Update binding when nested object changes

Question

I have a some XAML as follows (a simple Label and Button):

<Window x:Class="WpfApplication2.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:Test="clr-namespace:WpfApplication2">
<StackPanel DataContext="{Binding Path=TestPerson}">
    <Label Content="{Binding Path=Name}"></Label>
    <Button Content="Button" Click="button1_Click" />
</StackPanel>
</Window>

in the code behind I have:

public partial class MainWindow : Window, INotifyPropertyChanged
{
    private Person _person = new Person();
    public Person TestPerson { get { return _person; } }

    public MainWindow()
    {           
        DataContext = this;
        InitializeComponent();
    }

    private void button1_Click(object sender, RoutedEventArgs e)
    {
        _person.Name = "Bill";
        //_person = new Person() { Name = "Bill" };
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs("TestPerson"));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

and the class Person is:

public class Person
{
    string _name = "Bob";

    public string Name
    {
        get { return _name; }
        set { _name = value; }
    }
}

As it is, firing the Propertychanged event does not cause the Label's contents to change to Bill.

I've found I am able to overcome this by either:

Assigning a new object to _person (as in the commented out line)
Removing the DataContext from the StackPanel and have Label bind to Path=TestPerson.Name

I don't understand why I have to actually assign a NEW object to _person for the Label to update, or use the FULL path in the binding.

Is there a way to update the Label without supplying the full path (relying on the DataContext), and without assigning a new object to _person?

Fredrik Hedblad · Answer 1 · 2011-09-12T12:19:25.373

You raise PropertyChanged for the Person instance TestPerson. However, TestPerson hasn't changed, it is the Name property of TestPerson that has changed and that is the property the Label is binding to.

Edit: To answer why your first two versions work

Assigning a new object to _person (as in the commented out line)

Here you are actually changing the value of TestPerson and because DataContext is inherited by the children, the Label gets a new DataContext as well so that's why the Binding is updated.

Removing the DataContext from the StackPanel and have Label bind to Path=TestPerson.Name

This is something I've never seen. The same binding subscribes to PropertyChanged for both TestPerson and Name in Person so raising PropertyChanged for any of these properties will work.

If you want to overcome this without implementing INotifyPropertyChanged for Person, you can change set UpdateSourceTrigger to Explicit

<Label Name="label"
       Content="{Binding Path=Name, UpdateSourceTrigger=Explicit}"/>

And update the Binding manually whenever Name changes

private void button1_Click(object sender, RoutedEventArgs e)
{
    _person.Name = "Bill";
    BindingExpression be = label.GetBindingExpression(Label.ContentProperty);
    be.UpdateTarget();
    if (PropertyChanged != null)
    {
        PropertyChanged(this, new PropertyChangedEventArgs("TestPerson"));
    }
}

Otherwise, just implement INotifyPropertyChanged for Person as well and it will work

public class Person : INotifyPropertyChanged
{ 
    string _name = "Bob"; 

    public string Name 
    { 
        get { return _name; } 
        set
        {
            _name = value;
            OnPropertyChanged("Name");
        } 
    } 

    public event PropertyChangedEventHandler PropertyChanged;
    protected void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

Hmmm, I think I might have to implement INotifyPropertyChanged for each property in Person... — MrNick, Sep 12 '11 at 09:28
Meant to edit my comment above to this: The example I gave is somewhat contrived; my real code has a custom UserControl in place of the Label, and my equivalent of the Person class has something like 10 properties. I wanted a UserControl capable of displaying (and updating) all the Person properties but without all 10 properties using INotifyPropertyChanged, but I guess that's not possible. It's no big headache, but I like to understand why things don't work :) — MrNick, Sep 12 '11 at 10:04
Updated my answer with some details on why your two other versions work. I've never noticed the behavior of the last one (binding to `TestPerson.Name`) and I actually didn't expect this to work for both properties but apparently it did :) +1 for that finding — Fredrik Hedblad, Sep 12 '11 at 12:21

score 0 · Answer 2 · answered Sep 12 '11 at 07:17

You need a little change in your XAML...

In your code behind, instead of setting DataContext as this, set it in XAML via Binding...

     <StackPanel DataContext="{Binding RelativeSource={RelativeSource
                                            AncestorType= {x:Type Window},
                                                Mode=FindAncestor}, 
                                       Path=TestPerson}">
          <Label Content="{Binding Path=Name}"></Label>
          <Button Content="Button" Click="button1_Click" />
     </StackPanel>

Remove the

     DataContext = this;

from your code behind.

Let me know if this helps.

Update binding when nested object changes

2 Answers2