Question

I’ve got a custom Button class, that always performs the same action when it gets clicked (opening a specific window). I’m adding a Click event that can be assigned in the button’s XAML, like a regular button.

When it gets clicked, I want to execute the Click event handler if one has been assigned, otherwise I want to execute the default action. The problem is that there’s apparently no way to check if any handlers have been added to an event.

I thought a null check on the event would do it:

if (Click == null) 
{ 
    DefaultClickAction(); 
} 
else 
{ 
    RaiseEvent(new RoutedEventArgs(ClickEvent, this));;
}

…but that doesn’t compile. The compiler tells me that I can’t do anything other than += or -= to an event outside of the defining class, event though I’m trying to do this check INSIDE the defining class.

I’ve implemented the correct behavior myself, but it’s ugly and verbose and I can’t believe there isn’t a built-in way to do this. I must be missing something.

Here’s the relevant code:

public class MyButtonClass : Control
{
    //...

    public static readonly RoutedEvent ClickEvent =
        EventManager.RegisterRoutedEvent("Click",
                                         RoutingStrategy.Bubble,
                                         typeof(RoutedEventHandler),
                                         typeof(MyButtonClass));

    public event RoutedEventHandler Click
    {
        add { ClickHandlerCount++; AddHandler(ClickEvent, value); }
        remove { ClickHandlerCount--; RemoveHandler(ClickEvent, value); }
    }

    private int ClickHandlerCount = 0;

    private Boolean ClickHandlerExists
    {
        get { return ClickHandlerCount > 0; }
    }

    //...
}

Answer 1

No it is not possible. Actually the code you have assumes that they are handling the event on your control itself but you are declaring a Bubble event not a Direct event so technically something can be listening to the event further up the element chain. Also, technically one doesn’t need to use the CLR event to hook the event; one can just use the AddHandler method directly passing in your routed event and that is what someone would have to do to hook the event further up the chain. Lastly the CLR event won’t be used if someone registers a class handler for the event (to be notified whenever that event is raised for any instance of your MyButtonClass). If you look at something like the UIElement.BuildRouteHelper you will see all the steps that WPF goes through to establish the event route and the objects that will be invoked when the event is raised.

If you really need to know if there are any listeners then you are better off creating a CLR only event and not a routed event. Then you can check if your delegate is non-null.