Should I unsubscribe from events?

Question

I have 3 questions concerning events:

1. Should I always unsubscribe events that were subscribed?
2. What happens if I do NOT?
3. In the below examples, how would you unsubscribe from the subscribed events?

I have for example this code:

Ctor: Purpose: For database property updates

this.PropertyChanged += (o, e) =>
{
    switch (e.PropertyName)
    {
        case "FirstName": break;
        case "LastName": break;
    }
};

and this: Purpose: For GUI-binding wrap the model into viewmodels

ObservableCollection<Period> periods = _lpRepo.GetDailyLessonPlanner(data.DailyDate);
PeriodListViewModel = new ObservableCollection<PeriodViewModel>();

foreach (Period period in periods)
{
    PeriodViewModel periodViewModel = new PeriodViewModel(period,_lpRepo);
    foreach (DocumentListViewModel documentListViewModel in periodViewModel.DocumentViewModelList)
    {
        documentListViewModel.DeleteDocumentDelegate += new Action<List<Document>>(OnDeleteDocument);
        documentListViewModel.AddDocumentDelegate += new Action(OnAddDocument);
        documentListViewModel.OpenDocumentDelegate += new Action<int, string>(OnOpenDocument);
    }
    PeriodListViewModel.Add(periodViewModel);
}

Answer 1

1) It depends. Usually it's a good idea, but there are typical cases where you don't need to. Basically, if you are sure that the subscribing object is going to outlive the event source, you ought to unsubscribe, otherwise this would create an unnecessary reference.

If however your object is subscribing to its own events, like in the following:

<Window Loaded="self_Loaded" ...>...</Window>

--then you don't have to.

2) Subscribing to an event makes additional reference to the subscribing object. So if you don't unsubscribe, your object might be kept alive by this reference, making effectively a memory leak. By unsubscribing you are removing that reference. Note that in the case of self-subscription the problem doesn't arise.

3) You can do like that:

this.PropertyChanged += PropertyChangedHandler;
...
this.PropertyChanged -= PropertyChangedHandler;

where

void PropertyChangedHandler(object o, PropertyChangedEventArgs e)
{
    switch (e.PropertyName)
    {
        case "FirstName": break;
        case "LastName": break;
    }
}