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QuestionWhat is the correct code to invoke subscribers for an event? Pin
User 1106097923-Jul-16 21:16
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Let's assume we have something like this:
C#
public class Person : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged([CallerMemberName]string propertyName = null)
    {
        // Is it really needed, to make a copy?  *1) 
        var handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }
    // ....
}
*1) Copy of the subscribers really needed?
A lot of time in forums like CP I read we need to make a copy of the subscribers before invoke the handlers. Argumentation: This because theoretically the subscribers can unsubscribe in between...

My doubts, respectively I see only arguments why not to copy:
a.) In a some lot (in most?) of MSDN examples, no copy will be made. So, when MSDN does not do it why should I do it?
b.) In case someone unsubscribes he is not longer interested in the event. Why should I notify him then longer?
c.) My opinion is, that it can be dangerous to inform a subscriber while he unsubscribed and therefore does not expect notifications any longer.

[Edit] Thank you BillWoodruff Smile | :)
Ok, a problem I see now in case I do not make a copy:
In bewteen (if PropertyChanged != null) and invoking PropertyChanged(...) it is possible that one does unsubscribe. Probably this Point is solved in c#6 by using (PropertyChanged?.Invoke(...)

But still "c.)" above is critical I think...(?) and the subscriber has to be aware that even after unsubscribing he can get notifications.

Comments / corrections / notes will be very appreciated.
Thank you in advance.

modified 19-Jan-21 21:04pm.

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