Design pattern for handling multiple message types

You could create separate message handlers for each message type, and naively pass the message to each available handler until you find one that can handle it. Similar to the chain of responsibility pattern:

public interface IMessageHandler {
    bool HandleMessage( IMessage msg );
}

public class OrderMessageHandler : IMessageHandler {
    bool HandleMessage( IMessage msg ) {
       if ( !(msg is OrderMessage)) return false;

       // Handle the message and return true to indicate it was handled
       return true; 
    }
}

public class SomeOtherMessageHandler : IMessageHandler {
    bool HandleMessage( IMessage msg ) {
       if ( !(msg is SomeOtherMessage) ) return false;

       // Handle the message and return true to indicate it was handled
       return true;
    }
}

... etc ...

public class MessageProcessor {
    private List<IMessageHandler> handlers;

    public MessageProcessor() {
       handlers = new List<IMessageHandler>();
       handlers.add(new SomeOtherMessageHandler());
       handlers.add(new OrderMessageHandler());
    }

    public void ProcessMessage( IMessage msg ) {
       bool messageWasHandled
       foreach( IMessageHandler handler in handlers ) {
           if ( handler.HandleMessage(msg) ) {
               messageWasHandled = true;
               break;
           }
       }

       if ( !messageWasHandled ) {
          // Do some default processing, throw error, whatever.
       }
    }
}

You could also implement this as a map, with the message class name or message type id as a key and the appropriate handler instance as the value.

Others have suggested having the message object “handle” itself, but that just doesn’t feel right to me. Seems like it would be best to separate the handling of the message from the message itself.

Some other things I like about it:

1. You can inject the message handlers via spring or what-have-you rather than creating them in the constructor, making this very testable.

2. You can introduce topic-like behavior where you have multiple handlers for a single message by simply removing the “break” from the ProcessMessage loop.

3. By separating the message from the handler, you can have different handlers for the same message at different destinations (e.g. multiple MessageProcessor classes that handle the same messages differently)