A weird behavior of PropertyChanged UpdataSourceTrigger in WPF

Question

I have an entity like this:

public class Person
{
    public string Name { get; set; }

    public Person()
    {
        Name = "Godspeed";
    }
}

Then I have three textbox and a button in XAML:

<Window x:Class="WpfApplication19.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApplication19"
        Title="MainWindow" Height="350" Width="525">
    <Window.DataContext>
        <local:Person />
    </Window.DataContext>
    <StackPanel>
        <TextBox Text="{Binding Name,UpdateSourceTrigger=PropertyChanged}" />
        <TextBox Text="{Binding Name,UpdateSourceTrigger=PropertyChanged}" />
        <TextBox Text="{Binding Name,UpdateSourceTrigger=PropertyChanged}" />
        <Button Click="Button_Click">Click</Button>
    </StackPanel>
</Window>

The weird thing is that, the entity "Person" doesn't implement the INotifyPropertyChanged, but when one text box is changed, it modifies the source(Person object), but we didn't raised the property changed event but the rest two textboxes automatically changed.

When the button clicks, we update the source directly by code like:

((Person)DataContext).Name = "Godspeed";

It doesn't update. So what I think is that if the Person class implement the INotifyPropertyChanged, this behavior is normal, but now the class doesn't implement the interface, but it update the interface too. Please info me the reason if you have some clue. Thanks.

Fredrik Hedblad · Accepted Answer · 2011-08-12T08:57:24.457

The reason is PropertyDescriptor, see the following thread, the same question is being asked: How does the data binding system know when a property is changed?

Here is two of the answers

I think the magic lies in the binding system's use of PropertyDescriptor (SetValue presumably raises a ValueChanged - the PropertyDescriptor is likely shared, while the events are raised on a per-object basis).

I'm not at Microsoft, but I can confirm it. If PropertyDescriptor is used to update the value, as it will be, then relevant change notifications are automatically propagated.

Edit
You can verify this by naming the Person DataContext object

<Window.DataContext>
    <local:Person x:Name="person"/>
</Window.DataContext>

and add the following code to the MainWindow ctor

public MainWindow()
{
    InitializeComponent();

    PropertyDescriptorCollection properties = TypeDescriptor.GetProperties(person);
    PropertyDescriptor nameProperty = properties[0];
    nameProperty.AddValueChanged(person, OnPropertyChanged);
}
void OnPropertyChanged(object sender, EventArgs e)
{
    MessageBox.Show("Name Changed");
}

Once you change the value on any of the three TextBoxes, you'll end up in the event handler OnPropertyChanged.

score 1 · Answer 2 · answered Aug 12 '11 at 08:23

Well, as you said, you just have to implement INotifyPropertyChanged

The PropertyChanged event is used when you set a property from code and need to reflect this change to your UI (UI will cath the PropertyChanged event, and thanks to your UpdateSourceTrigger the UI will be updated). The other side (changing from UI) does not need any PropertyChanged, this is why you get this behavior

Just try it like that:

    public class Person : INotifyPropertyChanged
    {
            #region INotifyPropertyChanged Members

            /// <summary>
            /// Property Changed Event
            /// </summary>
            public event PropertyChangedEventHandler PropertyChanged;

            /// <summary>
            /// Property Changed
            /// </summary>
            /// <param name="propertyName"></param>
            protected void OnPropertyChanged(string propertyName)
            {
                if (PropertyChanged != null)
                    PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
            #endregion

    private string name;

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

}

Using this code, when you set the Name, the PropertyChanged event will be fired and therefore update UI accordingly :)

So what you mean is that the UpdateTrigger will automatically update the UI if I set propertychanged, not only change the source entity right? — Howard, Aug 12 '11 at 08:31
And the INotifyPropertyChanged interface only triggers the UI changing from the code behind right? — Howard, Aug 12 '11 at 08:32
That's it, yes. Basically, your UpdateTrigger is waiting to catch a PropertyChanged event to refresh the binding. Since you don't fire it anywhere, it won't trigger any update. You got the point of what I am saying here: PropertyChanged is use to say "Hey UI, please refresh yourself, I've changed!" :) — Damascus, Aug 12 '11 at 09:03

score 1 · Answer 3 · answered Aug 12 '11 at 08:48

It works not only with updatesourcetrigger=propertychanged, but with default (lost focus) value too. In addition to what @Meleak said, I want to point that it is good behaviour. Any changes made by ui are propagated to all binding targets. Binding engine wants to propagate this changes to all controls at once. If you make changes through code, and not implement INotifyPropertyChanged - changes made from code are not reflected at all. Again, for all controls with the same binding source. All controls works in the synchronized way with such implementation.

A weird behavior of PropertyChanged UpdataSourceTrigger in WPF

3 Answers3