Too much protocol - I need better specs - feel free to fork & adjust

SimpleMessaging.MessageBox.cs

using System;

namespace SimpleMessaging
{
    public class MessageBox
    {
        Func<dynamic,bool?> Handle;

        public MessageBox()
        {
            Handle = (msg) => null;
        }

        public void Subscribe(Predicate<dynamic> before = null,Predicate<dynamic> after = null)
        {
            var previousHandler = this.Handle;
            this.Handle =  (msg)=> { 
                bool? result= null;
                if (before!=null)
                    result = before(msg);
                if (result==false) 
                    result=previousHandler(msg);
                else if (result!=true && after!=null)
                    result=after(msg);
                return result;
            };
        }

        public bool Deliver(dynamic message)
        {
            bool? handled = Publish(message);
            switch(handled)
            {
                case null:
                    throw new InvalidOperationException(
                        "Missing a handler for the following message:", message.ToJson());
                default:
                    return handled.Value;
            }
        }

        public bool? Publish(dynamic message)
        {
            return Handle(message);
        }
    }
}

SimpleMessaging.Specs.cs

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Shouldly;


namespace SimpleMessaging.Specs
{
    [TestClass]
    public class Delivering_a_message_should_fail : 
        Given_a_messagebox_without_subscribers
    {
        [TestInitialize]
        [ExpectedException(typeof(InvalidOperationException))]
        public void When_delivering_a_message() 
        { 
            Succeeded = Sut.Deliver(Message); 
        }
    }

    [TestClass]
    public class Publishing_a_message_should_succeed :
        Given_a_messagebox_without_subscribers
    {
        [TestInitialize]
        public void When_publishing_a_message()
        {
            Succeeded = Sut.Publish(Message);
        }

        [TestMethod]
        public void The_publish_command_should_have_returned_null()
        {
            Succeeded.ShouldBe(null);
        }
    }

    [TestClass]
    public class Delivering_a_message_succeed_if_there_is_a_receiver:
        Given_a_messagebox_that_has_a_subscriber_for_the_message
    {
        [TestInitialize]
        public void When_delivering_a_message()
        {
            Succeeded = Sut.Deliver(Message);
        }
         
        [TestMethod]
        public void The_correct_message_should_be_received() 
        { 
            ReceivedMessage.ShouldBe(Message); 
        }

        [TestMethod]
        public void The_deliver_command_should_have_returned_success() 
        { 
            Succeeded.ShouldBe(true); 
        }
    }

    [TestClass]
    public class Publishing_a_message_should_succeed_and_the_message_should_match :
        Given_a_messagebox_that_has_a_subscriber_for_the_message
    {
        [TestInitialize]
        public void When_publishing_a_message()
        {
            Succeeded = Sut.Publish(Message);
        }

        [TestMethod]
        public void The_publish_command_should_have_returned_true()
        {
            Succeeded.ShouldBe(true);
        }

        [TestMethod]
        public void The_correct_message_should_be_received()
        {
            ReceivedMessage.ShouldBe(Message);
        }
    }


    public class Given_a_messagebox_without_subscribers
    {
        public bool? Succeeded = null;

        public class SayHello { public string Name = "Tom"; }

        public SayHello Message = new SayHello();

        protected MessageBox Sut = new MessageBox();
    }

    public class Given_a_messagebox_that_has_a_subscriber_for_the_message :
        Given_a_messagebox_without_subscribers
    {
        public SayHello ReceivedMessage = null;

        public Given_a_messagebox_that_has_a_subscriber_for_the_message()
        {
            Sut.Subscribe(x => { ReceivedMessage = x; return true; });
        }
    }
}