Question

I was looking through some old code today and found an event handler that looked like this:

public void HandleEvent(EventClassA eventObj)
{
    if(eventObj is EventSubClassA)
    {
        HandleEventSubClassA(eventObj as EventSubClassA);
    }
    else if(eventObj is EventSubClassB)
    {
        HandleEventSubClassB(eventObj as EventSubClassB);
    }
    else if(eventObj.GetType() == typeof(EventSubClassC))
    {
        HandleEventSubClassC(eventObj as EventSubClassC);
    }
    else if(eventObj is EventSubClassD)
    {
        HandleEventSubClassD(eventObj as EventSubClassD);
    }
}

I thought this was kind of ugly. So I refactored it like this:

delegate void EventHandler(dynamic eventObj);
private static readonly Dictionary<Type, EventHandler> EVENT_MAP = new Dictionary<Type, EventHandler>()
    {
        { typeof(EventSubClassA), HandleEventSubClassA },
        { typeof(EventSubClassB), HandleEventSubClassB },
        { typeof(EventSubClassC), HandleEventSubClassC },
        { typeof(EventSubClassD), HandleEventSubClassD }
    };

public void HandleEvent(EventClassA eventObj)
{
    EVENT_MAP[eventObj.GetType()](eventObj);
}

private void HandleEventSubClassA(dynamic evt)
{
    var eventObj = evt as EventSubClassA;
}

I had a coworker review the code and there were concerns about the way this solution worked compared to the previous solution. I have a hard time believing that the previous solution is the best solution for this case, so I’ve turned to StackOverflow.

Is there a better way to build this type of class?
Is there a pattern I’m not aware of that is designed for this?

Answer 1

You can use generics to make your existing solution slightly safer:

private static Dictionary<Type, Delegate> handlers;

static HandlerClass()
{
    handlers = new Dictionary<Type, Delegate>();
    AddHandler<EventSubClassA>(HandleEventSubClassA);
    AddHandler<EventSubClassB>(HandleEventSubClassB);
    ...
}

public static void AddHandler<T>(Action<T> handler) where T : EventClassA
{
    handlers[typeof(T)] = handler;
}

public void HandleEvent(EventClassA @event)
{
    Delegate handler;
    if(handlers.TryGetValue(@event.GetType(), out handler))
    {
        handler.DynamicInvoke(@event);
    }
}

Alternatively, if you can modify the classes in your event hierarchy you could implement the visitor pattern:

public interface IHandlers
{
    void HandleSubClassA(EventSubClassA a);
    void HandleSubClassB(EventSubClassB b);
    ...
}

public abstract class EventClassA
{
    public abstract void Visit(IHandlers handlers);
}

public class EventSubClassA : EventClassA
{
    public override void Visit(IHandlers handlers)
    {
        handlers.HandleSubClassA(this);
    }
}