/// <summary>
        /// Creates a test context manager.
        /// </summary>
        /// <param name="contextTracker">The test context tracker.</param>
        /// <param name="messageSink">The message sink to which test message are published.</param>
        /// <exception cref="ArgumentNullException">Thrown if <paramref name="contextTracker"/>
        /// or <paramref name="messageSink"/> is null.</exception>
        public ObservableTestContextManager(ITestContextTracker contextTracker,
            IMessageSink messageSink)
        {
            if (contextTracker == null)
                throw new ArgumentNullException("contextTracker");
            if (messageSink == null)
                throw new ArgumentNullException("messageSink");

            this.contextTracker = contextTracker;
            this.messageSink = messageSink;
        }
        /// <summary>
        /// Creates a test context manager.
        /// </summary>
        /// <param name="contextTracker">The test context tracker.</param>
        /// <param name="messageSink">The message sink to which test message are published.</param>
        /// <exception cref="ArgumentNullException">Thrown if <paramref name="contextTracker"/>
        /// or <paramref name="messageSink"/> is null.</exception>
        public ObservableTestContextManager(ITestContextTracker contextTracker,
                                            IMessageSink messageSink)
        {
            if (contextTracker == null)
            {
                throw new ArgumentNullException("contextTracker");
            }
            if (messageSink == null)
            {
                throw new ArgumentNullException("messageSink");
            }

            this.contextTracker = contextTracker;
            this.messageSink    = messageSink;
        }
 static TestContextTrackerAccessor()
 {
     RuntimeAccessor.InstanceChanged += delegate { cachedContextTracker = null; };
 }
 static TestContextTrackerAccessor()
 {
     RuntimeAccessor.InstanceChanged += delegate { cachedContextTracker = null; };
 }