IValueConverter with Bound Dependency Properties

Question

I need to determine the StringFormat of some bound TextBlocks at runtime based on the unit system identified in the object to be bound.

I Have a converter with a Dependency Property that I would like to Bind to. The Bound value is used in determining the conversion process.

public class UnitConverter : DependencyObject, IValueConverter
{
    public static readonly DependencyProperty IsMetricProperty =
        DependencyProperty.Register("IsMetric", typeof(bool), typeof(UnitConverter), new PropertyMetadata(true, ValueChanged));

    private static void ValueChanged(DependencyObject source, DependencyPropertyChangedEventArgs e)
    {
        ((UnitConverter)source).IsMetric = (bool)e.NewValue;
    }

    public bool IsMetric
    {
        get { return (bool)this.GetValue(IsMetricProperty); }
        set { this.SetValue(IsMetricProperty, value); }
    }

    object IValueConverter.Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (IsMetric)
            return string.Format("{0:0.0}", value);
        else
            return string.Format("{0:0.000}", value);
    }

    object IValueConverter.ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

I declare the converter

<my:UnitConverter x:Key="Units" IsMetric="{Binding Path=IsMetric}"/>

and bind the TextBlock

<TextBlock Text="{Binding Path=Breadth, Converter={StaticResource Units}}" Style="{StaticResource Values}"/>

Never the less, I get the following error:

System.Windows.Data Error: 2 : Cannot find governing FrameworkElement or FrameworkContentElement for target element. BindingExpression:Path=IsMetric; DataItem=null; target element is 'UnitConverter' (HashCode=62641008); target property is 'IsMetric' (type 'Boolean')

I guess this is initialising before I set the datacontext and therefore there is nothing to bind the IsMetric property to. How can I achieve the desired result?

Answer 1

Provided that Breadthand IsMetric are properties of the same data object, you might use a MultiBinding in conjunction with a multi value converter:

<TextBlock>
    <TextBlock.Text>
        <MultiBinding Converter="{StaticResource UnitMultiValueConverter}">
            <Binding Path="Breadth" />
            <Binding Path="IsMetric" />
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>

with a converter like this:

public class UnitMultiValueConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        double value = (double)values[0];
        bool isMetric = (bool)values[1];
        string format = isMetric ? "{0:0.0}" : "{0:0.000}";
        return string.Format(format, value);
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

The problem with your approach is that when the UnitConverter is declared as resource it does not have a DataContext, and it will never get one later on.

And one more important thing: the ValueChanged callback for UnitConverter.IsMetric is nonsense. It sets the same property again which was just changed.