Fix stream and socket handling.

Frameworks/MQTTnet.NetStandard/ManagedClient/IManagedMqttClient.cs

@@ -7,6 +7,7 @@ namespace MQTTnet.ManagedClient
 {
     public interface IManagedMqttClient : IApplicationMessageReceiver, IApplicationMessagePublisher, IDisposable
     {
+        bool IsStarted { get; }
         bool IsConnected { get; }
 
         event EventHandler<MqttClientConnectedEventArgs> Connected;

Frameworks/MQTTnet.NetStandard/ManagedClient/ManagedMqttClient.cs

@@ -40,6 +40,7 @@ namespace MQTTnet.ManagedClient
         }
 
         public bool IsConnected => _mqttClient.IsConnected;
+        public bool IsStarted => _connectionCancellationToken != null;
 
         public event EventHandler<MqttClientConnectedEventArgs> Connected;
         public event EventHandler<MqttClientDisconnectedEventArgs> Disconnected;

Frameworks/MQTTnet.NetStandard/ManagedClient/ManagedMqttClientExtensions.cs

@@ -21,6 +21,14 @@ namespace MQTTnet.ManagedClient
             return managedClient.SubscribeAsync(new TopicFilterBuilder().WithTopic(topic).WithQualityOfServiceLevel(qualityOfServiceLevel).Build());
         }
 
+        public static Task SubscribeAsync(this IManagedMqttClient managedClient, string topic)
+        {
+            if (managedClient == null) throw new ArgumentNullException(nameof(managedClient));
+            if (topic == null) throw new ArgumentNullException(nameof(topic));
+
+            return managedClient.SubscribeAsync(new TopicFilterBuilder().WithTopic(topic).Build());
+        }
+
         public static Task UnsubscribeAsync(this IManagedMqttClient managedClient, params string[] topicFilters)
         {
             if (managedClient == null) throw new ArgumentNullException(nameof(managedClient));

Frameworks/MQTTnet.NetStandard/ManagedClient/ManagedMqttClientOptionsBuilder.cs

@@ -47,7 +47,7 @@ namespace MQTTnet.ManagedClient
         {
             if (options == null) throw new ArgumentNullException(nameof(options));
 
-            if (_clientOptionsBuilder != null)
+            if (_clientOptionsBuilder == null)
             {
                 _clientOptionsBuilder = new MqttClientOptionsBuilder();
             }