Question

I am driving crazy with a custom Dependency Property. I already checked loads of threads here, but haven’t found any solution yet. What I want to do is to replace the value of the Property if the source provides a specific value (null for the given example). No matter what I try, the property value within the source remains null and is never updated.

Here is my Custom Control:

public class TextBoxEx : TextBox
{
    public TextBoxEx()
    {
        TrueValue = 0;
        this.TextChanged += (s, e) =>
        {
            TrueValue = Text.Length;
            SetCurrentValue(MyPropertyProperty, TrueValue);
            var x = BindingOperations.GetBindingExpression(this, MyPropertyProperty);
            if (x != null)
            {
                x.UpdateSource();
            }
        };
    }

    public int? TrueValue { get; set; }

    public int? MyProperty
    {
        get { return (int?)GetValue(MyPropertyProperty); }
        set { SetValue(MyPropertyProperty, value); }
    }

    public static readonly DependencyProperty MyPropertyProperty =
        DependencyProperty.Register("MyProperty", typeof(int?), typeof(TextBoxEx), new PropertyMetadata(null, PropertyChangedCallback));

    private static void PropertyChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (e.NewValue == null)
        {
            d.SetCurrentValue(MyPropertyProperty, (d as TextBoxEx).TrueValue);
        }
    }
}

Here is the DataContext i am Binding:

public class VM : INotifyPropertyChanged
{
    private int? _Bar = null;

    public int? Bar
    {
        get { return _Bar; }
        set
        {
            _Bar = value;
            OnPropertyChanged("Bar");
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

My Binding looks like this:

<local:TextBoxEx MyProperty="{Binding Bar, UpdateSourceTrigger=PropertyChanged}"/>

Remember: I need a TwoWay binding, so OneWayToSource does not work for me.

Any idea what I am not getting in here?

Answer 1

You just need to set the binding to two-way and it will work. But as that should be the default you can register the property accordingly using the following metadata:

... new FrameworkPropertyMetadata(null,
                                  FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
                                  PropertyChangedCallback)

Getting the expression in the TextChanged handler and updating source manually is not required so i would remove that code.

---

If you do not explicitly set a mode on a binding the default will be used, from the documentation:

Default: Uses the default Mode value of the binding target. The default value varies for each dependency property. In general, user-editable control properties, such as those of text boxes and check boxes, default to two-way bindings, whereas most other properties default to one-way bindings. A programmatic way to determine whether a dependency property binds one-way or two-way by default is to get the property metadata of the property using GetMetadata and then check the Boolean value of the BindsTwoWayByDefault property.