Problem with WPF Data Binding Defined in Code Not Updating UI Elements

Question

I need to define new UI Elements as well as data binding in code because they will be implemented after run-time. Here is a simplified version of what I am trying to do.

Data Model:

public class AddressBook : INotifyPropertyChanged
{
    private int _houseNumber;
    public int HouseNumber
    {
        get { return _houseNumber; }
        set { _houseNumber = value; NotifyPropertyChanged("HouseNumber"); }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected void NotifyPropertyChanged(string sProp)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(sProp));
        }
    }
}

Binding in Code:

AddressBook book = new AddressBook();
book.HouseNumber = 123;
TextBlock tb = new TextBlock();
Binding bind = new Binding("HouseNumber");
bind.Source = book;
bind.Mode = BindingMode.OneWay;
tb.SetBinding(TextBlock.TextProperty, bind); // Text block displays "123"
myGrid.Children.Add(tb);
book.HouseNumber = 456; // Text block displays "123" but PropertyChanged event fires

When the data is first bound, the text block is updated with the correct house number. Then, if I change the house number in code later, the book's PropertyChanged event fires, but the text block is not updated. Can anyone tell me why?

Thanks, Ben

Answer 1

Do you need to set the binding mode programatically? It may be defaulting to OneTime.

Answer 2

Are you sure you're updating the same object as you've used in the binding? At first glance nothing looks wrong, so check the simple things. :)

Answer 3

Does any code bypass the property, setting the field (_houseNumber) directly?

Answer 4

Make sure you're updating the AddressBook reference that was used in the binding, and not some other AddressBook reference.

I got the following to work with the AddressBook code you gave.

<StackPanel>
    <Button Click="Button_Click">Random</Button>
    <Grid x:Name="myGrid">
    </Grid>
</StackPanel>

Code behind:

public partial class Window1 : Window
{
    private AddressBook book;

    public Window1()
    {
        InitializeComponent();

        book = new AddressBook();
        book.HouseNumber = 13;

        TextBlock tb = new TextBlock();
        Binding bind = new Binding("HouseNumber");
        bind.Source = book;
        tb.SetBinding(TextBlock.TextProperty, bind);
        myGrid.Children.Add(tb);
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        Random rnd = new Random();
        book.HouseNumber = rnd.Next();
    }
}

Note the same reference is used in the update code.

Answer 5

I've just cut+pasted your code (and added a little), and it works fine for me:

    public Window1()
    {
        InitializeComponent();

        AddressBook book = new AddressBook();
        book.HouseNumber = 123;
        TextBlock tb = new TextBlock();
        Binding bind = new Binding("HouseNumber");
        bind.Source = book;
        bind.Mode = BindingMode.OneWay;
        tb.SetBinding(TextBlock.TextProperty, bind); // Text block displays "123"
        myGrid.Children.Add(tb);
        book.HouseNumber = 456; 
    }

    private void TestButton_Click(object sender, RoutedEventArgs e)
    {
        AddressBook book = (AddressBook)((TextBlock)myGrid.Children[0]).GetBindingExpression(TextBlock.TextProperty).DataItem;

        book.HouseNumber++;
    }

Displays 456 on startup, clicking the button makes the number in the TextBlock increment just fine.

Perhaps you are looking in the wrong place for the cause of the problem?

Answer 6

The root of it was that the string I passed to PropertyChangedEventArgs did not EXACTLY match the name of the property. I had something like this:

public int HouseNumber
{
    get { return _houseNumber; }
    set { _houseNumber = value; NotifyPropertyChanged("HouseNum"); }
}

Where it should be this:

public int HouseNumber
{
    get { return _houseNumber; }
    set { _houseNumber = value; NotifyPropertyChanged("HouseNumber"); }
}

Yikes! Thanks for the push in the right direction.