Question

I want to have a singleton that holds some values S1 and S2 that I can bind to. The goal is to have some UIElements update when its value changes. The problem is that I want to use the value inside of a reused DataTemplate. That means that I cannot bind directly to a dependency property of the singleton but this has to be set outside.

To correctly pass updates the values have to be DependencyProperty. Because I dont know to which property I have to bind I created another attachable property AttProperty of the same type as the values. Now I tried to bind the S1 to AttProperty but this gives me an error:

Additional information: A ‘Binding’ cannot be set on the
‘SetAttProperty’ property of type ‘TextBox’. A ‘Binding’ can only be
set on a DependencyProperty of a DependencyObject.

So how can I bind with an attachable DependencyProperty to another DependencyProperty?

Here is the code for the singleton I have so far (C#):

public class DO : DependencyObject
{
  // Singleton pattern (Expose a single shared instance, prevent creating additional instances)
  public static readonly DO Instance = new DO();
  private DO() { }
  public static readonly DependencyProperty S1Property = DependencyProperty.Register(
    "S1", typeof(string), typeof(DO),
    new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.AffectsRender));
  public string S1
  {
    get { return (string)GetValue(S1Property); }
    set { SetValue(S1Property, value); }
  }
  public static readonly DependencyProperty AttProperty = DependencyProperty.RegisterAttached( 
    "Att", typeof(string), typeof(DO), 
    new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.AffectsRender) );
  public static void SetAttProperty(DependencyObject depObj, string value)
  {
    depObj.SetValue(AttProperty, value);
  }
  public static string GetAttProperty(DependencyObject depObj)
  {
    return (string)depObj.GetValue(AttProperty);
  }
}

Here is the problematic thing (XAML):

<TextBox Name="Input" Text="" TextChanged="Input_TextChanged" local:DO.AttProperty="{Binding Source={x:Static local:DO.Instance}, Path=S1}" />

Update

With the changes of Bojin Li the errors go away. But one issue remains – if I now try to update the singleton with the help of the attached property like this:

<TextBox local:DO.Att="{Binding Source={x:Static local:DO.Instance}, Path=S1, Mode=TwoWay}" Text="{Binding Path=(local:DO.Att), RelativeSource={RelativeSource Self}, Mode=TwoWay}"/>

Why is the value not propagated to S1 in the singleton?

Answer 1

You need to implement INotifyPropertyChanged and wire the changes up to the dependency property changing.

public class DO : DependencyObject,INotifyPropertyChanged {
        // Singleton pattern (Expose a single shared instance, prevent creating additional instances) 
        public static readonly DO Instance = new DO();
        private DO() { }
        public static readonly DependencyProperty S1Property = DependencyProperty.Register(
          "S1", typeof(string), typeof(DO),
          new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.AffectsRender,onS1Changed));

        private static void onS1Changed(DependencyObject d, DependencyPropertyChangedEventArgs e) {
            DO item = d as DO;
            if (item != null) item.OnPropertyChanged(new PropertyChangedEventArgs("S1"));
        }

        public string S1 {
            get { return (string)GetValue(S1Property); }
            set { SetValue(S1Property, value); }
        }
        public static readonly DependencyProperty AttProperty = DependencyProperty.RegisterAttached(
          "Att", typeof(string), typeof(DO),
          new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.AffectsRender,onAttChanged));

        private static void onAttChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) {
            DO item = d as DO;
            if (item != null) item.OnPropertyChanged(new PropertyChangedEventArgs("Att"));
        }

        public static void SetAttProperty(DependencyObject depObj, string value) {
            depObj.SetValue(AttProperty, value);
        }
        public static string GetAttProperty(DependencyObject depObj) {
            return (string)depObj.GetValue(AttProperty);
        }

        public event PropertyChangedEventHandler PropertyChanged;

        public void OnPropertyChanged(PropertyChangedEventArgs e) {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null) handler(this, e);
        }
   }