Enable #nullable for kafka transport.

src/DotNetCore.CAP.Kafka/CAP.KafkaOptions.cs

@@ -37,11 +37,11 @@ namespace DotNetCore.CAP
         /// Initial list of brokers as a CSV list of broker host or host:port.
         /// </para>
         /// </summary>
-        public string Servers { get; set; }
+        public string Servers { get; set; } = default!;
 
         /// <summary>
         /// If you need to get offset and partition and so on.., you can use this function to write additional header into <see cref="CapHeader"/>
         /// </summary>
-        public Func<ConsumeResult<string, byte[]>, List<KeyValuePair<string, string>>> CustomHeaders { get; set; }
+        public Func<ConsumeResult<string, byte[]>, List<KeyValuePair<string, string>>>? CustomHeaders { get; set; }
     }
 }

src/DotNetCore.CAP.Kafka/DotNetCore.CAP.Kafka.csproj

@@ -2,14 +2,13 @@
 
   <PropertyGroup>
     <TargetFramework>netstandard2.1</TargetFramework>
-    <AssemblyName>DotNetCore.CAP.Kafka</AssemblyName>
+	<Nullable>enable</Nullable>
     <PackageTags>$(PackageTags);Kafka</PackageTags>
   </PropertyGroup>
 
   <PropertyGroup>
     <WarningsAsErrors>NU1605;NU1701</WarningsAsErrors>
     <NoWarn>NU1701;CS1591</NoWarn>
-    <DocumentationFile>bin\$(Configuration)\netstandard2.1\DotNetCore.CAP.Kafka.xml</DocumentationFile>
   </PropertyGroup>
 
   <ItemGroup>

src/DotNetCore.CAP.Kafka/ITransport.Kafka.cs

@@ -43,8 +43,8 @@ namespace DotNetCore.CAP.Kafka
                 var result = await producer.ProduceAsync(message.GetName(), new Message<string, byte[]>
                 {
                     Headers = headers,
-                    Key = message.Headers.TryGetValue(KafkaHeaders.KafkaKey, out string kafkaMessageKey) && !string.IsNullOrEmpty(kafkaMessageKey) ? kafkaMessageKey : message.GetId(),
-                    Value = message.Body
+                    Key = message.Headers.TryGetValue(KafkaHeaders.KafkaKey, out string? kafkaMessageKey) && !string.IsNullOrEmpty(kafkaMessageKey) ? kafkaMessageKey : message.GetId(),
+                    Value = message.Body!
                 });
 
                 if (result.Status == PersistenceStatus.Persisted || result.Status == PersistenceStatus.PossiblyPersisted)

src/DotNetCore.CAP.Kafka/KafkaConsumerClient.cs

@@ -21,7 +21,7 @@ namespace DotNetCore.CAP.Kafka
 
         private readonly string _groupId;
         private readonly KafkaOptions _kafkaOptions;
-        private IConsumer<string, byte[]> _consumerClient;
+        private IConsumer<string, byte[]>? _consumerClient;
 
         public KafkaConsumerClient(string groupId, IOptions<KafkaOptions> options)
         {
@@ -29,11 +29,11 @@ namespace DotNetCore.CAP.Kafka
             _kafkaOptions = options.Value ?? throw new ArgumentNullException(nameof(options));
         }
 
-        public event EventHandler<TransportMessage> OnMessageReceived;
+        public event EventHandler<TransportMessage>? OnMessageReceived;
 
-        public event EventHandler<LogMessageEventArgs> OnLog;
+        public event EventHandler<LogMessageEventArgs>? OnLog;
 
-        public BrokerAddress BrokerAddress => new BrokerAddress("Kafka", _kafkaOptions.Servers);
+        public BrokerAddress BrokerAddress => new ("Kafka", _kafkaOptions.Servers);
 
         public ICollection<string> FetchTopics(IEnumerable<string> topicNames)
         {
@@ -80,7 +80,7 @@ namespace DotNetCore.CAP.Kafka
 
             Connect();
 
-            _consumerClient.Subscribe(topics);
+            _consumerClient!.Subscribe(topics);
         }
 
         public void Listening(TimeSpan timeout, CancellationToken cancellationToken)
@@ -89,11 +89,11 @@ namespace DotNetCore.CAP.Kafka
 
             while (true)
             {
-                var consumerResult = _consumerClient.Consume(cancellationToken);
+                var consumerResult = _consumerClient!.Consume(cancellationToken);
 
                 if (consumerResult.IsPartitionEOF || consumerResult.Message.Value == null) continue;
 
-                var headers = new Dictionary<string, string>(consumerResult.Message.Headers.Count);
+                var headers = new Dictionary<string, string?>(consumerResult.Message.Headers.Count);
                 foreach (var header in consumerResult.Message.Headers)
                 {
                     var val = header.GetValueBytes();
@@ -119,12 +119,12 @@ namespace DotNetCore.CAP.Kafka
 
         public void Commit(object sender)
         {
-            _consumerClient.Commit((ConsumeResult<string, byte[]>)sender);
+            _consumerClient!.Commit((ConsumeResult<string, byte[]>)sender);
         }
 
-        public void Reject(object sender)
+        public void Reject(object? sender)
         {
-            _consumerClient.Assign(_consumerClient.Assignment);
+            _consumerClient!.Assign(_consumerClient.Assignment);
         }
 
         public void Dispose()