Double Dispatch Pattern

gistfile1.txt

namespace Messaging
{
    interface IHandler
    {

    }

    interface IMessageHandler<in MessageType> : IHandler
    {
        void ProcessMessage(MessageType message);
    }

    interface IMessage
    {
        void Dispatch(IHandler handler);
    }

    class MessageBase<MessageType> : IMessage where MessageType : class, IMessage
    {
        public void Dispatch(IHandler handler)
        {
            MessageType welcomeMessage = this as MessageType;
            if (welcomeMessage != null)
            {
                DynamicDispatch(handler, welcomeMessage);
            }
        }

        protected void DynamicDispatch(IHandler handler, MessageType self)
        {
            IMessageHandler<MessageType> handlerTarget = handler as IMessageHandler<MessageType>;
            if (handlerTarget != null)
            {
                handlerTarget.ProcessMessage(self);
            }
        }
    }

    class WelcomeMessage : MessageBase<WelcomeMessage>
    {
        public string WelcomeText { get; set; }
    }

    class WelcomeMessageHandler : IMessageHandler<WelcomeMessage>
    {
        public void ProcessMessage(WelcomeMessage message)
        {
            Console.WriteLine(message.WelcomeText);
        }
    }
}