Vfrdtyky

I have a ViewModel that it has some events to notify actions to anothers viewModels.

I have few events, but I will ask about one them, that it is this:

public event MyEventEventHandler MyEvent;

private void OnMyEvent(MyType param)

{

    MyEventEvent?.Invoke(param);

}

In a intermediate ViewModel, I relation a second view model that has a method to subscribe to this event. I do in this way:

MainViewModel myMainViewmodel = new MyViewModel();

SecondViewModel mySecondViewModel = new SecondViewModel();

myMainViewmodel.MyEvent += mySecondViewmodel.handleEvent;

In this case, it is to open a dialog, so the main view model is the dialog and the second view model is which calls to the dialog, so when I finish to open the dialog, how the second view model still is subcribed to the main ViewModel, the main ViewModel would not be recollected by the garbage collector.

My doubt is where I should unsubscribe the event, in the disponse of the main ViewModel or the finally block in the intermediate view model?

I was thinking if there would be someway to access to the collection of subcribers in the main ViewModel, so in the dipose I could do something like MyEvent.clear() or something like that to unsubcribe all the events, instead to unsubscribe one by one all of the in the finally block in the intermediate ViewModel.

In general, I would like to know the best way to unsubscribe events when I related two ViewModels.

Thanks.

You shouldn't make a type IDisposable only for one purpose of being able to unsubscribe from events. IDisposable means, a type contains unmanaged resources or instances of other IDisposable types.

If your view-model already implements IDisposable, you can of course unsubscribe from events there.

If not, you can create a CleanUp (or similar) method and unsubscribe there.

There's a very neat solution I use for such things. I call them one-shot event handlers (or auto-unsubscribing event handlers). This is where the new feature of C#7 "local functions" helps.

void IntermediateViewModel_CreateViewModels()

{

    MainViewModel myMainViewmodel = new MyViewModel();

    SecondViewModel mySecondViewModel = new SecondViewModel();

    myMainViewmodel.MyEvent += LocalEventHandler;



    void LocalEventHandler(object sender, EventArgs e)

    {

        // unsubscribe --> no memory leaks!

        myMainViewmodel.MyEvent -= LocalEventHandler;

        mySecondViewmodel.handleEvent(sender, e);            

    }

}

With such approach, when the MainViewModel.MyEvent fires, the local function LocalEventHandler is executed as event handler. You can unsubscribe from the event right away in that local function, so that there's no reference anymore (no memory leak, the view-model can be GCed).

From your use case I understood that you want to create a dialog view-model, display it using a view, and then immediately get rid of that view-model. Then, my sample perfectly fits to your requirements.