INotifyPropertyChange ~ PropertyChanged not firing when property is a collection and a new item is added to the collection

Question

I have a class that implements the INotifyPropertyChanged interface. Some of the properties of the class are of type List. For example:

public List<string> Answers
{
    get { return _answers;  }
    set
    {
      _answers = value;
      onPropertyChanged("Answers")
    }
}

...

private void onPropertyChanged(string propertyName)
{
    if(this.PropertyChanged != null)
        this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}

If I assign a new List<string> to Answer, then the PropertyChanged event fires as expected; but if I add a string string to the Answer list using the List Add method, then PropertyChanged event doesn't fire.

I was considering adding an AddAnswer() method to my class, which would handle calling the lists's Add method and would call onPropertyChanged() from there, but is that the right way to do it? Is there a more elegant way of doing it?

Cheers, KT

Answer 1

You should expose an ObservableCollection<string>, which implements the INotifyCollectionChange interface to raise its own change events.

You should also remove the property setter; Collection properties should be read only.

You should not raise the PropertyChanged event when the contents of the collection change.