Question

I’m trying to validate a UI change when Enter key is pressed. The UI element is a textbox, which is data binded to a string. My problem is that the data binding hasn’t updated TestText when Enter key is Up. It is only updated when I press the button which brings up a message box.

/// <summary>
/// Interaction logic for Window1.xaml
/// </summary>
public partial class Window1 : Window, INotifyPropertyChanged
{
    String _testText = new StringBuilder("One").ToString();
    public string TestText
    {
        get { return _testText; }
        set { if (value != _testText) { _testText = value; OnPropertyChanged("TestText"); } }
    }


    public Window1()
    {
        InitializeComponent();
        myGrid.DataContext = this;
    }

    private void OnPropertyChanged(string property)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(property));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private void onKeyUp(object sender, KeyEventArgs e)
    {
       if (e.Key != System.Windows.Input.Key.Enter) return;
       System.Diagnostics.Trace.WriteLine(TestText);
    }

    private void button1_Click(object sender, RoutedEventArgs e)
    {
        MessageBox.Show(TestText);
    }

}

Window XAML:

Window x:Class="VerificationTest.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Window1" Height="300" Width="300" KeyUp="onKeyUp"

TextBox XAML:

TextBox Name="myTextBox" Text="{Binding TestText}"

Button XAML:

Button Name="button1" Click="button1_Click"

Answer 1

In order to force the TextBox to commit the value back to the binding source you can do:

var binding = myTextBox.GetBindingExpression(TextBox.TextProperty);
binding.UpdateSource();

Optionally, you can configure the binding to update the source every time the Text property changes which would mean on every character you enter into the text box.

<TextBox Name="myTextBox"
         Text="{Binding TestText, UpdateSourceTrigger=PropertyChanged}" />

But this will raise a lot of property change notifications. What I have done in my applications was to create a class derived from TextBox to override the OnKeyDown method and when enter is pressed, I call the UpdateSource method as I described above and also call SelectAll on the TextBox to give the user an idea that I just “accepted” their input. Deriving a class from TextBox will let you reuse that behavior anywhere else you might want it in your application.