Question

The example given here is a simplification of the actual UserControl that I am trying to implement but it illustrates the structure and suffers from the same problem. The user control has a DependencyProperty Words that sets the text of textblock that is defined in the user control XAML.

    public partial class MyControl : UserControl
    {
        public static readonly DependencyProperty WordsProperty = DependencyProperty.Register("Words", typeof(string), typeof(MyControl));
        public MyControl()
        {
            InitializeComponent();
        }
        public string Words
        {
            get { return (string)GetValue(WordsProperty); }
            set
            {
                m_TextBlock.Text = value;
                SetValue(WordsProperty, value);
            }

An INotifyPropertyChanged ViewModelBase class derived ViewModel is assigned to the mainWindow DataContext. The ModelText property set calls OnPropertyChanged.

class MainWindow : ViewModelBase
{
    private string m_ModelString;
    public string ModelText
    {
        get { return m_ModelString; }
        set
        {
            m_ModelString = value;
            base.OnPropertyChanged("ModelText");
        }
    }
}

In the MainWindow XAML binding is made to the UserControl and a TextBlock

 <Window x:Class="Binding.View.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="218" Width="266" xmlns:my="clr-namespace:Binding.View">
    <Grid>
        <my:MyControl Words="{Binding ModelText}" HorizontalAlignment="Left" Margin="39,29,0,0" x:Name="myControl1" VerticalAlignment="Top" Height="69" Width="179" Background="#FF96FF96" />
        <TextBlock Height="21" HorizontalAlignment="Left" Margin="59,116,0,0" Name="textBlock1" Text="{Binding ModelText}" VerticalAlignment="Top" Width="104" Background="Yellow" />
    </Grid>
</Window>

The binding works for the textblock but not for the user control. Why can’t UserControl DependencyProperty be bound in the same way as Control Properties?

Answer 1

The culprit is:

m_TextBlock.Text = value;

WPF does not directly use properties backed by a DP.

If you want to update m_TextBlock.Text when the WordsProperty is modified, either bind that textblock to Words, or use the PropertyChangedCallback in UIPropertyMetadata:

 public static readonly DependencyProperty WordsProperty = 
         DependencyProperty.Register("Words", 
                                    typeof(string), 
                                    typeof(MyControl),
                                    new UIPRopertyMetadata(
                 new PropertyChangedCallback(
                     (dpo, dpce) => 
                     {
                         //Probably going to need to first cast dpo to MyControl
                         //And then assign its m_TextBlock property's text.
                         m_TextBlock.Text = dpce.NewValue as string;
                     })));

Consider dpo the sender, and dpce the event args.