WPF - Dependency Property of Custom Control lost Binding at 2 way mode - wpf

Question

I have this Custom Control
XAML:
<UserControl x:Class="WpfApplication1.UC"
...
x:Name="uc">
<StackPanel HorizontalAlignment="Left" VerticalAlignment="Top" Orientation="Horizontal">
<TextBox Text="{Binding Test, ElementName=uc}" Width="50" HorizontalAlignment="Left"/>
</StackPanel>
</UserControl>
C#
public partial class UC : UserControl
{
public static readonly DependencyProperty TestProperty;
public string Test
{
get
{
return (string)GetValue(TestProperty);
}
set
{
SetValue(TestProperty, value);
}
}
static UC()
{
TestProperty = DependencyProperty.Register("Test",typeof(string),
typeof(UC), new FrameworkPropertyMetadata("", FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));
}
public UC()
{
InitializeComponent();
}
}
And this is how i used that custom control:
<DockPanel>
<ItemsControl ItemsSource="{Binding Path=DataList}"
DockPanel.Dock="Left">
<ItemsControl.ItemTemplate>
<DataTemplate>
<Button Content="{Binding}" CommandParameter="{Binding}" Click="Button_Click"/>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
<local:UC Test="{Binding SelectedString, Mode=OneWay}"/>
</DockPanel>
--
public partial class MainWindow : Window, INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
private ObservableCollection<string> _dataList;
public ObservableCollection<string> DataList
{
get { return _dataList; }
set
{
_dataList = value;
OnPropertyChanged("DataList");
}
}
private string _selectedString;
public string SelectedString
{
get { return _selectedString; }
set
{
_selectedString = value;
OnPropertyChanged("SelectedString");
}
}
public MainWindow()
{
InitializeComponent();
this.DataList = new ObservableCollection<string>();
this.DataList.Add("1111");
this.DataList.Add("2222");
this.DataList.Add("3333");
this.DataList.Add("4444");
this.DataContext = this;
}
private void Button_Click(object sender, RoutedEventArgs e)
{
this.SelectedString = (sender as Button).CommandParameter.ToString();
}
}
If I do not change text of UC, everything is ok. When I click each button in the left panel, button's content is displayed on UC.
But when I change text of UC (ex: to 9999), Test property lost binding. When I click each button in the left panel, text of UC is the same that was changed (9999). In debug I see that SelectedString is changed by each button click but UC's text is not.
I can 'fix' this problem by using this <TextBox Text="{Binding Test, ElementName=uc, Mode=OneWay}" Width="50" HorizontalAlignment="Left"/> in the UC.
But I just want to understand the problem, can someone help me to explain it please.

Answer 1

Setting the value of the target of a OneWay binding clears the binding. The binding <TextBox Text="{Binding Test, ElementName=uc}" is two way, and when the text changes it updates the Test property as well. But the Test property is the Target of a OneWay binding, and that binding is cleared.
Your 'fix' works because as a OneWay binding, it never updates Test and the binding is never cleared. Depending on what you want, you could also change the UC binding to <local:UC Test="{Binding SelectedString, Mode=TwoWay}"/> Two Way bindings are not cleared when the source or target is updated through another method.

Answer 2

The issue is with below line
<local:UC Test="{Binding SelectedString, Mode=OneWay}"/>
The mode is set as oneway for SelectString binding so text will be updated when the value from code base changes. To change either the source property or the target property to automatically update the binding source as TwoWay.
<local:UC Test="{Binding SelectedString, Mode=TwoWay}"/>