ScopedEventHandler

One idea I’ve had lately for Spackle.NET is to add a class called ScopedEventHandler. The motivation for this class comes from this code:

var provider = new EventProvider();

var handler = new EventHandler((sender, e) => 
  Console.Out.WriteLine("Caught broadcast via manual."));
provider.Broadcast += handler;

try
{
  provider.Speak();
}
finally
{
  provider.Broadcast -= handler;
}

If you don’t do the “-=” thing, you can leak memory. You could use WeakReference, but let’s stick with the traditional approach but have a disposable object do that for us:

var provider = new EventProvider();

var handler = new EventHandler((sender, e) => 
  Console.Out.WriteLine("Caught broadcast via scoped."));

using(var scope = new ScopedEventHandler(provider, "Broadcast", handler))
{
  provider.Speak();            
}

Now, you may be wondering why I’m passing in a string here, rather than the actual event. Let’s take a look at EventProvider:

public class EventProvider
{
  public event EventHandler Broadcast;
    
  public void Speak()
  {
    if(this.Broadcast != null)
    {
      this.Broadcast(this, new EventArgs());
    }
  }
}

What I wanted to do was this:

using(var scope = new ScopedEventHandler(provider.Broadcast, handler))

Where the implementation of the constructor looked like this:

public ScopedEventHandler(EventHandler provider, EventHandler listener)
{
  provider += listener;
}

But that lead to this compilation error:

Error 1 The event 'Playground.EventProvider.Broadcast' can only appear on the left hand side of += or -= (except when used from within the type 'Playground.EventProvider').

This is compiler error CS0070.

So I’m stuck with Reflection:

public ScopedEventHandler(object provider, string eventName, Delegate listener)
{
  var eventInfo = provider.GetType().GetEvent(eventName);
  eventInfo.AddEventHandler(provider, listener);
}

The end result is the client code is clearer and easier in my mind, but the API seems kind of dorky. However, I don’t see another way around this. Any suggestions?

Published: 12.02.2008 11:23:50 AM CST

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Jason Bock

ScopedEventHandler