WPF – Converter as DependencyObject

Question

It is not possible to bind a value to ConverterParametr of Binding. Binding can be only set on a DependencyProperty of a DependencyObject.

I'm curious about implementing IValueConverter converter as DependencyObject.

public class AddConverter : DependencyObject, IValueConverter
{
    public static readonly DependencyProperty AddIntegerProperty =
        DependencyProperty.Register(nameof(AddInteger),
            typeof(int),
            typeof(AddConverter),
            new PropertyMetadata(0));

    public int AddInteger
    {
        get => (int)GetValue(AddIntegerProperty);
        set => SetValue(AddIntegerProperty, value);
    }

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (!(value is int intValue)) return 0;
        return intValue + AddInteger;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        int intValue;
        try
        {
            intValue = System.Convert.ToInt32(value);
        }
        catch (Exception)
        {
            return 0;
        }

        return intValue - AddInteger;
    }
}

Let's us it in example View.

<TextBox>
    <TextBox.Text>
        <Binding Path="MyIntegerProperty">
            <Binding.Converter>
                <local:AddConverter AddInteger="{Binding MyAddIntegerProperty, Mode=OneWay}" />
            </Binding.Converter>
        </Binding>
    </TextBox.Text>
</TextBox>

The result is that AddInteger still returns the default value. What is the reason that there is no change of AddInteger Dependency Property via provided Binding?

---

Footnote: The MultiBinding does not help me because of ConvertBack method consists only of value provided by a control. This stuff should also be solved in ViewModel, but I'm curious about a solution with a converter.

Answer 1

The problem is, first, that the converter can't inherit the DataContext where it is, so the binding can't work: You'll see "Framework mentor not found" in the VS output if you put a trace on the binding you have (see appendix A). This is also why you can't just derive from FrameworkElement and use RelativeSource={RelativeSource AncestorType=Whatever}: You're out of the visual tree. There are no ancestors. Also, even if there were a framework mentor, a DependencyObject can't provide a source for the binding. The source would have to be explicit. Only classes that inherit from FrameworkElement can inherit a DataContext.

So I stole a BindingProxy class (from this answer) and used that to provide a source for the binding. This is slightly clumsyish, but the other alternative I thought of was to inherit the converter from Freezable, essentially giving it the properties of a BindingProxy, and creating the converter in resources. It worked but I prefer the way this is organized.

public class BindingProxy : Freezable
{
    #region Overrides of Freezable

    protected override Freezable CreateInstanceCore()
    {
        return new BindingProxy();
    }

    #endregion

    #region Data Property
    public Object Data
    {
        get { return (Object)GetValue(DataProperty); }
        set { SetValue(DataProperty, value); }
    }

    public static readonly DependencyProperty DataProperty =
        DependencyProperty.Register(nameof(Data), typeof(Object), typeof(BindingProxy),
            new PropertyMetadata(null));
    #endregion Data Property
}

XAML

<StackPanel.Resources>
    <local:BindingProxy
        x:Key="VMProxy"
        Data="{Binding}"
        />
</StackPanel.Resources>
<TextBlock>
    <TextBlock.Text>
        <Binding Path="MyIntegerProperty">
            <Binding.Converter>
                <local:AddConverter
                    AddInteger="{Binding Data.MyAddIntegerProperty, Source={StaticResource VMProxy}}" 
                    />
            </Binding.Converter>
        </Binding>
    </TextBlock.Text>
</TextBlock>

---

Appendix A

AddInteger="{Binding MyAddIntegerProperty, Mode=OneWay, 
    PresentationTraceSources.TraceLevel=High}"

System.Windows.Data Warning: 56 : Created BindingExpression (hash=14964341) for Binding (hash=21653700)
System.Windows.Data Warning: 58 :   Path: 'MyAddIntegerProperty'
System.Windows.Data Warning: 60 : BindingExpression (hash=14964341): Default mode resolved to OneWay
System.Windows.Data Warning: 61 : BindingExpression (hash=14964341): Default update trigger resolved to PropertyChanged
System.Windows.Data Warning: 62 : BindingExpression (hash=14964341): Attach to WpfApp2.AddConverter2.AddInteger (hash=57434139)
System.Windows.Data Warning: 64 : BindingExpression (hash=14964341): Use Framework mentor <null>
System.Windows.Data Warning: 67 : BindingExpression (hash=14964341): Resolving source 
System.Windows.Data Warning: 69 : BindingExpression (hash=14964341): Framework mentor not found
System.Windows.Data Warning: 65 : BindingExpression (hash=14964341): Resolve source deferred
System.Windows.Data Warning: 67 : BindingExpression (hash=14964341): Resolving source 
System.Windows.Data Warning: 69 : BindingExpression (hash=14964341): Framework mentor not found