Why doesn't this data binding work?

Question

I have a ViewModel class that contains a list of points, and I am trying to bind it to a Polyline. The Polyline picks up the initial list of points, but does not notice when additional points are added even though I implement INotifyPropertyChanged. What's wrong?

<StackPanel>
    <Button Click="Button_Click">Add!</Button>
    <Polyline x:Name="_line" Points="{Binding Pts}" Stroke="Black" StrokeThickness="5"/>
</StackPanel>

C# side:

// code-behind
_line.DataContext = new ViewModel();
private void Button_Click(object sender, RoutedEventArgs e)
{
 // The problem is here: NOTHING HAPPENS ON-SCREEN!
 ((ViewModel)_line.DataContext).AddPoint();
}

// ViewModel class
public class ViewModel : INotifyPropertyChanged
{
 public event PropertyChangedEventHandler PropertyChanged;

 public PointCollection Pts { get; set; }

 public ViewModel()
 {
  Pts = new PointCollection();
  Pts.Add(new Point(1, 1));
  Pts.Add(new Point(11, 11));
 }

 public void AddPoint()
 {
  Pts.Add(new Point(25, 13));
  if (PropertyChanged != null)
   PropertyChanged(this, new PropertyChangedEventArgs("Pts"));
 }
}

Answer 1

It is quite likely that since it is binding to the collection, it will need something like ObservableCollection<T>. What happens if you switch from PointCollection to ObservableCollection<Point>?

Answer 2

Change your PointCollections Property to a dependency property:

public PointCollection Pts
     {
      get { return (PointCollection)GetValue(PtsProperty); }
      set { SetValue(PtsProperty, value); }
     }

     // Using a DependencyProperty as the backing store for Pts.  This enables animation, styling, binding, etc...
     public static readonly DependencyProperty PtsProperty =
      DependencyProperty.Register("Pts", typeof(PointCollection), typeof(ViewModel), new UIPropertyMetadata(new PointCollection()));

BTW Doing this, you won't need to fire the PropertyChanged event.

Oh sorry, and your object needs to inherit from DependencyObject

    public class ViewModel : DependencyObject 
{ 
//... 
}

Answer 3

i got it to work as a POCO by implementing INotifyCollectionChanged instead of INotifyPropertyChanged.