reddal reddal - 1 month ago 8
C# Question

Is it possible to write a class that hides the difference between adding WeakEventManager handlers and traditional ones?

I have a C# (.NET 4.6.1) project that uses a lot of events. I'm tempted to move all the event handlers to the new WeakEventManager pattern - to avoid endlessly worrying about deregistering handlers to avoid memory leaks.

However I have to do lots of testing for performance and want a way to switch between the two methods easily. So far I've been using conditional compilation to do this along the lines of :

#if WeakEvents
WeakEventManager<EventSource,EArgs>.AddHandler(source, "TheEvent", handler);
#else
source.TheEvent += handler;
#endif


This works, but its messy. Ideally I'd like to create a class that hides this away. ie create a class which internally can use either method. Then I can change all my source to attach handlers with the new class and switch easily (or even move to some new method in the future).

However I can't see how to write that class - because I can't pass the event as a parameter - and there would have to be some reflection going on with the handler/name which is beyond me.

Is there an easy way to do that?

Evk Evk
Answer

Here is one way to do that:

static class EventHelper {
    public static void Subscribe<TSource, TEventArgs>(TSource source, Expression<Func<TSource, EventHandler<TEventArgs>>> eventRef, EventHandler<TEventArgs> handler) where TEventArgs : EventArgs {
        if (source == null)
            throw new ArgumentNullException(nameof(source));
        var memberExp = eventRef.Body as MemberExpression;
        if (memberExp == null)
            throw new ArgumentException("eventRef should be member access expression");

        var eventName = memberExp.Member.Name;
    #if WeakEvents
        WeakEventManager<TSource, TEventArgs>.AddHandler(source, eventName, handler);            
    #else
        // some reflection here to get to the event
        var ev = source.GetType().GetEvent(eventName);
        if (ev == null)
            throw new ArgumentException($"There is no event with name {eventName} on type {source.GetType()}");
        ev.AddMethod.Invoke(source, new object[] { handler });
    #endif
    }
}

Usage is as simple as:

// second parameter is event reference
EventHelper.Subscribe(source, s => s.TheEvent, Handler);