Force Propagation of Coerced Value

Question

tl;dr: Coerced values are not propagated across data bindings. How can I force the update across the data binding when code-behind doesn't know the other side of the binding?

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I'm using a CoerceValueCallback on a WPF dependency property and I'm stuck at the issue that coerced values don't get propagated through to bindings.

Window1.xaml.cs

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Media;

namespace CoerceValueTest
{
    public class SomeControl : UserControl
    {
        public SomeControl()
        {
            StackPanel sp = new StackPanel();

            Button bUp = new Button();
            bUp.Content = "+";
            bUp.Click += delegate(object sender, RoutedEventArgs e) {
                Value += 2;
            };

            Button bDown = new Button();
            bDown.Content = "-";
            bDown.Click += delegate(object sender, RoutedEventArgs e) {
                Value -= 2;
            };

            TextBlock tbValue = new TextBlock();
            tbValue.SetBinding(TextBlock.TextProperty,
                               new Binding("Value") {
                                Source = this
                               });

            sp.Children.Add(bUp);
            sp.Children.Add(tbValue);
            sp.Children.Add(bDown);

            this.Content = sp;
        }

        public static readonly DependencyProperty ValueProperty = DependencyProperty.Register("Value",
                                                                                              typeof(int),
                                                                                              typeof(SomeControl),
                                                                                              new PropertyMetadata(0, ProcessValueChanged, CoerceValue));

        private static object CoerceValue(DependencyObject d, object baseValue)
        {
            if ((int)baseValue % 2 == 0) {
                return baseValue;
            } else {
                return DependencyProperty.UnsetValue;
            }
        }

        private static void ProcessValueChanged(object source, DependencyPropertyChangedEventArgs e)
        {
            ((SomeControl)source).ProcessValueChanged(e);
        }

        private void ProcessValueChanged(DependencyPropertyChangedEventArgs e)
        {
            OnValueChanged(EventArgs.Empty);
        }

        protected virtual void OnValueChanged(EventArgs e)
        {
            if (e == null) {
                throw new ArgumentNullException("e");
            }

            if (ValueChanged != null) {
                ValueChanged(this, e);
            }
        }

        public event EventHandler ValueChanged;

        public int Value {
            get {
                return (int)GetValue(ValueProperty);
            }
            set {
                SetValue(ValueProperty, value);
            }
        }
    }

    public class SomeBiggerControl : UserControl
    {
        public SomeBiggerControl()
        {
            Border parent = new Border();
            parent.BorderThickness = new Thickness(2);
            parent.Margin = new Thickness(2);
            parent.Padding = new Thickness(3);
            parent.BorderBrush = Brushes.DarkRed;

            SomeControl ctl = new SomeControl();
            ctl.SetBinding(SomeControl.ValueProperty,
                           new Binding("Value") {
                            Source = this,
                            Mode = BindingMode.TwoWay
                           });
            parent.Child = ctl;

            this.Content = parent;
        }

        public static readonly DependencyProperty ValueProperty = DependencyProperty.Register("Value",
                                                                                              typeof(int),
                                                                                              typeof(SomeBiggerControl),
                                                                                              new PropertyMetadata(0));

        public int Value {
            get {
                return (int)GetValue(ValueProperty);
            }
            set {
                SetValue(ValueProperty, value);
            }
        }
    }

    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();
        }
    }
}

Window1.xaml

<Window x:Class="CoerceValueTest.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="CoerceValueTest" Height="300" Width="300"
    xmlns:local="clr-namespace:CoerceValueTest"
    >
    <StackPanel>
        <local:SomeBiggerControl x:Name="sc"/>
        <TextBox Text="{Binding Value, ElementName=sc, Mode=TwoWay}" Name="tb"/>
        <Button Content=" "/>
    </StackPanel>
</Window>

i.e. two user controls, one nested inside the other, and the outer one of those in a window. The inner user control has a Value dependency property that is bound to a Value dependency property of the outer control. In the window, a TextBox.Text property is bound to the Value property of the outer control.

The inner control has a CoerceValueCallback registered with its Value property whose effect is that this Value property can only be assigned even numbers.

Note that this code is simplified for demonstration purposes. The real version doesn't initialize anything in the constructor; the two controls actually have control templates that do everything that's done in the respective constructors here. That is, in the real code, the outer control doesn't know the inner control.

When writing an even number into the text box and changing the focus (e.g. by focusing the dummy button below the text box), both Value properties get duly updated. When writing an odd number into the text box, however, the Value property of the inner control doesn't change, while the Value property of the outer control, as well as the TextBox.Text property, show the odd number.

My question is: How can I force an update in the text box (and ideally also in the outer control's Value property, while we're at it)?

I have found an SO question on the same problem, but doesn't really provide a solution. It alludes to using a property changed event handler to reset the value, but as far as I can see, that would mean duplicating the evaluation code to the outer control ... which is not really viable, as my actual evaluation code relies on some information basically only known (without much effort) to the inner control.

Moreover, this blogpost suggests invoking UpdateTarget on the binding in TextBox.Text in the CoerceValueCallback, but first, as implied above, my inner control cannot possibly have any knowledge about the text box, and second, I would probably have to call UpdateSource first on the binding of the Value property of the inner control. I don't see where to do that, though, as within the CoerceValue method, the coerced value has not yet been set (so it's too early to update the binding), while in the case that the value is reset by CoerceValue, the property value will just remain what it was, hence a property changed callback will not get invoked (as also implied in this discussion).

One possible workaround I had thought of was replacing the dependency property in SomeControl with a conventional property and an INotifyPropertyChanged implementation (so I can manually trigger the PropertyChanged event even if the value has been coerced). However, this would mean that I cannot declare a binding on that property any more, so it's not a really useful solution.

Answer 1

I have been looking for an answer to this rather nasty bug myself for a while. One way to do it, without the need to force an UpdateTarget on the bindings is this:

• Remove your CoerceValue callback.
• Shift the logic of the CoerceValue callback into your ProcessValueChanged callback.
• Assign your coerced value to your Value property, when applicable (when the number is odd)
• You will end up with the ProcessValueChanged callback being hit twice, but your coerced value will end up being effectively pushed to your binding.

Base on your code, your dependency property declaration would become this:

public static readonly DependencyProperty ValueProperty = 
                       DependencyProperty.Register("Value",
                                                   typeof(int),
                                                   typeof(SomeControl),
                                                   new PropertyMetadata(0, ProcessValueChanged, null));

And then, your ProcessValueChanged would become this:

private static void ProcessValueChanged(object source, DependencyPropertyChangedEventArgs e)
    {
        int baseValue = (int) e.NewValue;
        SomeControl someControl = source as SomeControl;
        if (baseValue % 2 != 0) 
        {
            someControl.Value = DependencyProperty.UnsetValue;
        }
        else
        {
            someControl.ProcessValueChanged(e);
        }
    }

I slightly modified your logic, to prevent raising the event when the value needs to be coerced. As mentionned before, assigning to someControl.Value the coerced value will cause your ProcessValueChanged to be called twice in a row. Putting the else statement would only raise the events with valid values once.

I hope this helps!