string ToString()
     {

Source/MQTTnet/Diagnostics/Logger/MqttNetNullLogger.cs

@@ -15,7 +15,7 @@ public sealed class MqttNetNullLogger : IMqttNetLogger
 
     public bool IsEnabled { get; }
 
-    public void Publish(MqttNetLogLevel level, string source, string message, object[] parameters, Exception exception)
+    public void Publish(MqttNetLogLevel level, string source, string message, object?[]? parameters, Exception? exception)
     {
     }
 }

Source/MQTTnet/Diagnostics/Logger/MqttNetSourceLogger.cs

@@ -12,7 +12,7 @@ public sealed class MqttNetSourceLogger(IMqttNetLogger logger, string source)
 
     public bool IsEnabled => _logger.IsEnabled;
 
-    public void Publish(MqttNetLogLevel logLevel, string message, object[] parameters, Exception exception)
+    public void Publish(MqttNetLogLevel logLevel, string message, object?[]? parameters, Exception? exception)
     {
         _logger.Publish(logLevel, source, message, parameters, exception);
     }

Source/MQTTnet/Disconnecting/MqttClientDisconnectOptions.cs

@@ -19,7 +19,7 @@ public sealed class MqttClientDisconnectOptions
     ///     Gets or sets the reason string.
     ///     <remarks>MQTT 5.0.0+ feature.</remarks>
     /// </summary>
-    public string ReasonString { get; set; }
+    public string? ReasonString { get; set; }
 
     /// <summary>
     ///     Gets or sets the session expiry interval.
@@ -31,5 +31,5 @@ public sealed class MqttClientDisconnectOptions
     ///     Gets or sets the user properties.
     ///     <remarks>MQTT 5.0.0+ feature.</remarks>
     /// </summary>
-    public List<MqttUserProperty> UserProperties { get; set; }
+    public List<MqttUserProperty>? UserProperties { get; set; }
 }

Source/MQTTnet/Disconnecting/MqttClientDisconnectOptionsBuilder.cs

@@ -10,9 +10,9 @@ namespace MQTTnet;
 public sealed class MqttClientDisconnectOptionsBuilder
 {
     MqttClientDisconnectOptionsReason _reason = MqttClientDisconnectOptionsReason.NormalDisconnection;
-    string _reasonString;
+    string? _reasonString;
     uint _sessionExpiryInterval;
-    List<MqttUserProperty> _userProperties;
+    List<MqttUserProperty>? _userProperties;
 
     public MqttClientDisconnectOptions Build()
     {

Source/MQTTnet/Disconnecting/MqttClientDisconnectedEventArgs.cs

@@ -12,11 +12,11 @@ public sealed class MqttClientDisconnectedEventArgs : EventArgs
 {
     public MqttClientDisconnectedEventArgs(
         bool clientWasConnected,
-        MqttClientConnectResult connectResult,
+        MqttClientConnectResult? connectResult,
         MqttClientDisconnectReason reason,
-        string reasonString,
+        string? reasonString,
-        List<MqttUserProperty> userProperties,
+        List<MqttUserProperty>? userProperties,
-        Exception exception)
+        Exception? exception)
     {
         ClientWasConnected = clientWasConnected;
         ConnectResult = connectResult;
@@ -32,9 +32,9 @@ public sealed class MqttClientDisconnectedEventArgs : EventArgs
     ///     Gets the authentication result.
     ///     <remarks>MQTT 5.0.0+ feature.</remarks>
     /// </summary>
-    public MqttClientConnectResult ConnectResult { get; }
+    public MqttClientConnectResult? ConnectResult { get; }
 
-    public Exception Exception { get; }
+    public Exception? Exception { get; }
 
     /// <summary>
     ///     Gets or sets the reason.
@@ -42,7 +42,7 @@ public sealed class MqttClientDisconnectedEventArgs : EventArgs
     /// </summary>
     public MqttClientDisconnectReason Reason { get; }
 
-    public string ReasonString { get; }
+    public string? ReasonString { get; }
 
-    public List<MqttUserProperty> UserProperties { get; }
+    public List<MqttUserProperty>? UserProperties { get; }
 }

Source/MQTTnet/Exceptions/MqttClientUnexpectedDisconnectReceivedException.cs

@@ -9,17 +9,17 @@ using MQTTnet.Protocol;
 
 namespace MQTTnet.Exceptions;
 
-public sealed class MqttClientUnexpectedDisconnectReceivedException(MqttDisconnectPacket disconnectPacket, Exception innerException = null) : MqttCommunicationException(
+public sealed class MqttClientUnexpectedDisconnectReceivedException(MqttDisconnectPacket disconnectPacket, Exception? innerException = null) : MqttCommunicationException(
     $"Unexpected DISCONNECT (Reason code={disconnectPacket.ReasonCode}) received.",
     innerException)
 {
     public MqttDisconnectReasonCode? ReasonCode { get; } = disconnectPacket.ReasonCode;
 
-    public string ReasonString { get; } = disconnectPacket.ReasonString;
+    public string? ReasonString { get; } = disconnectPacket.ReasonString;
 
-    public string ServerReference { get; } = disconnectPacket.ServerReference;
+    public string? ServerReference { get; } = disconnectPacket.ServerReference;
 
     public uint? SessionExpiryInterval { get; } = disconnectPacket.SessionExpiryInterval;
 
-    public List<MqttUserProperty> UserProperties { get; } = disconnectPacket.UserProperties;
+    public List<MqttUserProperty>? UserProperties { get; } = disconnectPacket.UserProperties;
 }

Source/MQTTnet/Exceptions/MqttCommunicationException.cs

@@ -8,12 +8,12 @@ namespace MQTTnet.Exceptions;
 
 public class MqttCommunicationException : Exception
 {
-    public MqttCommunicationException(Exception innerException)
+    public MqttCommunicationException(Exception? innerException)
         : base(innerException?.Message ?? "MQTT communication failed.", innerException)
     {
     }
 
-    public MqttCommunicationException(string message, Exception innerException = null)
+    public MqttCommunicationException(string message, Exception? innerException = null)
         : base(message, innerException)
     {
     }

Source/MQTTnet/Formatter/IMqttPacketFormatter.cs

@@ -9,7 +9,7 @@ namespace MQTTnet.Formatter;
 
 public interface IMqttPacketFormatter
 {
-    MqttPacket Decode(ReceivedMqttPacket receivedPacket);
+    MqttPacket? Decode(ReceivedMqttPacket receivedPacket);
 
     MqttPacketBuffer Encode(MqttPacket packet);
 }

Source/MQTTnet/Formatter/MqttBufferReader.cs

@@ -131,7 +131,7 @@ public sealed class MqttBufferReader
 
     public void SetBuffer(ArraySegment<byte> buffer)
     {
-        SetBuffer(buffer.Array, buffer.Offset, buffer.Count);
+        SetBuffer(buffer.Array!, buffer.Offset, buffer.Count);
     }
 
     public void SetBuffer(byte[] buffer, int offset, int length)

Source/MQTTnet/Formatter/MqttBufferWriter.cs

@@ -110,7 +110,7 @@ public sealed class MqttBufferWriter
         WriteBinary(propertyWriter._buffer, 0, propertyWriter.Length);
     }
 
-    public void WriteBinary(byte[] value)
+    public void WriteBinary(byte[]? value)
     {
         if (value == null || value.Length == 0)
         {
@@ -158,7 +158,7 @@ public sealed class MqttBufferWriter
         IncreasePosition(1);
     }
 
-    public void WriteString(string value)
+    public void WriteString(string? value)
     {
         if (string.IsNullOrEmpty(value))
         {

Source/MQTTnet/Formatter/MqttPacketBuffer.cs

@@ -40,7 +40,7 @@ public readonly struct MqttPacketBuffer
         }
 
         var buffer = GC.AllocateUninitializedArray<byte>(Length);
-        MqttMemoryHelper.Copy(Packet.Array, Packet.Offset, buffer, 0, Packet.Count);
+        MqttMemoryHelper.Copy(Packet.Array!, Packet.Offset, buffer, 0, Packet.Count);
         MqttMemoryHelper.Copy(Payload, 0, buffer, Packet.Count, (int)Payload.Length);
 
         return buffer;

Source/MQTTnet/Formatter/MqttPacketFormatterAdapter.cs

@@ -17,7 +17,7 @@ public sealed class MqttPacketFormatterAdapter
     readonly MqttBufferReader _bufferReader = new();
     readonly MqttBufferWriter _bufferWriter;
 
-    IMqttPacketFormatter _formatter;
+    IMqttPacketFormatter? _formatter;
 
     public MqttPacketFormatterAdapter(MqttBufferWriter mqttBufferWriter)
     {
@@ -37,10 +37,10 @@ public sealed class MqttPacketFormatterAdapter
         _bufferWriter.Cleanup();
     }
 
-    public MqttPacket Decode(ReceivedMqttPacket receivedMqttPacket)
+    public MqttPacket? Decode(ReceivedMqttPacket receivedMqttPacket)
     {
         ThrowIfFormatterNotSet();
-        return _formatter.Decode(receivedMqttPacket);
+        return _formatter!.Decode(receivedMqttPacket);
     }
 
     public void DetectProtocolVersion(ReceivedMqttPacket receivedMqttPacket)
@@ -52,7 +52,7 @@ public sealed class MqttPacketFormatterAdapter
     public MqttPacketBuffer Encode(MqttPacket packet)
     {
         ThrowIfFormatterNotSet();
-        return _formatter.Encode(packet);
+        return _formatter!.Encode(packet);
     }
 
     public static IMqttPacketFormatter GetMqttPacketFormatter(MqttProtocolVersion protocolVersion, MqttBufferWriter bufferWriter)
@@ -80,7 +80,7 @@ public sealed class MqttPacketFormatterAdapter
             throw new MqttProtocolViolationException("CONNECT packet must have at least 7 bytes.");
         }
 
-        _bufferReader.SetBuffer(receivedMqttPacket.Body.Array, receivedMqttPacket.Body.Offset, receivedMqttPacket.Body.Count);
+        _bufferReader.SetBuffer(receivedMqttPacket.Body.Array!, receivedMqttPacket.Body.Offset, receivedMqttPacket.Body.Count);
 
         var protocolName = _bufferReader.ReadString();
         var protocolLevel = _bufferReader.ReadByte();

Source/MQTTnet/Formatter/MqttUnsubscribePacketFactory.cs

@@ -18,10 +18,7 @@ public static class MqttUnsubscribePacketFactory
             UserProperties = clientUnsubscribeOptions.UserProperties
         };
 
-        if (clientUnsubscribeOptions.TopicFilters != null)
+        packet.TopicFilters!.AddRange(clientUnsubscribeOptions.TopicFilters);
-        {
-            packet.TopicFilters.AddRange(clientUnsubscribeOptions.TopicFilters);
-        }
 
         return packet;
     }

Source/MQTTnet/Formatter/V3/MqttV3PacketFormatter.cs

@@ -29,7 +29,7 @@ public sealed class MqttV3PacketFormatter : IMqttPacketFormatter
         _mqttProtocolVersion = mqttProtocolVersion;
     }
 
-    public MqttPacket Decode(ReceivedMqttPacket receivedPacket)
+    public MqttPacket? Decode(ReceivedMqttPacket receivedPacket)
     {
         if (receivedPacket.TotalLength == 0)
         {
@@ -126,7 +126,7 @@ public sealed class MqttV3PacketFormatter : IMqttPacketFormatter
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var packet = new MqttConnAckPacket();
 
@@ -140,7 +140,7 @@ public sealed class MqttV3PacketFormatter : IMqttPacketFormatter
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var packet = new MqttConnAckPacket();
 
@@ -156,7 +156,7 @@ public sealed class MqttV3PacketFormatter : IMqttPacketFormatter
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var protocolName = _bufferReader.ReadString();
         var protocolVersion = _bufferReader.ReadByte();
@@ -224,7 +224,7 @@ public sealed class MqttV3PacketFormatter : IMqttPacketFormatter
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         return new MqttPubAckPacket
         {
@@ -236,7 +236,7 @@ public sealed class MqttV3PacketFormatter : IMqttPacketFormatter
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         return new MqttPubCompPacket
         {
@@ -248,7 +248,7 @@ public sealed class MqttV3PacketFormatter : IMqttPacketFormatter
     {
         ThrowIfBodyIsEmpty(receivedMqttPacket.Body);
 
-        _bufferReader.SetBuffer(receivedMqttPacket.Body.Array, receivedMqttPacket.Body.Offset, receivedMqttPacket.Body.Count);
+        _bufferReader.SetBuffer(receivedMqttPacket.Body.Array!, receivedMqttPacket.Body.Offset, receivedMqttPacket.Body.Count);
 
         var retain = (receivedMqttPacket.FixedHeader & 0x1) > 0;
         var qualityOfServiceLevel = (MqttQualityOfServiceLevel)((receivedMqttPacket.FixedHeader >> 1) & 0x3);
@@ -283,7 +283,7 @@ public sealed class MqttV3PacketFormatter : IMqttPacketFormatter
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         return new MqttPubRecPacket
         {
@@ -295,7 +295,7 @@ public sealed class MqttV3PacketFormatter : IMqttPacketFormatter
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         return new MqttPubRelPacket
         {
@@ -307,7 +307,7 @@ public sealed class MqttV3PacketFormatter : IMqttPacketFormatter
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var packet = new MqttSubAckPacket
         {
@@ -327,7 +327,7 @@ public sealed class MqttV3PacketFormatter : IMqttPacketFormatter
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var packet = new MqttSubscribePacket
         {
@@ -352,7 +352,7 @@ public sealed class MqttV3PacketFormatter : IMqttPacketFormatter
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         return new MqttUnsubAckPacket
         {
@@ -364,7 +364,7 @@ public sealed class MqttV3PacketFormatter : IMqttPacketFormatter
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var packet = new MqttUnsubscribePacket
         {
@@ -373,7 +373,7 @@ public sealed class MqttV3PacketFormatter : IMqttPacketFormatter
 
         while (!_bufferReader.EndOfStream)
         {
-            packet.TopicFilters.Add(_bufferReader.ReadString());
+            packet.TopicFilters!.Add(_bufferReader.ReadString());
         }
 
         return packet;
@@ -694,9 +694,9 @@ public sealed class MqttV3PacketFormatter : IMqttPacketFormatter
 
         bufferWriter.WriteTwoByteInteger(packet.PacketIdentifier);
 
-        if (packet.ReasonCodes.Count != 0)
+        if (packet.ReasonCodes?.Count > 0)
         {
-            foreach (var packetSubscribeReturnCode in packet.ReasonCodes)
+            foreach (var packetSubscribeReturnCode in packet.ReasonCodes!)
             {
                 if (packetSubscribeReturnCode == MqttSubscribeReasonCode.GrantedQoS0)
                 {
@@ -759,7 +759,7 @@ public sealed class MqttV3PacketFormatter : IMqttPacketFormatter
 
     static byte EncodeUnsubscribePacket(MqttUnsubscribePacket packet, MqttBufferWriter bufferWriter)
     {
-        if (packet.TopicFilters.Count == 0)
+        if (packet.TopicFilters?.Count == 0)
         {
             throw new MqttProtocolViolationException("At least one topic filter must be set [MQTT-3.10.3-2].");
         }

Source/MQTTnet/Formatter/V5/MqttV5PacketDecoder.cs

@@ -15,7 +15,7 @@ public sealed class MqttV5PacketDecoder
 {
     readonly MqttBufferReader _bufferReader = new();
 
-    public MqttPacket Decode(ReceivedMqttPacket receivedMqttPacket)
+    public MqttPacket? Decode(ReceivedMqttPacket receivedMqttPacket)
     {
         if (receivedMqttPacket.TotalLength == 0)
         {
@@ -51,7 +51,7 @@ public sealed class MqttV5PacketDecoder
 
     MqttAuthPacket DecodeAuthPacket(ArraySegment<byte> body)
     {
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var packet = new MqttAuthPacket();
 
@@ -95,7 +95,7 @@ public sealed class MqttV5PacketDecoder
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var acknowledgeFlags = _bufferReader.ReadByte();
 
@@ -198,7 +198,7 @@ public sealed class MqttV5PacketDecoder
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var packet = new MqttConnectPacket
         {
@@ -346,7 +346,7 @@ public sealed class MqttV5PacketDecoder
             };
         }
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var packet = new MqttDisconnectPacket
         {
@@ -383,7 +383,7 @@ public sealed class MqttV5PacketDecoder
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var packet = new MqttPubAckPacket
         {
@@ -420,7 +420,7 @@ public sealed class MqttV5PacketDecoder
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var packet = new MqttPubCompPacket
         {
@@ -458,7 +458,7 @@ public sealed class MqttV5PacketDecoder
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var retain = (header & 1) > 0;
         var qos = (MqttQualityOfServiceLevel)((header >> 1) & 3);
@@ -533,7 +533,7 @@ public sealed class MqttV5PacketDecoder
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var packet = new MqttPubRecPacket
         {
@@ -570,7 +570,7 @@ public sealed class MqttV5PacketDecoder
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var packet = new MqttPubRelPacket
         {
@@ -607,7 +607,7 @@ public sealed class MqttV5PacketDecoder
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var packet = new MqttSubAckPacket
         {
@@ -643,7 +643,7 @@ public sealed class MqttV5PacketDecoder
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var packet = new MqttSubscribePacket
         {
@@ -693,7 +693,7 @@ public sealed class MqttV5PacketDecoder
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var packet = new MqttUnsubAckPacket
         {
@@ -730,7 +730,7 @@ public sealed class MqttV5PacketDecoder
     {
         ThrowIfBodyIsEmpty(body);
 
-        _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
+        _bufferReader.SetBuffer(body.Array!, body.Offset, body.Count);
 
         var packet = new MqttUnsubscribePacket
         {
@@ -747,7 +747,7 @@ public sealed class MqttV5PacketDecoder
 
         while (!_bufferReader.EndOfStream)
         {
-            packet.TopicFilters.Add(_bufferReader.ReadString());
+            packet.TopicFilters!.Add(_bufferReader.ReadString());
         }
 
         return packet;

Source/MQTTnet/Formatter/V5/MqttV5PacketEncoder.cs

@@ -30,7 +30,9 @@ public sealed class MqttV5PacketEncoder(MqttBufferWriter bufferWriter)
         var fixedHeader = EncodePacket(packet);
         var remainingLength = (uint)_bufferWriter.Length - reservedHeaderSize;
 
-        if (packet is MqttPublishPacket publishPacket)
+        var publishPacket = packet as MqttPublishPacket;
+
+        if (publishPacket != null)
         {
             var payload = publishPacket.Payload;
             remainingLength += (uint)payload.Length;
@@ -405,7 +407,7 @@ public sealed class MqttV5PacketEncoder(MqttBufferWriter bufferWriter)
         _propertiesWriter.WriteTo(_bufferWriter);
         _propertiesWriter.Reset();
 
-        foreach (var reasonCode in packet.ReasonCodes)
+        foreach (var reasonCode in packet.ReasonCodes!)
         {
             _bufferWriter.WriteByte((byte)reasonCode);
         }
@@ -476,7 +478,7 @@ public sealed class MqttV5PacketEncoder(MqttBufferWriter bufferWriter)
         _propertiesWriter.WriteTo(_bufferWriter);
         _propertiesWriter.Reset();
 
-        foreach (var reasonCode in packet.ReasonCodes)
+        foreach (var reasonCode in packet.ReasonCodes!)
         {
             _bufferWriter.WriteByte((byte)reasonCode);
         }
@@ -500,7 +502,7 @@ public sealed class MqttV5PacketEncoder(MqttBufferWriter bufferWriter)
         _propertiesWriter.WriteTo(_bufferWriter);
         _propertiesWriter.Reset();
 
-        foreach (var topicFilter in packet.TopicFilters)
+        foreach (var topicFilter in packet.TopicFilters!)
         {
             _bufferWriter.WriteString(topicFilter);
         }

Source/MQTTnet/Formatter/V5/MqttV5PacketFormatter.cs

@@ -18,7 +18,7 @@ public sealed class MqttV5PacketFormatter : IMqttPacketFormatter
         _encoder = new MqttV5PacketEncoder(bufferWriter);
     }
 
-    public MqttPacket Decode(ReceivedMqttPacket receivedPacket)
+    public MqttPacket? Decode(ReceivedMqttPacket receivedPacket)
     {
         return _decoder.Decode(receivedPacket);
     }

Source/MQTTnet/Formatter/V5/MqttV5PropertiesReader.cs

@@ -35,7 +35,7 @@ public struct MqttV5PropertiesReader
         CurrentPropertyId = MqttPropertyId.None;
     }
 
-    public List<MqttUserProperty> CollectedUserProperties { get; private set; }
+    public List<MqttUserProperty>? CollectedUserProperties { get; private set; }
 
     public MqttPropertyId CurrentPropertyId { get; private set; }
 

Source/MQTTnet/Formatter/V5/MqttV5PropertiesWriter.cs

@@ -9,14 +9,9 @@ using MQTTnet.Protocol;
 
 namespace MQTTnet.Formatter.V5;
 
-public sealed class MqttV5PropertiesWriter
+public sealed class MqttV5PropertiesWriter(MqttBufferWriter bufferWriter)
 {
-    readonly MqttBufferWriter _bufferWriter;
+    readonly MqttBufferWriter _bufferWriter = bufferWriter ?? throw new ArgumentNullException(nameof(bufferWriter));
-
-    public MqttV5PropertiesWriter(MqttBufferWriter bufferWriter)
-    {
-        _bufferWriter = bufferWriter ?? throw new ArgumentNullException(nameof(bufferWriter));
-    }
 
     public int Length => _bufferWriter.Length;
 
@@ -26,27 +21,27 @@ public sealed class MqttV5PropertiesWriter
         _bufferWriter.Cleanup();
     }
 
-    public void WriteAssignedClientIdentifier(string value)
+    public void WriteAssignedClientIdentifier(string? value)
     {
         Write(MqttPropertyId.AssignedClientIdentifier, value);
     }
 
-    public void WriteAuthenticationData(byte[] value)
+    public void WriteAuthenticationData(byte[]? value)
     {
         Write(MqttPropertyId.AuthenticationData, value);
     }
 
-    public void WriteAuthenticationMethod(string value)
+    public void WriteAuthenticationMethod(string? value)
     {
         Write(MqttPropertyId.AuthenticationMethod, value);
     }
 
-    public void WriteContentType(string value)
+    public void WriteContentType(string? value)
     {
         Write(MqttPropertyId.ContentType, value);
     }
 
-    public void WriteCorrelationData(byte[] value)
+    public void WriteCorrelationData(byte[]? value)
     {
         Write(MqttPropertyId.CorrelationData, value);
     }
@@ -96,7 +91,7 @@ public sealed class MqttV5PropertiesWriter
         Write(MqttPropertyId.PayloadFormatIndicator, (byte)value);
     }
 
-    public void WriteReasonString(string value)
+    public void WriteReasonString(string? value)
     {
         Write(MqttPropertyId.ReasonString, value);
     }
@@ -134,12 +129,12 @@ public sealed class MqttV5PropertiesWriter
         Write(MqttPropertyId.RequestResponseInformation, true);
     }
 
-    public void WriteResponseInformation(string value)
+    public void WriteResponseInformation(string? value)
     {
         Write(MqttPropertyId.ResponseInformation, value);
     }
 
-    public void WriteResponseTopic(string value)
+    public void WriteResponseTopic(string? value)
     {
         Write(MqttPropertyId.ResponseTopic, value);
     }
@@ -165,7 +160,7 @@ public sealed class MqttV5PropertiesWriter
         Write(MqttPropertyId.ServerKeepAlive, value);
     }
 
-    public void WriteServerReference(string value)
+    public void WriteServerReference(string? value)
     {
         Write(MqttPropertyId.ServerReference, value);
     }
@@ -197,7 +192,7 @@ public sealed class MqttV5PropertiesWriter
         WriteAsVariableByteInteger(MqttPropertyId.SubscriptionIdentifier, value);
     }
 
-    public void WriteSubscriptionIdentifiers(ICollection<uint> value)
+    public void WriteSubscriptionIdentifiers(ICollection<uint>? value)
     {
         if (value == null)
         {
@@ -251,7 +246,7 @@ public sealed class MqttV5PropertiesWriter
         Write(MqttPropertyId.TopicAliasMaximum, value);
     }
 
-    public void WriteUserProperties(List<MqttUserProperty> userProperties)
+    public void WriteUserProperties(List<MqttUserProperty>? userProperties)
     {
         if (userProperties == null || userProperties.Count == 0)
         {
@@ -306,7 +301,7 @@ public sealed class MqttV5PropertiesWriter
         _bufferWriter.WriteTwoByteInteger(value);
     }
 
-    void Write(MqttPropertyId id, string value)
+    void Write(MqttPropertyId id, string? value)
     {
         if (string.IsNullOrEmpty(value))
         {
@@ -317,7 +312,7 @@ public sealed class MqttV5PropertiesWriter
         _bufferWriter.WriteString(value);
     }
 
-    void Write(MqttPropertyId id, byte[] value)
+    void Write(MqttPropertyId id, byte[]? value)
     {
         if (value == null)
         {

Source/MQTTnet/IMqttClient.cs

@@ -19,7 +19,7 @@ public interface IMqttClient : IDisposable
 
     bool IsConnected { get; }
 
-    MqttClientOptions Options { get; }
+    MqttClientOptions? Options { get; }
 
     Task<MqttClientConnectResult> ConnectAsync(MqttClientOptions options, CancellationToken cancellationToken = default);
 

Source/MQTTnet/Implementations/CrossPlatformSocket.cs

@@ -16,7 +16,7 @@ public sealed class CrossPlatformSocket : IDisposable
 {
     readonly Socket _socket;
 
-    NetworkStream _networkStream;
+    NetworkStream? _networkStream;
 
     public CrossPlatformSocket(AddressFamily addressFamily, ProtocolType protocolType)
     {
@@ -50,13 +50,13 @@ public sealed class CrossPlatformSocket : IDisposable
         set => _socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive, value ? 1 : 0);
     }
 
-    public LingerOption LingerState
+    public LingerOption? LingerState
     {
         get => _socket.LingerState;
-        set => _socket.LingerState = value;
+        set => _socket.LingerState = value!;
     }
 
-    public EndPoint LocalEndPoint => _socket.LocalEndPoint;
+    public EndPoint? LocalEndPoint => _socket.LocalEndPoint;
 
     public bool NoDelay
     {
@@ -70,7 +70,7 @@ public sealed class CrossPlatformSocket : IDisposable
         set => _socket.ReceiveBufferSize = value;
     }
 
-    public EndPoint RemoteEndPoint => _socket.RemoteEndPoint;
+    public EndPoint? RemoteEndPoint => _socket.RemoteEndPoint;
 
     public bool ReuseAddress
     {
@@ -108,7 +108,7 @@ public sealed class CrossPlatformSocket : IDisposable
         set => _socket.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveTime, value);
     }
 
-    public async Task<CrossPlatformSocket> AcceptAsync(CancellationToken cancellationToken)
+    public async Task<CrossPlatformSocket?> AcceptAsync(CancellationToken cancellationToken)
     {
         try
         {

Source/MQTTnet/Implementations/MqttClientAdapterFactory.cs

@@ -12,7 +12,7 @@ namespace MQTTnet.Implementations;
 
 public sealed class MqttClientAdapterFactory : IMqttClientAdapterFactory
 {
-    public IMqttChannelAdapter CreateClientAdapter(MqttClientOptions options, MqttPacketInspector packetInspector, IMqttNetLogger logger)
+    public IMqttChannelAdapter CreateClientAdapter(MqttClientOptions options, MqttPacketInspector? packetInspector, IMqttNetLogger logger)
     {
         ArgumentNullException.ThrowIfNull(options);
 

Source/MQTTnet/Implementations/MqttTcpChannel.cs

@@ -20,10 +20,10 @@ namespace MQTTnet.Implementations;
 
 public sealed class MqttTcpChannel : IMqttChannel
 {
-    readonly MqttClientOptions _clientOptions;
+    readonly MqttClientOptions? _clientOptions;
-    readonly MqttClientTcpOptions _tcpOptions;
+    readonly MqttClientTcpOptions? _tcpOptions;
 
-    Stream _stream;
+    Stream? _stream;
 
     public MqttTcpChannel()
     {
@@ -32,7 +32,7 @@ public sealed class MqttTcpChannel : IMqttChannel
     public MqttTcpChannel(MqttClientOptions clientOptions) : this()
     {
         _clientOptions = clientOptions ?? throw new ArgumentNullException(nameof(clientOptions));
-        _tcpOptions = (MqttClientTcpOptions)clientOptions.ChannelOptions;
+        _tcpOptions = (MqttClientTcpOptions)clientOptions.ChannelOptions!;
 
         IsSecureConnection = clientOptions.ChannelOptions?.TlsOptions?.UseTls == true;
     }
@@ -47,18 +47,18 @@ public sealed class MqttTcpChannel : IMqttChannel
         ClientCertificate = clientCertificate;
     }
 
-    public X509Certificate2 ClientCertificate { get; }
+    public X509Certificate2? ClientCertificate { get; }
 
-    public EndPoint RemoteEndPoint { get; private set; }
+    public EndPoint? RemoteEndPoint { get; private set; }
 
     public bool IsSecureConnection { get; }
 
     public async Task ConnectAsync(CancellationToken cancellationToken)
     {
-        CrossPlatformSocket socket = null;
+        CrossPlatformSocket? socket = null;
         try
         {
-            if (_tcpOptions.AddressFamily == AddressFamily.Unspecified)
+            if (_tcpOptions!.AddressFamily == AddressFamily.Unspecified)
             {
                 socket = new CrossPlatformSocket(_tcpOptions.ProtocolType);
             }
@@ -74,7 +74,7 @@ public sealed class MqttTcpChannel : IMqttChannel
 
             socket.ReceiveBufferSize = _tcpOptions.BufferSize;
             socket.SendBufferSize = _tcpOptions.BufferSize;
-            socket.SendTimeout = (int)_clientOptions.Timeout.TotalMilliseconds;
+            socket.SendTimeout = (int)_clientOptions!.Timeout.TotalMilliseconds;
 
             if (_tcpOptions.ProtocolType == ProtocolType.Tcp)
             {
@@ -87,7 +87,7 @@ public sealed class MqttTcpChannel : IMqttChannel
                 socket.LingerState = _tcpOptions.LingerState;
             }
 
-            if (_tcpOptions.DualMode.HasValue)
+            if (_tcpOptions.DualMode != null)
             {
                 // It is important to avoid setting the flag if no specific value is set by the user
                 // because on IPv4 only networks the setter will always throw an exception. Regardless
@@ -95,7 +95,7 @@ public sealed class MqttTcpChannel : IMqttChannel
                 socket.DualMode = _tcpOptions.DualMode.Value;
             }
 
-            await socket.ConnectAsync(_tcpOptions.RemoteEndpoint, cancellationToken).ConfigureAwait(false);
+            await socket.ConnectAsync(_tcpOptions.RemoteEndpoint!, cancellationToken).ConfigureAwait(false);
 
             cancellationToken.ThrowIfCancellationRequested();
 
@@ -285,13 +285,13 @@ public sealed class MqttTcpChannel : IMqttChannel
         object sender,
         string targetHost,
         X509CertificateCollection localCertificates,
-        X509Certificate remoteCertificate,
+        X509Certificate? remoteCertificate,
         string[] acceptableIssuers)
     {
         var certificateSelectionHandler = _tcpOptions?.TlsOptions?.CertificateSelectionHandler;
         if (certificateSelectionHandler != null)
         {
-            var eventArgs = new MqttClientCertificateSelectionEventArgs(targetHost, localCertificates, remoteCertificate, acceptableIssuers, _tcpOptions);
+            var eventArgs = new MqttClientCertificateSelectionEventArgs(targetHost, localCertificates, remoteCertificate, acceptableIssuers, _tcpOptions!);
             return certificateSelectionHandler(eventArgs);
         }
 
@@ -300,15 +300,15 @@ public sealed class MqttTcpChannel : IMqttChannel
             return localCertificates[0];
         }
 
-        return null;
+        return null!;
     }
 
-    bool InternalUserCertificateValidationCallback(object sender, X509Certificate x509Certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
+    bool InternalUserCertificateValidationCallback(object? sender, X509Certificate? x509Certificate, X509Chain? chain, SslPolicyErrors sslPolicyErrors)
     {
         var certificateValidationHandler = _tcpOptions?.TlsOptions?.CertificateValidationHandler;
         if (certificateValidationHandler != null)
         {
-            var eventArgs = new MqttClientCertificateValidationEventArgs(x509Certificate, chain, sslPolicyErrors, _tcpOptions);
+            var eventArgs = new MqttClientCertificateValidationEventArgs(x509Certificate, chain, sslPolicyErrors, _tcpOptions!);
             return certificateValidationHandler(eventArgs);
         }
 
@@ -320,8 +320,8 @@ public sealed class MqttTcpChannel : IMqttChannel
         return sslPolicyErrors == SslPolicyErrors.None;
     }
 
-    X509CertificateCollection LoadCertificates()
+    X509CertificateCollection? LoadCertificates()
     {
-        return _tcpOptions.TlsOptions.ClientCertificatesProvider?.GetCertificates();
+        return _tcpOptions!.TlsOptions.ClientCertificatesProvider?.GetCertificates();
     }
 }

Source/MQTTnet/Implementations/MqttWebSocketChannel.cs

@@ -16,17 +16,17 @@ namespace MQTTnet.Implementations;
 
 public sealed class MqttWebSocketChannel : IMqttChannel
 {
-    readonly MqttClientWebSocketOptions _options;
+    readonly MqttClientWebSocketOptions? _options;
 
-    AsyncLock _sendLock = new();
+    AsyncLock? _sendLock = new();
-    WebSocket _webSocket;
+    WebSocket? _webSocket;
 
     public MqttWebSocketChannel(MqttClientWebSocketOptions options)
     {
         _options = options ?? throw new ArgumentNullException(nameof(options));
     }
 
-    public MqttWebSocketChannel(WebSocket webSocket, EndPoint remoteEndPoint, bool isSecureConnection, X509Certificate2 clientCertificate)
+    public MqttWebSocketChannel(WebSocket webSocket, EndPoint? remoteEndPoint, bool isSecureConnection, X509Certificate2 clientCertificate)
     {
         _webSocket = webSocket ?? throw new ArgumentNullException(nameof(webSocket));
 
@@ -35,15 +35,15 @@ public sealed class MqttWebSocketChannel : IMqttChannel
         ClientCertificate = clientCertificate;
     }
 
-    public X509Certificate2 ClientCertificate { get; }
+    public X509Certificate2? ClientCertificate { get; }
 
-    public EndPoint RemoteEndPoint { get; }
+    public EndPoint? RemoteEndPoint { get; }
 
     public bool IsSecureConnection { get; private set; }
 
     public async Task ConnectAsync(CancellationToken cancellationToken)
     {
-        var uri = _options.Uri;
+        var uri = _options!.Uri;
         if (!uri.StartsWith("ws://", StringComparison.OrdinalIgnoreCase) && !uri.StartsWith("wss://", StringComparison.OrdinalIgnoreCase))
         {
             if (_options.TlsOptions?.UseTls == true)
@@ -96,7 +96,7 @@ public sealed class MqttWebSocketChannel : IMqttChannel
 
     public async Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
     {
-        var response = await _webSocket.ReceiveAsync(new ArraySegment<byte>(buffer, offset, count), cancellationToken).ConfigureAwait(false);
+        var response = await _webSocket!.ReceiveAsync(new ArraySegment<byte>(buffer, offset, count), cancellationToken).ConfigureAwait(false);
         return response.Count;
     }
 
@@ -111,7 +111,7 @@ public sealed class MqttWebSocketChannel : IMqttChannel
         {
             length -= segment.Length;
             bool endOfPacket = isEndOfPacket && length == 0;
-            await _webSocket.SendAsync(segment, WebSocketMessageType.Binary, endOfPacket, cancellationToken).ConfigureAwait(false);
+            await _webSocket!.SendAsync(segment, WebSocketMessageType.Binary, endOfPacket, cancellationToken).ConfigureAwait(false);
         }
     }
 
@@ -133,9 +133,9 @@ public sealed class MqttWebSocketChannel : IMqttChannel
         }
     }
 
-    WebProxy CreateProxy()
+    WebProxy? CreateProxy()
     {
-        if (string.IsNullOrEmpty(_options.ProxyOptions?.Address))
+        if (string.IsNullOrEmpty(_options!.ProxyOptions?.Address))
         {
             return null;
         }
@@ -165,7 +165,7 @@ public sealed class MqttWebSocketChannel : IMqttChannel
 
     void SetupClientWebSocket(ClientWebSocket clientWebSocket)
     {
-        if (_options.ProxyOptions != null)
+        if (_options!.ProxyOptions != null)
         {
             clientWebSocket.Options.Proxy = CreateProxy();
         }

Source/MQTTnet/Internal/AsyncEventInvocator.cs

@@ -9,10 +9,10 @@ namespace MQTTnet.Internal;
 
 public readonly struct AsyncEventInvocator<TEventArgs>
 {
-    readonly Action<TEventArgs> _handler;
+    readonly Action<TEventArgs>? _handler;
-    readonly Func<TEventArgs, Task> _asyncHandler;
+    readonly Func<TEventArgs, Task>? _asyncHandler;
 
-    public AsyncEventInvocator(Action<TEventArgs> handler, Func<TEventArgs, Task> asyncHandler)
+    public AsyncEventInvocator(Action<TEventArgs>? handler, Func<TEventArgs, Task>? asyncHandler)
     {
         _handler = handler;
         _asyncHandler = asyncHandler;

Source/MQTTnet/Internal/AsyncQueue.cs

@@ -70,7 +70,7 @@ public sealed class AsyncQueue<TItem> : IDisposable
         {
             try
             {
-                Task task = null;
+                Task? task = null;
                 lock (_syncRoot)
                 {
                     if (_isDisposed)

Source/MQTTnet/Internal/AsyncQueueDequeueResult.cs

@@ -6,7 +6,7 @@ namespace MQTTnet.Internal;
 
 public sealed class AsyncQueueDequeueResult<TItem>(bool isSuccess, TItem item)
 {
-    public static readonly AsyncQueueDequeueResult<TItem> NonSuccess = new(false, default);
+    public static readonly AsyncQueueDequeueResult<TItem> NonSuccess = new(false, default!);
 
     public bool IsSuccess { get; } = isSuccess;
 

Source/MQTTnet/Internal/AsyncSignal.cs

@@ -14,7 +14,7 @@ public sealed class AsyncSignal : IDisposable
 
     bool _isDisposed;
     bool _isSignaled;
-    AsyncSignalWaiter _waiter;
+    AsyncSignalWaiter? _waiter;
 
     public void Dispose()
     {

Source/MQTTnet/Internal/BlockingQueue.cs

@@ -15,7 +15,7 @@ public sealed class BlockingQueue<TItem> : IDisposable
     readonly LinkedList<TItem> _items = [];
     readonly object _syncRoot = new();
 
-    ManualResetEventSlim _gate = new(false);
+    ManualResetEventSlim? _gate = new(false);
 
     public int Count
     {

Source/MQTTnet/Internal/MqttPacketBus.cs

@@ -104,7 +104,7 @@ public sealed class MqttPacketBus : IDisposable
         _signal.Dispose();
     }
 
-    public MqttPacketBusItem DropFirstItem(MqttPacketBusPartition partition)
+    public MqttPacketBusItem? DropFirstItem(MqttPacketBusPartition partition)
     {
         lock (_syncRoot)
         {

Source/MQTTnet/Internal/MqttPacketBusItem.cs

@@ -8,18 +8,13 @@ using MQTTnet.Packets;
 
 namespace MQTTnet.Internal;
 
-public sealed class MqttPacketBusItem
+public sealed class MqttPacketBusItem(MqttPacket packet)
 {
     readonly AsyncTaskCompletionSource<MqttPacket> _promise = new();
 
-    public MqttPacketBusItem(MqttPacket packet)
+    public event EventHandler? Completed;
-    {
-        Packet = packet ?? throw new ArgumentNullException(nameof(packet));
-    }
 
-    public event EventHandler Completed;
+    public MqttPacket Packet { get; } = packet ?? throw new ArgumentNullException(nameof(packet));
-
-    public MqttPacket Packet { get; }
 
     public void Cancel()
     {

Source/MQTTnet/Internal/TaskExtensions.cs

@@ -10,19 +10,19 @@ namespace MQTTnet.Internal;
 
 public static class TaskExtensions
 {
-    public static void RunInBackground(this Task task, MqttNetSourceLogger logger = null)
+    public static void RunInBackground(this Task? task, MqttNetSourceLogger? logger = null)
     {
         task?.ContinueWith(
             t =>
             {
                 // Consume the exception first so that we get no exception regarding the not observed exception.
                 var exception = t.Exception;
-                logger?.Error(exception, "Unhandled exception in background task.");
+                logger?.Error(exception!, "Unhandled exception in background task.");
             },
             TaskContinuationOptions.OnlyOnFaulted);
     }
 
-    public static async Task WaitAsync(this Task task, Task sender, MqttNetSourceLogger logger)
+    public static async Task WaitAsync(this Task? task, Task? sender, MqttNetSourceLogger logger)
     {
         ArgumentNullException.ThrowIfNull(logger);
 

Source/MQTTnet/LowLevelClient/LowLevelMqttClient.cs

@@ -21,7 +21,7 @@ public sealed class LowLevelMqttClient : ILowLevelMqttClient
     readonly MqttNetSourceLogger _logger;
     readonly IMqttNetLogger _rootLogger;
 
-    IMqttChannelAdapter _adapter;
+    IMqttChannelAdapter? _adapter;
 
     public LowLevelMqttClient(IMqttClientAdapterFactory clientAdapterFactory, IMqttNetLogger logger)
     {
@@ -48,7 +48,7 @@ public sealed class LowLevelMqttClient : ILowLevelMqttClient
             throw new InvalidOperationException("Low level MQTT client is already connected. Disconnect first before connecting again.");
         }
 
-        MqttPacketInspector packetInspector = null;
+        MqttPacketInspector? packetInspector = null;
         if (_inspectPacketEvent.HasHandlers)
         {
             packetInspector = new MqttPacketInspector(_inspectPacketEvent, _rootLogger);

Source/MQTTnet/MQTTnet.csproj

@@ -46,6 +46,8 @@
         <NuGetAuditLevel>low</NuGetAuditLevel>
         <AnalysisLevel>latest-Recommended</AnalysisLevel>
         <IsAotCompatible>true</IsAotCompatible>
+        <LangVersion>default</LangVersion>
+        <Nullable>enable</Nullable>
     </PropertyGroup>
 
     <PropertyGroup Condition="'$(Configuration)'=='Debug'">

Source/MQTTnet/MqttApplicationMessage.cs

@@ -18,7 +18,7 @@ public sealed class MqttApplicationMessage
     ///     The content type must be a UTF-8 encoded string. The content type value identifies the kind of UTF-8 encoded
     ///     payload.
     /// </summary>
-    public string ContentType { get; set; }
+    public string? ContentType { get; set; }
 
     /// <summary>
     ///     Gets or sets the correlation data.
@@ -26,7 +26,7 @@ public sealed class MqttApplicationMessage
     ///     published message.
     ///     Hint: MQTT 5 feature only.
     /// </summary>
-    public byte[] CorrelationData { get; set; }
+    public byte[]? CorrelationData { get; set; }
 
     /// <summary>
     ///     If the DUP flag is set to 0, it indicates that this is the first occasion that the Client or Server has attempted
@@ -94,7 +94,7 @@ public sealed class MqttApplicationMessage
     ///     the request/response pattern between clients that is common in web applications.
     ///     Hint: MQTT 5 feature only.
     /// </summary>
-    public string ResponseTopic { get; set; }
+    public string? ResponseTopic { get; set; }
 
     /// <summary>
     ///     Gets or sets a value indicating whether the message should be retained or not.
@@ -112,7 +112,7 @@ public sealed class MqttApplicationMessage
     ///     the client when need to forward PUBLISH packets matching this subscription to this client.
     ///     Hint: MQTT 5 feature only.
     /// </summary>
-    public List<uint> SubscriptionIdentifiers { get; set; }
+    public List<uint>? SubscriptionIdentifiers { get; set; }
 
     /// <summary>
     ///     Gets or sets the MQTT topic.
@@ -121,7 +121,7 @@ public sealed class MqttApplicationMessage
     ///     The topic consists of one or more topic levels. Each topic level is separated by a forward slash (topic level
     ///     separator).
     /// </summary>
-    public string Topic { get; set; }
+    public required string Topic { get; set; }
 
     /// <summary>
     ///     Gets or sets the topic alias.
@@ -141,5 +141,5 @@ public sealed class MqttApplicationMessage
     ///     The feature is very similar to the HTTP header concept.
     ///     Hint: MQTT 5 feature only.
     /// </summary>
-    public List<MqttUserProperty> UserProperties { get; set; }
+    public List<MqttUserProperty>? UserProperties { get; set; }
 }

Source/MQTTnet/MqttApplicationMessageBuilder.cs

@@ -18,19 +18,19 @@ namespace MQTTnet;
 
 public sealed class MqttApplicationMessageBuilder
 {
-    string _contentType;
+    string? _contentType;
-    byte[] _correlationData;
+    byte[]? _correlationData;
     uint _messageExpiryInterval;
 
     MqttPayloadFormatIndicator _payloadFormatIndicator;
     ReadOnlySequence<byte> _payload;
     MqttQualityOfServiceLevel _qualityOfServiceLevel = MqttQualityOfServiceLevel.AtMostOnce;
-    string _responseTopic;
+    string? _responseTopic;
     bool _retain;
-    List<uint> _subscriptionIdentifiers;
+    List<uint>? _subscriptionIdentifiers;
-    string _topic;
+    string? _topic;
     ushort _topicAlias;
-    List<MqttUserProperty> _userProperties;
+    List<MqttUserProperty>? _userProperties;
 
     public MqttApplicationMessage Build()
     {
@@ -41,7 +41,7 @@ public sealed class MqttApplicationMessageBuilder
 
         var applicationMessage = new MqttApplicationMessage
         {
-            Topic = _topic,
+            Topic = _topic!,
             Payload = _payload,
             QualityOfServiceLevel = _qualityOfServiceLevel,
             Retain = _retain,
@@ -88,7 +88,7 @@ public sealed class MqttApplicationMessageBuilder
         return this;
     }
 
-    public MqttApplicationMessageBuilder WithPayload(byte[] payload)
+    public MqttApplicationMessageBuilder WithPayload(byte[]? payload)
     {
         _payload = payload == null || payload.Length == 0 ? EmptyBuffer.ReadOnlySequence : new ReadOnlySequence<byte>(payload);
         return this;
@@ -100,7 +100,7 @@ public sealed class MqttApplicationMessageBuilder
         return this;
     }
 
-    public MqttApplicationMessageBuilder WithPayload(IEnumerable<byte> payload)
+    public MqttApplicationMessageBuilder WithPayload(IEnumerable<byte>? payload)
     {
         if (payload == null)
         {
@@ -120,12 +120,12 @@ public sealed class MqttApplicationMessageBuilder
         return WithPayload(payload.ToArray());
     }
 
-    public MqttApplicationMessageBuilder WithPayload(Stream payload)
+    public MqttApplicationMessageBuilder WithPayload(Stream? payload)
     {
         return payload == null ? WithPayload(default(byte[])) : WithPayload(payload, payload.Length - payload.Position);
     }
 
-    public MqttApplicationMessageBuilder WithPayload(Stream payload, long length)
+    public MqttApplicationMessageBuilder WithPayload(Stream? payload, long length)
     {
         if (payload == null || length == 0)
         {
@@ -148,7 +148,7 @@ public sealed class MqttApplicationMessageBuilder
         return WithPayload(payloadBuffer);
     }
 
-    public MqttApplicationMessageBuilder WithPayload(string payload)
+    public MqttApplicationMessageBuilder WithPayload(string? payload)
     {
         if (string.IsNullOrEmpty(payload))
         {

Source/MQTTnet/MqttApplicationMessageExtensions.cs

@@ -9,7 +9,7 @@ namespace MQTTnet;
 
 public static class MqttApplicationMessageExtensions
 {
-    public static string ConvertPayloadToString(this MqttApplicationMessage applicationMessage)
+    public static string? ConvertPayloadToString(this MqttApplicationMessage applicationMessage)
     {
         ArgumentNullException.ThrowIfNull(applicationMessage);
 

Source/MQTTnet/MqttClient.cs

@@ -29,7 +29,7 @@ public sealed class MqttClient : Disposable, IMqttClient
     readonly MqttPacketIdentifierProvider _packetIdentifierProvider = new();
     readonly IMqttNetLogger _rootLogger;
 
-    IMqttChannelAdapter _adapter;
+    IMqttChannelAdapter? _adapter;
 
     bool _cleanDisconnectInitiated;
     volatile int _connectionStatus;
@@ -37,18 +37,18 @@ public sealed class MqttClient : Disposable, IMqttClient
     // The value for this field can be set from two different enums.
     // They contain the same values but the set is reduced in one case.
     int _disconnectReason;
-    string _disconnectReasonString;
+    string? _disconnectReasonString;
-    List<MqttUserProperty> _disconnectUserProperties;
+    List<MqttUserProperty>? _disconnectUserProperties;
 
-    Task _keepAlivePacketsSenderTask;
+    Task? _keepAlivePacketsSenderTask;
     DateTime _lastPacketSentTimestamp;
 
-    CancellationTokenSource _mqttClientAlive;
+    CancellationTokenSource? _mqttClientAlive;
-    MqttPacketDispatcher _packetDispatcher;
+    MqttPacketDispatcher? _packetDispatcher;
-    Task _packetReceiverTask;
+    Task? _packetReceiverTask;
-    AsyncQueue<MqttPublishPacket> _publishPacketReceiverQueue;
+    AsyncQueue<MqttPublishPacket>? _publishPacketReceiverQueue;
-    Task _publishPacketReceiverTask;
+    Task? _publishPacketReceiverTask;
-    MqttDisconnectPacket _unexpectedDisconnectPacket;
+    MqttDisconnectPacket? _unexpectedDisconnectPacket;
 
     public MqttClient(IMqttClientAdapterFactory channelFactory, IMqttNetLogger logger)
     {
@@ -89,7 +89,7 @@ public sealed class MqttClient : Disposable, IMqttClient
 
     public bool IsConnected => (MqttClientConnectionStatus)_connectionStatus == MqttClientConnectionStatus.Connected;
 
-    public MqttClientOptions Options { get; private set; }
+    public MqttClientOptions? Options { get; private set; }
 
     public async Task<MqttClientConnectResult> ConnectAsync(MqttClientOptions options, CancellationToken cancellationToken = default)
     {
@@ -102,7 +102,7 @@ public sealed class MqttClient : Disposable, IMqttClient
             throw new InvalidOperationException("Not allowed to connect while connect/disconnect is pending.");
         }
 
-        MqttClientConnectResult connectResult = null;
+        MqttClientConnectResult? connectResult = null;
 
         try
         {
@@ -202,9 +202,9 @@ public sealed class MqttClient : Disposable, IMqttClient
             _disconnectReason = (int)options.Reason;
             _cleanDisconnectInitiated = true;
 
-            if (Options.ValidateFeatures)
+            if (Options!.ValidateFeatures)
             {
-                MqttClientDisconnectOptionsValidator.ThrowIfNotSupported(options, _adapter.PacketFormatterAdapter.ProtocolVersion);
+                MqttClientDisconnectOptionsValidator.ThrowIfNotSupported(options, _adapter!.PacketFormatterAdapter.ProtocolVersion);
             }
 
             // Sending the DISCONNECT may fail due to connection issues. The resulting exception
@@ -240,7 +240,7 @@ public sealed class MqttClient : Disposable, IMqttClient
         }
         else
         {
-            using var timeout = new CancellationTokenSource(Options.Timeout);
+            using var timeout = new CancellationTokenSource(Options!.Timeout);
             await Request<MqttPingRespPacket>(MqttPingReqPacket.Instance, timeout.Token).ConfigureAwait(false);
         }
     }
@@ -254,9 +254,9 @@ public sealed class MqttClient : Disposable, IMqttClient
         ThrowIfDisposed();
         ThrowIfNotConnected();
 
-        if (Options.ValidateFeatures)
+        if (Options!.ValidateFeatures)
         {
-            MqttApplicationMessageValidator.ThrowIfNotSupported(applicationMessage, _adapter.PacketFormatterAdapter.ProtocolVersion);
+            MqttApplicationMessageValidator.ThrowIfNotSupported(applicationMessage, _adapter!.PacketFormatterAdapter.ProtocolVersion);
         }
 
         var publishPacket = MqttPublishPacketFactory.Create(applicationMessage);
@@ -292,7 +292,7 @@ public sealed class MqttClient : Disposable, IMqttClient
         var authPacket = new MqttAuthPacket
         {
             // This must always be equal to the value from the CONNECT packet. So we use it here to ensure that.
-            AuthenticationMethod = Options.AuthenticationMethod,
+            AuthenticationMethod = Options!.AuthenticationMethod,
             AuthenticationData = data.AuthenticationData,
             ReasonString = data.ReasonString,
             ReasonCode = data.ReasonCode,
@@ -314,9 +314,9 @@ public sealed class MqttClient : Disposable, IMqttClient
         ThrowIfDisposed();
         ThrowIfNotConnected();
 
-        if (Options.ValidateFeatures)
+        if (Options!.ValidateFeatures)
         {
-            MqttClientSubscribeOptionsValidator.ThrowIfNotSupported(options, _adapter.PacketFormatterAdapter.ProtocolVersion);
+            MqttClientSubscribeOptionsValidator.ThrowIfNotSupported(options, _adapter!.PacketFormatterAdapter.ProtocolVersion);
         }
 
         var subscribePacket = MqttSubscribePacketFactory.Create(options);
@@ -348,9 +348,9 @@ public sealed class MqttClient : Disposable, IMqttClient
         ThrowIfDisposed();
         ThrowIfNotConnected();
 
-        if (Options.ValidateFeatures)
+        if (Options!.ValidateFeatures)
         {
-            MqttClientUnsubscribeOptionsValidator.ThrowIfNotSupported(options, _adapter.PacketFormatterAdapter.ProtocolVersion);
+            MqttClientUnsubscribeOptionsValidator.ThrowIfNotSupported(options, _adapter!.PacketFormatterAdapter.ProtocolVersion);
         }
 
         var unsubscribePacket = MqttUnsubscribePacketFactory.Create(options);
@@ -465,8 +465,8 @@ public sealed class MqttClient : Disposable, IMqttClient
 
     Task HandleEnhancedAuthentication(MqttAuthPacket authPacket, CancellationToken cancellationToken)
     {
-        var eventArgs = new MqttEnhancedAuthenticationEventArgs(authPacket, _adapter, cancellationToken);
+        var eventArgs = new MqttEnhancedAuthenticationEventArgs(authPacket, _adapter!, cancellationToken);
-        return Options.EnhancedAuthenticationHandler.HandleEnhancedAuthenticationAsync(eventArgs);
+        return Options!.EnhancedAuthenticationHandler!.HandleEnhancedAuthenticationAsync(eventArgs);
     }
 
     void Cleanup()
@@ -498,10 +498,10 @@ public sealed class MqttClient : Disposable, IMqttClient
 
     async Task<MqttClientConnectResult> ConnectInternal(IMqttChannelAdapter channelAdapter, CancellationToken cancellationToken)
     {
-        var backgroundCancellationToken = _mqttClientAlive.Token;
+        var backgroundCancellationToken = _mqttClientAlive!.Token;
 
         using var effectiveCancellationToken = CancellationTokenSource.CreateLinkedTokenSource(backgroundCancellationToken, cancellationToken);
-        _logger.Verbose("Trying to connect with server '{0}'", Options.ChannelOptions);
+        _logger.Verbose("Trying to connect with server '{0}'", Options!.ChannelOptions);
         await channelAdapter.ConnectAsync(effectiveCancellationToken.Token).ConfigureAwait(false);
         _logger.Verbose("Connection with server established");
 
@@ -520,7 +520,7 @@ public sealed class MqttClient : Disposable, IMqttClient
         return connectResult;
     }
 
-    async Task DisconnectCore(Task sender, Exception exception, MqttClientConnectResult connectResult, bool clientWasConnected)
+    async Task DisconnectCore(Task? sender, Exception? exception, MqttClientConnectResult? connectResult, bool clientWasConnected)
     {
         TryInitiateDisconnect();
 
@@ -528,7 +528,7 @@ public sealed class MqttClient : Disposable, IMqttClient
         {
             if (_adapter != null)
             {
-                _logger.Verbose("Disconnecting [Timeout={0}]", Options.Timeout);
+                _logger.Verbose("Disconnecting [Timeout={0}]", Options!.Timeout);
 
                 using var timeout = new CancellationTokenSource(Options.Timeout);
                 await _adapter.DisconnectAsync(timeout.Token).ConfigureAwait(false);
@@ -543,7 +543,7 @@ public sealed class MqttClient : Disposable, IMqttClient
 
         try
         {
-            _packetDispatcher?.Dispose(new MqttClientDisconnectedException(exception));
+            _packetDispatcher?.Dispose(new MqttClientDisconnectedException(exception!));
 
             var receiverTask = _packetReceiverTask.WaitAsync(sender, _logger);
             var publishPacketReceiverTask = _publishPacketReceiverTask.WaitAsync(sender, _logger);
@@ -577,7 +577,7 @@ public sealed class MqttClient : Disposable, IMqttClient
         }
     }
 
-    Task DisconnectInternal(Task sender, Exception exception, MqttClientConnectResult connectResult)
+    Task DisconnectInternal(Task? sender, Exception? exception, MqttClientConnectResult? connectResult)
     {
         var clientWasConnected = IsConnected;
 
@@ -615,7 +615,7 @@ public sealed class MqttClient : Disposable, IMqttClient
     {
         try
         {
-            _publishPacketReceiverQueue.Enqueue(publishPacket);
+            _publishPacketReceiverQueue!.Enqueue(publishPacket);
         }
         catch (Exception exception)
         {
@@ -626,7 +626,7 @@ public sealed class MqttClient : Disposable, IMqttClient
     async Task<MqttApplicationMessageReceivedEventArgs> HandleReceivedApplicationMessage(MqttPublishPacket publishPacket)
     {
         var applicationMessage = MqttApplicationMessageFactory.Create(publishPacket);
-        var eventArgs = new MqttApplicationMessageReceivedEventArgs(Options.ClientId, applicationMessage, publishPacket, AcknowledgeReceivedPublishPacket);
+        var eventArgs = new MqttApplicationMessageReceivedEventArgs(Options!.ClientId, applicationMessage, publishPacket, AcknowledgeReceivedPublishPacket);
         await _events.ApplicationMessageReceivedEvent.InvokeAsync(eventArgs).ConfigureAwait(false);
 
         return eventArgs;
@@ -645,7 +645,7 @@ public sealed class MqttClient : Disposable, IMqttClient
 
     Task ProcessReceivedAuthPacket(MqttAuthPacket authPacket, CancellationToken cancellationToken)
     {
-        if (Options.EnhancedAuthenticationHandler == null)
+        if (Options!.EnhancedAuthenticationHandler == null)
         {
             // From RFC: If the re-authentication fails, the Client or Server SHOULD send DISCONNECT with an appropriate Reason Code
             // as described in section 4.13, and MUST close the Network Connection [MQTT-4.12.1-2].
@@ -661,7 +661,7 @@ public sealed class MqttClient : Disposable, IMqttClient
             return CompletedTask.Instance;
         }
 
-        var eventArgs = new MqttEnhancedAuthenticationEventArgs(authPacket, _adapter, cancellationToken);
+        var eventArgs = new MqttEnhancedAuthenticationEventArgs(authPacket, _adapter!, cancellationToken);
         return Options.EnhancedAuthenticationHandler.HandleEnhancedAuthenticationAsync(eventArgs);
     }
 
@@ -673,7 +673,7 @@ public sealed class MqttClient : Disposable, IMqttClient
         _unexpectedDisconnectPacket = disconnectPacket;
 
         // Also dispatch disconnect to waiting threads to generate a proper exception.
-        _packetDispatcher.Dispose(new MqttClientUnexpectedDisconnectReceivedException(disconnectPacket));
+        _packetDispatcher!.Dispose(new MqttClientUnexpectedDisconnectReceivedException(disconnectPacket));
 
         return DisconnectInternal(_packetReceiverTask, null, null);
     }
@@ -684,7 +684,7 @@ public sealed class MqttClient : Disposable, IMqttClient
         {
             try
             {
-                var publishPacketDequeueResult = await _publishPacketReceiverQueue.TryDequeueAsync(cancellationToken).ConfigureAwait(false);
+                var publishPacketDequeueResult = await _publishPacketReceiverQueue!.TryDequeueAsync(cancellationToken).ConfigureAwait(false);
                 if (!publishPacketDequeueResult.IsSuccess)
                 {
                     return;
@@ -713,7 +713,7 @@ public sealed class MqttClient : Disposable, IMqttClient
 
     Task ProcessReceivedPubRecPacket(MqttPubRecPacket pubRecPacket, CancellationToken cancellationToken)
     {
-        if (_packetDispatcher.TryDispatch(pubRecPacket))
+        if (_packetDispatcher!.TryDispatch(pubRecPacket))
         {
             return CompletedTask.Instance;
         }
@@ -776,11 +776,11 @@ public sealed class MqttClient : Disposable, IMqttClient
         return MqttClientPublishResultFactory.Create(pubRecPacket, pubCompPacket);
     }
 
-    async Task<MqttPacket> Receive(CancellationToken cancellationToken)
+    async Task<MqttPacket?> Receive(CancellationToken cancellationToken)
     {
-        var packetTask = _adapter.ReceivePacketAsync(cancellationToken);
+        var packetTask = _adapter!.ReceivePacketAsync(cancellationToken);
 
-        MqttPacket packet;
+        MqttPacket? packet;
         if (packetTask.IsCompleted)
         {
             packet = packetTask.Result;
@@ -863,7 +863,7 @@ public sealed class MqttClient : Disposable, IMqttClient
             packetIdentifier = packetWithIdentifier.PacketIdentifier;
         }
 
-        using var packetAwaitable = _packetDispatcher.AddAwaitable<TResponsePacket>(packetIdentifier);
+        using var packetAwaitable = _packetDispatcher?.AddAwaitable<TResponsePacket>(packetIdentifier);
         try
         {
             await Send(requestPacket, cancellationToken).ConfigureAwait(false);
@@ -871,12 +871,12 @@ public sealed class MqttClient : Disposable, IMqttClient
         catch (Exception exception)
         {
             _logger.Warning(exception, "Error when sending {0} request packet", requestPacket.GetType().Name);
-            packetAwaitable.Fail(exception);
+            packetAwaitable?.Fail(exception);
         }
 
         try
         {
-            return await packetAwaitable.WaitOneAsync(cancellationToken).ConfigureAwait(false);
+            return await packetAwaitable!.WaitOneAsync(cancellationToken).ConfigureAwait(false);
         }
         catch (Exception exception)
         {
@@ -895,7 +895,7 @@ public sealed class MqttClient : Disposable, IMqttClient
 
         _lastPacketSentTimestamp = DateTime.UtcNow;
 
-        return _adapter.SendPacketAsync(packet, cancellationToken);
+        return _adapter!.SendPacketAsync(packet, cancellationToken);
     }
 
     void ThrowIfConnected(string message)
@@ -971,13 +971,13 @@ public sealed class MqttClient : Disposable, IMqttClient
                     await ProcessReceivedAuthPacket(authPacket, cancellationToken).ConfigureAwait(false);
                     break;
                 case MqttPingRespPacket _:
-                    _packetDispatcher.TryDispatch(packet);
+                    _packetDispatcher!.TryDispatch(packet);
                     break;
                 case MqttPingReqPacket _:
                     throw new MqttProtocolViolationException("The PINGREQ Packet is sent from a client to the server only.");
                 default:
                 {
-                    if (!_packetDispatcher.TryDispatch(packet))
+                    if (!_packetDispatcher!.TryDispatch(packet))
                     {
                         throw new MqttProtocolViolationException($"Received packet '{packet}' at an unexpected time.");
                     }
@@ -1018,7 +1018,7 @@ public sealed class MqttClient : Disposable, IMqttClient
         {
             _logger.Verbose("Start sending keep alive packets");
 
-            var keepAlivePeriod = Options.KeepAlivePeriod;
+            var keepAlivePeriod = Options!.KeepAlivePeriod;
 
             while (!cancellationToken.IsCancellationRequested)
             {

Source/MQTTnet/MqttClientExtensions.cs

@@ -18,9 +18,9 @@ public static class MqttClientExtensions
     public static Task DisconnectAsync(
         this IMqttClient client,
         MqttClientDisconnectOptionsReason reason = MqttClientDisconnectOptionsReason.NormalDisconnection,
-        string reasonString = null,
+        string? reasonString = null,
         uint sessionExpiryInterval = 0,
-        List<MqttUserProperty> userProperties = null,
+        List<MqttUserProperty>? userProperties = null,
         CancellationToken cancellationToken = default)
     {
         ArgumentNullException.ThrowIfNull(client);
@@ -39,7 +39,7 @@ public static class MqttClientExtensions
     public static Task<MqttClientPublishResult> PublishBinaryAsync(
         this IMqttClient mqttClient,
         string topic,
-        IEnumerable<byte> payload = null,
+        IEnumerable<byte>? payload = null,
         MqttQualityOfServiceLevel qualityOfServiceLevel = MqttQualityOfServiceLevel.AtMostOnce,
         bool retain = false,
         CancellationToken cancellationToken = default)
@@ -79,7 +79,7 @@ public static class MqttClientExtensions
     public static Task<MqttClientPublishResult> PublishStringAsync(
         this IMqttClient mqttClient,
         string topic,
-        string payload = null,
+        string? payload = null,
         MqttQualityOfServiceLevel qualityOfServiceLevel = MqttQualityOfServiceLevel.AtMostOnce,
         bool retain = false,
         CancellationToken cancellationToken = default)
@@ -133,7 +133,7 @@ public static class MqttClientExtensions
     public static async Task<bool> TryDisconnectAsync(
         this IMqttClient client,
         MqttClientDisconnectOptionsReason reason = MqttClientDisconnectOptionsReason.NormalDisconnection,
-        string reasonString = null)
+        string? reasonString = null)
     {
         ArgumentNullException.ThrowIfNull(client);
 

Source/MQTTnet/MqttTopicFilterBuilder.cs

@@ -25,7 +25,7 @@ public sealed class MqttTopicFilterBuilder
     /// In MQTT, the word topic refers to an UTF-8 string that the broker uses to filter messages for each connected client.
     /// The topic consists of one or more topic levels. Each topic level is separated by a forward slash (topic level separator).
     /// </summary>
-    string _topic;
+    string? _topic;
     bool _noLocal;
     bool _retainAsPublished;
     MqttRetainHandling _retainHandling = MqttRetainHandling.SendAtSubscribe;

Source/MQTTnet/Options/DefaultMqttCertificatesProvider.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.Collections.Generic;
 using System.Security.Cryptography.X509Certificates;
 
@@ -9,18 +10,17 @@ namespace MQTTnet;
 
 public sealed class DefaultMqttCertificatesProvider : IMqttClientCertificatesProvider
 {
-    readonly X509Certificate2Collection _certificates;
+    readonly X509Certificate2Collection _certificates = new();
 
     public DefaultMqttCertificatesProvider(X509Certificate2Collection certificates)
     {
-        _certificates = certificates;
+        _certificates = certificates ?? throw new ArgumentNullException(nameof(certificates));
     }
 
-    public DefaultMqttCertificatesProvider(IEnumerable<X509Certificate> certificates)
+    public DefaultMqttCertificatesProvider(IEnumerable<X509Certificate>? certificates)
     {
         if (certificates != null)
         {
-            _certificates = [];
             foreach (var certificate in certificates)
             {
                 _certificates.Add(certificate);

Source/MQTTnet/Options/IMqttClientChannelOptions.cs

@@ -6,5 +6,5 @@ namespace MQTTnet;
 
 public interface IMqttClientChannelOptions
 {
-    MqttClientTlsOptions TlsOptions { get; }
+    MqttClientTlsOptions? TlsOptions { get; }
 }

Source/MQTTnet/Options/IMqttClientCredentialsProvider.cs

@@ -6,6 +6,6 @@ namespace MQTTnet;
 
 public interface IMqttClientCredentialsProvider
 {
-    byte[] GetPassword(MqttClientOptions clientOptions);
+    byte[]? GetPassword(MqttClientOptions clientOptions);
     string GetUserName(MqttClientOptions clientOptions);
 }

Source/MQTTnet/Options/MqttClientCertificateSelectionEventArgs.cs

@@ -11,23 +11,26 @@ public sealed class MqttClientCertificateSelectionEventArgs : EventArgs
 {
     public MqttClientCertificateSelectionEventArgs(
         string targetHost,
-        X509CertificateCollection localCertificates,
+        X509CertificateCollection? localCertificates,
-        X509Certificate remoteCertificate,
+        X509Certificate? remoteCertificate,
         string[] acceptableIssuers,
         MqttClientTcpOptions tcpOptions)
     {
         TargetHost = targetHost;
         LocalCertificates = localCertificates;
-        RemoveCertificate = remoteCertificate;
+        RemoteCertificate = remoteCertificate;
         AcceptableIssuers = acceptableIssuers;
         TcpOptions = tcpOptions ?? throw new ArgumentNullException(nameof(tcpOptions));
     }
 
     public string[] AcceptableIssuers { get; }
 
-    public X509CertificateCollection LocalCertificates { get; }
+    public X509CertificateCollection? LocalCertificates { get; }
 
-    public X509Certificate RemoveCertificate { get; }
+    [Obsolete("Typo! Use RemoteCertificate instead!")]
+    public X509Certificate? RemoveCertificate => RemoteCertificate;
+
+    public X509Certificate? RemoteCertificate { get; }
 
     public string TargetHost { get; }
 

Source/MQTTnet/Options/MqttClientCertificateValidationEventArgs.cs

@@ -10,7 +10,7 @@ namespace MQTTnet;
 
 public sealed class MqttClientCertificateValidationEventArgs : EventArgs
 {
-    public MqttClientCertificateValidationEventArgs(X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors, IMqttClientChannelOptions clientOptions)
+    public MqttClientCertificateValidationEventArgs(X509Certificate? certificate, X509Chain? chain, SslPolicyErrors sslPolicyErrors, IMqttClientChannelOptions clientOptions)
     {
         Certificate = certificate;
         Chain = chain;
@@ -18,9 +18,9 @@ public sealed class MqttClientCertificateValidationEventArgs : EventArgs
         ClientOptions = clientOptions ?? throw new ArgumentNullException(nameof(clientOptions));
     }
 
-    public X509Certificate Certificate { get; }
+    public X509Certificate? Certificate { get; }
 
-    public X509Chain Chain { get; }
+    public X509Chain? Chain { get; }
 
     public IMqttClientChannelOptions ClientOptions { get; }
 

Source/MQTTnet/Options/MqttClientCredentials.cs

@@ -6,16 +6,16 @@ namespace MQTTnet;
 
 public sealed class MqttClientCredentials : IMqttClientCredentialsProvider
 {
-    readonly byte[] _password;
+    readonly byte[]? _password;
     readonly string _userName;
 
-    public MqttClientCredentials(string userName, byte[] password = null)
+    public MqttClientCredentials(string userName, byte[]? password = null)
     {
         _userName = userName;
         _password = password;
     }
 
-    public byte[] GetPassword(MqttClientOptions clientOptions)
+    public byte[]? GetPassword(MqttClientOptions clientOptions)
     {
         return _password;
     }

Source/MQTTnet/Options/MqttClientDefaultCertificateValidationHandler.cs

@@ -17,7 +17,7 @@ public sealed class MqttClientDefaultCertificateValidationHandler
             return true;
         }
 
-        if (eventArgs.Chain.ChainStatus.Any(
+        if (eventArgs.Chain!.ChainStatus.Any(
                 c => c.Status is X509ChainStatusFlags.RevocationStatusUnknown or X509ChainStatusFlags.Revoked or X509ChainStatusFlags.OfflineRevocation))
         {
             if (eventArgs.ClientOptions?.TlsOptions?.IgnoreCertificateRevocationErrors != true)

Source/MQTTnet/Options/MqttClientOptions.cs

@@ -24,15 +24,15 @@ public sealed class MqttClientOptions
     ///     Gets or sets the authentication data.
     ///     <remarks>MQTT 5.0.0+ feature.</remarks>
     /// </summary>
-    public byte[] AuthenticationData { get; set; }
+    public byte[]? AuthenticationData { get; set; }
 
     /// <summary>
     ///     Gets or sets the authentication method.
     ///     <remarks>MQTT 5.0.0+ feature.</remarks>
     /// </summary>
-    public string AuthenticationMethod { get; set; }
+    public string? AuthenticationMethod { get; set; }
 
-    public IMqttClientChannelOptions ChannelOptions { get; set; }
+    public IMqttClientChannelOptions? ChannelOptions { get; set; }
 
     /// <summary>
     ///     Gets or sets a value indicating whether clean sessions are used or not.
@@ -52,14 +52,14 @@ public sealed class MqttClientOptions
     /// </summary>
     public string ClientId { get; set; } = Guid.NewGuid().ToString("N");
 
-    public IMqttClientCredentialsProvider Credentials { get; set; }
+    public IMqttClientCredentialsProvider? Credentials { get; set; }
 
     /// <summary>
     ///     Gets or sets the handler for AUTH packets.
     ///     This can happen when connecting or at any time while being already connected.
     ///     <remarks>MQTT 5.0.0+ feature.</remarks>
     /// </summary>
-    public IMqttEnhancedAuthenticationHandler EnhancedAuthenticationHandler { get; set; }
+    public IMqttEnhancedAuthenticationHandler? EnhancedAuthenticationHandler { get; set; }
 
     /// <summary>
     ///     Gets or sets the keep alive period.
@@ -143,7 +143,7 @@ public sealed class MqttClientOptions
     ///     The feature is very similar to the HTTP header concept.
     ///     <remarks>MQTT 5.0.0+ feature.</remarks>
     /// </summary>
-    public List<MqttUserProperty> UserProperties { get; set; }
+    public List<MqttUserProperty>? UserProperties { get; set; }
 
     /// <summary>
     ///     When this feature is enabled the client will check if used properties are supported in the selected protocol
@@ -158,13 +158,13 @@ public sealed class MqttClientOptions
     ///     Gets or sets the content type of the will message.
     ///     <remarks>MQTT 5.0.0+ feature.</remarks>
     /// </summary>
-    public string WillContentType { get; set; }
+    public string? WillContentType { get; set; }
 
     /// <summary>
     ///     Gets or sets the correlation data of the will message.
     ///     <remarks>MQTT 5.0.0+ feature.</remarks>
     /// </summary>
-    public byte[] WillCorrelationData { get; set; }
+    public byte[]? WillCorrelationData { get; set; }
 
     /// <summary>
     ///     Gets or sets the will delay interval.
@@ -182,7 +182,7 @@ public sealed class MqttClientOptions
     /// <summary>
     ///     Gets or sets the payload of the will message.
     /// </summary>
-    public byte[] WillPayload { get; set; }
+    public byte[]? WillPayload { get; set; }
 
     /// <summary>
     ///     Gets or sets the payload format indicator of the will message.
@@ -199,7 +199,7 @@ public sealed class MqttClientOptions
     ///     Gets or sets the response topic of the will message.
     ///     <remarks>MQTT 5.0.0+ feature.</remarks>
     /// </summary>
-    public string WillResponseTopic { get; set; }
+    public string? WillResponseTopic { get; set; }
 
     /// <summary>
     ///     Gets or sets the retain flag of the will message.
@@ -209,13 +209,13 @@ public sealed class MqttClientOptions
     /// <summary>
     ///     Gets or sets the topic of the will message.
     /// </summary>
-    public string WillTopic { get; set; }
+    public string? WillTopic { get; set; }
 
     /// <summary>
     ///     Gets or sets the user properties of the will message.
     ///     <remarks>MQTT 5.0.0+ feature.</remarks>
     /// </summary>
-    public List<MqttUserProperty> WillUserProperties { get; set; }
+    public List<MqttUserProperty>? WillUserProperties { get; set; }
 
     /// <summary>
     ///     Gets or sets the default and initial size of the packet write buffer.

Source/MQTTnet/Options/MqttClientOptionsBuilder.cs

@@ -18,10 +18,10 @@ public sealed class MqttClientOptionsBuilder
     readonly MqttClientOptions _options = new();
 
     int? _port;
-    EndPoint _remoteEndPoint;
+    EndPoint? _remoteEndPoint;
-    MqttClientTcpOptions _tcpOptions;
+    MqttClientTcpOptions? _tcpOptions;
-    MqttClientTlsOptions _tlsOptions;
+    MqttClientTlsOptions? _tlsOptions;
-    MqttClientWebSocketOptions _webSocketOptions;
+    MqttClientWebSocketOptions? _webSocketOptions;
 
     public MqttClientOptions Build()
     {
@@ -38,7 +38,7 @@ public sealed class MqttClientOptionsBuilder
 
         if (_tcpOptions != null)
         {
-            _tcpOptions.TlsOptions = tlsOptions;
+            _tcpOptions.TlsOptions = tlsOptions!;
 
             if (_remoteEndPoint == null)
             {
@@ -49,7 +49,7 @@ public sealed class MqttClientOptionsBuilder
             {
                 if (dns.Port == 0)
                 {
-                    if (_port.HasValue)
+                    if (_port != null)
                     {
                         _remoteEndPoint = new DnsEndPoint(dns.Host, _port.Value, dns.AddressFamily);
                     }
@@ -64,7 +64,7 @@ public sealed class MqttClientOptionsBuilder
             {
                 if (ip.Port == 0)
                 {
-                    if (_port.HasValue)
+                    if (_port != null)
                     {
                         _remoteEndPoint = new IPEndPoint(ip.Address, _port.Value);
                     }
@@ -82,10 +82,10 @@ public sealed class MqttClientOptionsBuilder
         }
         else if (_webSocketOptions != null)
         {
-            _webSocketOptions.TlsOptions = tlsOptions;
+            _webSocketOptions.TlsOptions = tlsOptions!;
         }
 
-        _options.ChannelOptions = (IMqttClientChannelOptions)_tcpOptions ?? _webSocketOptions;
+        _options.ChannelOptions = (IMqttClientChannelOptions?)_tcpOptions ?? _webSocketOptions;
 
         MqttClientOptionsValidator.ThrowIfNotSupported(_options);
 
@@ -94,7 +94,7 @@ public sealed class MqttClientOptionsBuilder
 
     public MqttClientOptionsBuilder WithAddressFamily(AddressFamily addressFamily)
     {
-        _tcpOptions.AddressFamily = addressFamily;
+        _tcpOptions!.AddressFamily = addressFamily;
         return this;
     }
 
@@ -174,9 +174,9 @@ public sealed class MqttClientOptionsBuilder
         return WithConnectionUri(new Uri(uri, UriKind.Absolute));
     }
 
-    public MqttClientOptionsBuilder WithCredentials(string username, string password)
+    public MqttClientOptionsBuilder WithCredentials(string username, string? password)
     {
-        byte[] passwordBuffer = null;
+        byte[]? passwordBuffer = null;
 
         if (password != null)
         {
@@ -186,7 +186,7 @@ public sealed class MqttClientOptionsBuilder
         return WithCredentials(username, passwordBuffer);
     }
 
-    public MqttClientOptionsBuilder WithCredentials(string username, byte[] password = null)
+    public MqttClientOptionsBuilder WithCredentials(string username, byte[]? password = null)
     {
         return WithCredentials(new MqttClientCredentials(username, password));
     }
@@ -205,7 +205,7 @@ public sealed class MqttClientOptionsBuilder
         return this;
     }
 
-    public MqttClientOptionsBuilder WithEnhancedAuthentication(string method, byte[] data = null)
+    public MqttClientOptionsBuilder WithEnhancedAuthentication(string method, byte[]? data = null)
     {
         _options.AuthenticationMethod = method;
         _options.AuthenticationData = data;
@@ -249,7 +249,7 @@ public sealed class MqttClientOptionsBuilder
 
     public MqttClientOptionsBuilder WithProtocolType(ProtocolType protocolType)
     {
-        _tcpOptions.ProtocolType = protocolType;
+        _tcpOptions!.ProtocolType = protocolType;
         return this;
     }
 
@@ -405,7 +405,7 @@ public sealed class MqttClientOptionsBuilder
         return this;
     }
 
-    public MqttClientOptionsBuilder WithWillPayload(byte[] willPayload)
+    public MqttClientOptionsBuilder WithWillPayload(byte[]? willPayload)
     {
         _options.WillPayload = willPayload;
         return this;
@@ -423,11 +423,11 @@ public sealed class MqttClientOptionsBuilder
         return this;
     }
 
-    public MqttClientOptionsBuilder WithWillPayload(string willPayload)
+    public MqttClientOptionsBuilder WithWillPayload(string? willPayload)
     {
         if (string.IsNullOrEmpty(willPayload))
         {
-            return WithWillPayload((byte[])null);
+            return WithWillPayload((byte[]?)null);
         }
 
         _options.WillPayload = Encoding.UTF8.GetBytes(willPayload);

Source/MQTTnet/Options/MqttClientTcpOptions.cs

@@ -26,7 +26,7 @@ public sealed class MqttClientTcpOptions : IMqttClientChannelOptions
     ///     Gets the local endpoint (network card) which is used by the client.
     ///     Set it to _null_ to let the OS select the network card.
     /// </summary>
-    public EndPoint LocalEndpoint { get; set; }
+    public EndPoint? LocalEndpoint { get; set; }
 
     /// <summary>
     ///     Enables or disables the Nagle algorithm for the socket.
@@ -42,7 +42,7 @@ public sealed class MqttClientTcpOptions : IMqttClientChannelOptions
     /// </summary>
     public ProtocolType ProtocolType { get; set; } = ProtocolType.Tcp;
 
-    public EndPoint RemoteEndpoint { get; set; }
+    public EndPoint? RemoteEndpoint { get; set; }
 
     public MqttClientTlsOptions TlsOptions { get; set; } = new();
 

Source/MQTTnet/Options/MqttClientTlsOptions.cs

@@ -12,9 +12,9 @@ namespace MQTTnet;
 
 public sealed class MqttClientTlsOptions
 {
-    public Func<MqttClientCertificateValidationEventArgs, bool> CertificateValidationHandler { get; set; }
+    public Func<MqttClientCertificateValidationEventArgs, bool>? CertificateValidationHandler { get; set; }
 
-    public Func<MqttClientCertificateSelectionEventArgs, X509Certificate> CertificateSelectionHandler { get; set; }
+    public Func<MqttClientCertificateSelectionEventArgs, X509Certificate>? CertificateSelectionHandler { get; set; }
 
     public bool UseTls { get; set; }
 
@@ -32,11 +32,11 @@ public sealed class MqttClientTlsOptions
     ///     with the server and requires certificates for authentication.
     ///     The implementation may return different certificates each time.
     /// </summary>
-    public IMqttClientCertificatesProvider ClientCertificatesProvider { get; set; }
+    public IMqttClientCertificatesProvider? ClientCertificatesProvider { get; set; }
 
-    public List<SslApplicationProtocol> ApplicationProtocols { get; set; }
+    public List<SslApplicationProtocol>? ApplicationProtocols { get; set; }
 
-    public CipherSuitesPolicy CipherSuitesPolicy { get; set; }
+    public CipherSuitesPolicy? CipherSuitesPolicy { get; set; }
 
     public EncryptionPolicy EncryptionPolicy { get; set; } = EncryptionPolicy.RequireEncryption;
 
@@ -46,9 +46,9 @@ public sealed class MqttClientTlsOptions
     ///     Gets or sets the target host.
     ///     If the value is null or empty the same host as the TCP socket host will be used.
     /// </summary>
-    public string TargetHost { get; set; }
+    public string? TargetHost { get; set; }
 
     public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls12 | SslProtocols.Tls13;
 
-    public X509Certificate2Collection TrustChain { get; set; }
+    public X509Certificate2Collection? TrustChain { get; set; }
 }

Source/MQTTnet/Options/MqttClientWebSocketOptions.cs

@@ -11,11 +11,11 @@ namespace MQTTnet;
 
 public sealed class MqttClientWebSocketOptions : IMqttClientChannelOptions
 {
-    public CookieContainer CookieContainer { get; set; }
+    public CookieContainer? CookieContainer { get; set; }
 
-    public WebSocketDeflateOptions DangerousDeflateOptions { get; set; }
+    public WebSocketDeflateOptions? DangerousDeflateOptions { get; set; }
 
-    public ICredentials Credentials { get; set; }
+    public ICredentials? Credentials { get; set; }
 
     /// <summary>
     ///     Gets or sets the keep alive interval for the Web Socket connection.
@@ -23,15 +23,15 @@ public sealed class MqttClientWebSocketOptions : IMqttClientChannelOptions
     /// </summary>
     public TimeSpan KeepAliveInterval { get; set; } = WebSocket.DefaultKeepAliveInterval;
 
-    public MqttClientWebSocketProxyOptions ProxyOptions { get; set; }
+    public MqttClientWebSocketProxyOptions? ProxyOptions { get; set; }
 
-    public IDictionary<string, string> RequestHeaders { get; set; }
+    public IDictionary<string, string>? RequestHeaders { get; set; }
 
-    public ICollection<string> SubProtocols { get; set; } = ["mqtt"];
+    public ICollection<string>? SubProtocols { get; set; } = ["mqtt"];
 
     public MqttClientTlsOptions TlsOptions { get; set; } = new();
 
-    public string Uri { get; set; }
+    public required string Uri { get; set; }
 
     /// <summary>
     ///     Gets or sets whether the default (system) credentials should be used when connecting via Web Socket connection.
@@ -41,6 +41,6 @@ public sealed class MqttClientWebSocketOptions : IMqttClientChannelOptions
 
     public override string ToString()
     {
-        return Uri;
+        return Uri ?? string.Empty;
     }
 }

Source/MQTTnet/Options/MqttClientWebSocketOptionsBuilder.cs

@@ -11,10 +11,19 @@ namespace MQTTnet;
 
 public sealed class MqttClientWebSocketOptionsBuilder
 {
-    readonly MqttClientWebSocketOptions _webSocketOptions = new();
+    readonly MqttClientWebSocketOptions _webSocketOptions = new()
+    {
+        Uri = string.Empty
+    };
 
     public MqttClientWebSocketOptions Build()
     {
+
+        if (string.IsNullOrEmpty(_webSocketOptions.Uri))
+        {
+            throw new InvalidOperationException("The URI is not set.");
+        }
+
         return _webSocketOptions;
     }
 

Source/MQTTnet/Options/MqttClientWebSocketProxyOptions.cs

@@ -6,17 +6,17 @@ namespace MQTTnet;
 
 public sealed class MqttClientWebSocketProxyOptions
 {
-    public string Address { get; set; }
+    public string? Address { get; set; }
 
-    public string[] BypassList { get; set; }
+    public string[]? BypassList { get; set; }
 
     public bool BypassOnLocal { get; set; }
 
-    public string Domain { get; set; }
+    public string? Domain { get; set; }
 
-    public string Password { get; set; }
+    public string? Password { get; set; }
 
     public bool UseDefaultCredentials { get; set; }
 
-    public string Username { get; set; }
+    public string? Username { get; set; }
 }

Source/MQTTnet/Options/MqttEnhancedAuthenticationEventArgs.cs

@@ -30,13 +30,13 @@ public class MqttEnhancedAuthenticationEventArgs : EventArgs
     ///     Gets the authentication data.
     ///     Hint: MQTT 5 feature only.
     /// </summary>
-    public byte[] AuthenticationData => _initialAuthPacket.AuthenticationData;
+    public byte[]? AuthenticationData => _initialAuthPacket.AuthenticationData;
 
     /// <summary>
     ///     Gets the authentication method.
     ///     Hint: MQTT 5 feature only.
     /// </summary>
-    public string AuthenticationMethod => _initialAuthPacket.AuthenticationMethod;
+    public string AuthenticationMethod => _initialAuthPacket.AuthenticationMethod!;
 
     public CancellationToken CancellationToken { get; }
 
@@ -50,7 +50,7 @@ public class MqttEnhancedAuthenticationEventArgs : EventArgs
     ///     Gets the reason string.
     ///     Hint: MQTT 5 feature only.
     /// </summary>
-    public string ReasonString => _initialAuthPacket.ReasonString;
+    public string? ReasonString => _initialAuthPacket.ReasonString;
 
     /// <summary>
     ///     Gets the user properties.
@@ -61,7 +61,7 @@ public class MqttEnhancedAuthenticationEventArgs : EventArgs
     ///     The feature is very similar to the HTTP header concept.
     ///     Hint: MQTT 5 feature only.
     /// </summary>
-    public List<MqttUserProperty> UserProperties => _initialAuthPacket.UserProperties;
+    public List<MqttUserProperty>? UserProperties => _initialAuthPacket.UserProperties;
 
     public async Task<ReceiveMqttEnhancedAuthenticationDataResult> ReceiveAsync(CancellationToken cancellationToken = default)
     {

Source/MQTTnet/Options/MqttEnhancedAuthenticationExchangeData.cs

@@ -18,7 +18,7 @@ public class MqttEnhancedAuthenticationExchangeData
     ///     method.
     ///     Hint: MQTT 5 feature only.
     /// </summary>
-    public byte[] AuthenticationData { get; set; }
+    public byte[]? AuthenticationData { get; set; }
 
     /// <summary>
     ///     Gets or sets the reason code.
@@ -30,7 +30,7 @@ public class MqttEnhancedAuthenticationExchangeData
     ///     Gets or sets the reason string.
     ///     Hint: MQTT 5 feature only.
     /// </summary>
-    public string ReasonString { get; set; }
+    public string? ReasonString { get; set; }
 
     /// <summary>
     ///     Gets or sets the user properties.
@@ -41,5 +41,5 @@ public class MqttEnhancedAuthenticationExchangeData
     ///     The feature is very similar to the HTTP header concept.
     ///     Hint: MQTT 5 feature only.
     /// </summary>
-    public List<MqttUserProperty> UserProperties { get; set; }
+    public List<MqttUserProperty>? UserProperties { get; set; }
 }

Source/MQTTnet/Options/ReceiveMqttEnhancedAuthenticationDataResult.cs

@@ -10,13 +10,13 @@ namespace MQTTnet;
 
 public sealed class ReceiveMqttEnhancedAuthenticationDataResult
 {
-    public byte[] AuthenticationData { get; init; }
+    public byte[]? AuthenticationData { get; init; }
 
-    public string AuthenticationMethod { get; init; }
+    public string? AuthenticationMethod { get; init; }
 
     public MqttAuthenticateReasonCode ReasonCode { get; init; }
 
-    public string ReasonString { get; init; }
+    public string? ReasonString { get; init; }
 
-    public List<MqttUserProperty> UserProperties { get; init; }
+    public List<MqttUserProperty>? UserProperties { get; init; }
 }

Source/MQTTnet/Options/SendMqttEnhancedAuthenticationDataOptions.cs

@@ -9,9 +9,9 @@ namespace MQTTnet;
 
 public sealed class SendMqttEnhancedAuthenticationDataOptions
 {
-    public byte[] Data { get; init; }
+    public byte[]? Data { get; init; }
 
-    public string ReasonString { get; init; }
+    public string? ReasonString { get; init; }
 
-    public List<MqttUserProperty> UserProperties { get; init; }
+    public List<MqttUserProperty>? UserProperties { get; init; }
 }

Source/MQTTnet/PacketDispatcher/MqttPacketAwaitableFilter.cs

@@ -8,7 +8,7 @@ namespace MQTTnet.PacketDispatcher;
 
 public sealed class MqttPacketAwaitableFilter
 {
-    public Type Type { get; set; }
+    public required Type Type { get; init; }
 
-    public ushort Identifier { get; set; }
+    public ushort Identifier { get; init; }
 }

Source/MQTTnet/Packets/MqttAuthPacket.cs

@@ -10,13 +10,13 @@ namespace MQTTnet.Packets;
 /// <summary>Added in MQTTv5.0.0.</summary>
 public sealed class MqttAuthPacket : MqttPacket
 {
-    public byte[] AuthenticationData { get; set; }
+    public byte[]? AuthenticationData { get; set; }
 
-    public string AuthenticationMethod { get; set; }
+    public string? AuthenticationMethod { get; set; }
 
     public MqttAuthenticateReasonCode ReasonCode { get; set; }
 
-    public string ReasonString { get; set; }
+    public string? ReasonString { get; set; }
 
-    public List<MqttUserProperty> UserProperties { get; set; }
+    public List<MqttUserProperty>? UserProperties { get; set; }
 }

Source/MQTTnet/Packets/MqttConnAckPacket.cs

@@ -12,11 +12,11 @@ public sealed class MqttConnAckPacket : MqttPacket
     /// <summary>
     ///     Added in MQTTv5.
     /// </summary>
-    public string AssignedClientIdentifier { get; set; }
+    public string? AssignedClientIdentifier { get; set; }
 
-    public byte[] AuthenticationData { get; set; }
+    public byte[]? AuthenticationData { get; set; }
 
-    public string AuthenticationMethod { get; set; }
+    public string? AuthenticationMethod { get; set; }
 
     /// <summary>
     ///     Added in MQTTv3.1.1.
@@ -32,11 +32,11 @@ public sealed class MqttConnAckPacket : MqttPacket
     /// </summary>
     public MqttConnectReasonCode ReasonCode { get; set; }
 
-    public string ReasonString { get; set; }
+    public string? ReasonString { get; set; }
 
     public ushort ReceiveMaximum { get; set; }
 
-    public string ResponseInformation { get; set; }
+    public string? ResponseInformation { get; set; }
 
     public bool RetainAvailable { get; set; }
 
@@ -44,7 +44,7 @@ public sealed class MqttConnAckPacket : MqttPacket
 
     public ushort ServerKeepAlive { get; set; }
 
-    public string ServerReference { get; set; }
+    public string? ServerReference { get; set; }
 
     public uint SessionExpiryInterval { get; set; }
 
@@ -54,7 +54,7 @@ public sealed class MqttConnAckPacket : MqttPacket
 
     public ushort TopicAliasMaximum { get; set; }
 
-    public List<MqttUserProperty> UserProperties { get; set; }
+    public List<MqttUserProperty>? UserProperties { get; set; }
 
     public bool WildcardSubscriptionAvailable { get; set; }
 

Source/MQTTnet/Packets/MqttConnectPacket.cs

@@ -9,24 +9,24 @@ namespace MQTTnet.Packets;
 
 public sealed class MqttConnectPacket : MqttPacket
 {
-    public byte[] AuthenticationData { get; set; }
+    public byte[]? AuthenticationData { get; set; }
 
-    public string AuthenticationMethod { get; set; }
+    public string? AuthenticationMethod { get; set; }
 
     /// <summary>
     ///     Also called "Clean Start" in MQTTv5.
     /// </summary>
     public bool CleanSession { get; set; }
 
-    public string ClientId { get; set; }
+    public string? ClientId { get; set; }
 
-    public byte[] WillCorrelationData { get; set; }
+    public byte[]? WillCorrelationData { get; set; }
 
     public ushort KeepAlivePeriod { get; set; }
 
     public uint MaximumPacketSize { get; set; }
 
-    public byte[] Password { get; set; }
+    public byte[]? Password { get; set; }
 
     public ushort ReceiveMaximum { get; set; }
 
@@ -34,23 +34,23 @@ public sealed class MqttConnectPacket : MqttPacket
 
     public bool RequestResponseInformation { get; set; }
 
-    public string WillResponseTopic { get; set; }
+    public string? WillResponseTopic { get; set; }
 
     public uint SessionExpiryInterval { get; set; }
 
     public ushort TopicAliasMaximum { get; set; }
 
-    public string Username { get; set; }
+    public string? Username { get; set; }
 
-    public List<MqttUserProperty> UserProperties { get; set; }
+    public List<MqttUserProperty>? UserProperties { get; set; }
 
-    public string WillContentType { get; set; }
+    public string? WillContentType { get; set; }
 
     public uint WillDelayInterval { get; set; }
 
     public bool WillFlag { get; set; }
 
-    public byte[] WillMessage { get; set; }
+    public byte[]? WillMessage { get; set; }
 
     public uint WillMessageExpiryInterval { get; set; }
 
@@ -60,9 +60,9 @@ public sealed class MqttConnectPacket : MqttPacket
 
     public bool WillRetain { get; set; }
 
-    public string WillTopic { get; set; }
+    public string? WillTopic { get; set; }
 
-    public List<MqttUserProperty> WillUserProperties { get; set; }
+    public List<MqttUserProperty>? WillUserProperties { get; set; }
 
     public bool TryPrivate { get; set; }
 

Source/MQTTnet/Packets/MqttDisconnectPacket.cs

@@ -17,12 +17,12 @@ public sealed class MqttDisconnectPacket : MqttPacket
     /// <summary>
     ///     Added in MQTTv5.
     /// </summary>
-    public string ReasonString { get; set; }
+    public string? ReasonString { get; set; }
 
     /// <summary>
     ///     Added in MQTTv5.
     /// </summary>
-    public string ServerReference { get; set; }
+    public string? ServerReference { get; set; }
 
     /// <summary>
     ///     Added in MQTTv5.
@@ -32,7 +32,7 @@ public sealed class MqttDisconnectPacket : MqttPacket
     /// <summary>
     ///     Added in MQTTv5.
     /// </summary>
-    public List<MqttUserProperty> UserProperties { get; set; }
+    public List<MqttUserProperty>? UserProperties { get; set; }
 
     public override string ToString()
     {

Source/MQTTnet/Packets/MqttPubAckPacket.cs

@@ -17,12 +17,12 @@ public sealed class MqttPubAckPacket : MqttPacketWithIdentifier
     /// <summary>
     ///     Added in MQTTv5.
     /// </summary>
-    public string ReasonString { get; set; }
+    public string? ReasonString { get; set; }
 
     /// <summary>
     ///     Added in MQTTv5.
     /// </summary>
-    public List<MqttUserProperty> UserProperties { get; set; }
+    public List<MqttUserProperty>? UserProperties { get; set; }
 
     public override string ToString()
     {

Source/MQTTnet/Packets/MqttPubCompPacket.cs

@@ -17,12 +17,12 @@ public sealed class MqttPubCompPacket : MqttPacketWithIdentifier
     /// <summary>
     ///     Added in MQTTv5.
     /// </summary>
-    public string ReasonString { get; set; }
+    public string? ReasonString { get; set; }
 
     /// <summary>
     ///     Added in MQTTv5.
     /// </summary>
-    public List<MqttUserProperty> UserProperties { get; set; }
+    public List<MqttUserProperty>? UserProperties { get; set; }
 
     public override string ToString()
     {

Source/MQTTnet/Packets/MqttPubRecPacket.cs

@@ -17,12 +17,12 @@ public sealed class MqttPubRecPacket : MqttPacketWithIdentifier
     /// <summary>
     ///     Added in MQTTv5.
     /// </summary>
-    public string ReasonString { get; set; }
+    public string? ReasonString { get; set; }
 
     /// <summary>
     ///     Added in MQTTv5.
     /// </summary>
-    public List<MqttUserProperty> UserProperties { get; set; }
+    public List<MqttUserProperty>? UserProperties { get; set; }
 
     public override string ToString()
     {

Source/MQTTnet/Packets/MqttPubRelPacket.cs

@@ -17,12 +17,12 @@ public sealed class MqttPubRelPacket : MqttPacketWithIdentifier
     /// <summary>
     ///     Added in MQTTv5.
     /// </summary>
-    public string ReasonString { get; set; }
+    public string? ReasonString { get; set; }
 
     /// <summary>
     ///     Added in MQTTv5.
     /// </summary>
-    public List<MqttUserProperty> UserProperties { get; set; }
+    public List<MqttUserProperty>? UserProperties { get; set; }
 
     public override string ToString()
     {