Microsoft.AspNet.SignalR.Messaging.Topic.AddSubscription C# (CSharp) Method

        public void AddSubscription(ISubscription subscription)
        {
            if (subscription == null)
            {
                throw new ArgumentNullException("subscription");
            }

            try
            {
                SubscriptionLock.EnterWriteLock();

                MarkUsed();

                Subscriptions.Add(subscription);
                
                // Created -> HasSubscriptions
                Interlocked.CompareExchange(ref State,
                                            TopicState.HasSubscriptions,
                                            TopicState.NoSubscriptions);
            }
            finally
            {
                SubscriptionLock.ExitWriteLock();
            }
        }

        public void TopicStateHasSubscriptionsIfMoreThanOne()
        {
            var topic = new Topic(100, TimeSpan.Zero);
            var sub1 = new Mock<ISubscription>();
            sub1.Setup(m => m.Identity).Returns("1");

            topic.AddSubscription(new Mock<ISubscription>().Object);
            topic.AddSubscription(sub1.Object);
            topic.RemoveSubscription(sub1.Object);

            Assert.Equal(TopicState.HasSubscriptions, topic.State);
        }