c# - WPF Binding to variable / DependencyProperty

Question

I'm playing around with WPF Binding and variables. Apparently one can only bind DependencyProperties. I have come up with the following, which works perfectly fine: The code-behind file:

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    public string Test
    {
        get { return (string)this.GetValue(TestProperty); }
        set { this.SetValue(TestProperty, value); }
        //set { this.SetValue(TestProperty, "BBB"); }
    }
    public static readonly DependencyProperty TestProperty = DependencyProperty.Register(
      "Test", typeof(string), typeof(MainWindow), new PropertyMetadata("CCC"));

    private void button1_Click(object sender, RoutedEventArgs e)
    {
        MessageBox.Show(Test);
        Test = "AAA";
        MessageBox.Show(Test);
    }
}

XAML:

<Window x:Class="WpfApplication3.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:diag="clr-namespace:System.Diagnostics;assembly=WindowsBase"
    Title="MainWindow" Height="350" Width="525"
    DataContext="{Binding RelativeSource={RelativeSource Self}}">
<Grid>
    <TextBox Height="31" HorizontalAlignment="Left" Margin="84,86,0,0" Name="textBox1" VerticalAlignment="Top" Width="152" 
             Text="{Binding Test, Mode=TwoWay, diag:PresentationTraceSources.TraceLevel=High}"/>
    <Button Content="Button" Height="23" HorizontalAlignment="Left" Margin="320,85,0,0" Name="button1" VerticalAlignment="Top" Width="75" Click="button1_Click" />
    <TextBox Height="31" HorizontalAlignment="Left" Margin="84,138,0,0" Name="textBox2" Text="{Binding Test, Mode=TwoWay}" VerticalAlignment="Top" Width="152" />
</Grid>

The two TextBoxes update one an other. And the Button sets them to "AAA".

But now I replaced the Setter function with the one that is commented out (simulating some manipulation of the given value). I would expect that whenever the property value is changed it will be reset to "BBB". It does so when you press the button, that is when you set the property in code. But it does for some reason not affect the WPF Bindings, that is you can change the TextBox contents and thus the property, but apparently the Setter is never called. I wonder why that is so, and how one would go about to achive the expected behaviour.

Answer 1

The CLR Property wrapper for a Dependency Property is never guaranteed to be called and therefore, you should never place any additional logic there. Whenever you need additional logic when a DP is changed, you should use the property changed callback.

In your case..

public string Test
{
    get { return (string)this.GetValue(TestProperty); }
    set { this.SetValue(TestProperty, value); }
}

public static readonly DependencyProperty TestProperty =
    DependencyProperty.Register("Test",
    typeof(string),
    typeof(MainWindow),
    new PropertyMetadata("CCC", TestPropertyChanged));

private static void TestPropertyChanged(DependencyObject source, DependencyPropertyChangedEventArgs e)
{
    MainWindow mainWindow = source as MainWindow;
    string newValue = e.NewValue as string;
    // Do additional logic
}