INotifyPropertyChanged

I had a conversation with Martin about INotifyPropertyChanged and it made me realise there was an (other) aspect of Binding that I hadn't appreciated.

So I created a little project to test my assumptions.

1) Business Class implements INotifyPropertyChanged

public class Person : INotifyPropertyChanged
{
  private string _surname = "";
  public string Surname
  {
    get { return _surname; }
    set
    {
      _surname = value;
      OnPropertyChanged("Surname");
    }
  }
  public event PropertyChangedEventHandler PropertyChanged;
  protected void OnPropertyChanged(string propertyName)
  {
    if (PropertyChanged != null)
    {
      PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
  }
}

2) Model exposing two instances of Person. Model does not implement INPC

public class Model
{
  public Person APerson { get; set; }
  public Person AnotherPerson { get; set; }
  public Model()
  {
    APerson = new Person();
    AnotherPerson = new Person();
  }
}

3) Bind to only one of the two instances of Person in the Model

<TextBlock Text="{Binding Model.APerson.Surname}"></TextBlock>
<Button Click="Button_Click">Go</Button>

4) Wire it all up

public partial class Window2 : Window
{
  private Model _model = new Model();
  public Model Model
  {
    get { return _model; }
    set { _model = value; }
  }
  public Window2()
  {
    InitializeComponent();
    DataContext = this;
  }
  private void Button_Click(object sender, RoutedEventArgs e)
  {
    Model.APerson.Surname = "Smith";
    Model.AnotherPerson.Surname = "Jones";
  }
}

Result:
PropertyChanged is only hooked up for APerson and not for AnotherPerson because nothing is bound to AnotherPerson in the xaml.

Conclusion:
I might need to revisit the changes I made to AbstractModel hooking up PropertyChanged and DataError. Currently they are hooked up when the Model is instantiated, regardless of whether there is any binding in the xaml.