Add subscribe options builder.

Source/MQTTnet/Client/Subscribing/MqttClientSubscribeOptions.cs

@@ -9,6 +9,6 @@ namespace MQTTnet.Client.Subscribing
 
         public uint? SubscriptionIdentifier { get; set; }
 
-        public List<MqttUserProperty> UserProperties { get; set; } = new List<MqttUserProperty>();
+        public List<MqttUserProperty> UserProperties { get; set; }
     }
 }

Source/MQTTnet/Client/Subscribing/MqttClientSubscribeOptionsBuilder.cs

@@ -0,0 +1,87 @@
+using System;
+using System.Collections.Generic;
+using MQTTnet.Packets;
+using MQTTnet.Protocol;
+
+namespace MQTTnet.Client.Subscribing
+{
+    public class MqttClientSubscribeOptionsBuilder
+    {
+        private readonly MqttClientSubscribeOptions _subscribeOptions = new MqttClientSubscribeOptions();
+
+        public MqttClientSubscribeOptionsBuilder WithUserProperty(string name, string value)
+        {
+            if (name == null) throw new ArgumentNullException(nameof(name));
+            if (value == null) throw new ArgumentNullException(nameof(value));
+
+            if (_subscribeOptions.UserProperties == null)
+            {
+                _subscribeOptions.UserProperties = new List<MqttUserProperty>();
+            }
+
+            _subscribeOptions.UserProperties.Add(new MqttUserProperty(name, value));
+
+            return this;
+        }
+
+        public MqttClientSubscribeOptionsBuilder WithSubscriptionIdentifier(uint? subscriptionIdentifier)
+        {
+            _subscribeOptions.SubscriptionIdentifier = subscriptionIdentifier;
+
+            return this;
+        }
+
+        public MqttClientSubscribeOptionsBuilder WithTopicFilter(
+            string topic,
+            MqttQualityOfServiceLevel qualityOfServiceLevel = MqttQualityOfServiceLevel.AtMostOnce,
+            bool? noLocal = null,
+            bool? retainAsPublished = null,
+            MqttRetainHandling? retainHandling = null)
+        {
+            return WithTopicFilter(new TopicFilter
+            {
+                Topic = topic,
+                QualityOfServiceLevel = qualityOfServiceLevel,
+                NoLocal = noLocal,
+                RetainAsPublished = retainAsPublished,
+                RetainHandling = retainHandling
+            });
+        }
+
+        public MqttClientSubscribeOptionsBuilder WithTopicFilter(Action<TopicFilterBuilder> topicFilterBuilder)
+        {
+            if (topicFilterBuilder == null) throw new ArgumentNullException(nameof(topicFilterBuilder));
+
+            var internalTopicFilterBuilder = new TopicFilterBuilder();
+            topicFilterBuilder(internalTopicFilterBuilder);
+
+            return WithTopicFilter(internalTopicFilterBuilder);
+        }
+
+        public MqttClientSubscribeOptionsBuilder WithTopicFilter(TopicFilterBuilder topicFilterBuilder)
+        {
+            if (topicFilterBuilder == null) throw new ArgumentNullException(nameof(topicFilterBuilder));
+
+            return WithTopicFilter(topicFilterBuilder.Build());
+        }
+
+        public MqttClientSubscribeOptionsBuilder WithTopicFilter(TopicFilter topicFilter)
+        {
+            if (topicFilter == null) throw new ArgumentNullException(nameof(topicFilter));
+
+            if (_subscribeOptions.TopicFilters == null)
+            {
+                _subscribeOptions.TopicFilters = new List<TopicFilter>();
+            }
+
+            _subscribeOptions.TopicFilters.Add(topicFilter);
+
+            return this;
+        }
+
+        public MqttClientSubscribeOptions Build()
+        {
+            return _subscribeOptions;
+        }
+    }
+}