Bind a Converter Parameter in re-usable User Control

Question

I am trying to create a re-usable user control (for data entry) in which there are two text boxes and they are linked to each by an IValueConvertor.

The following XAML is the original, normal code. This is what I am trying to reproduce in a user control.

<WrapPanel>
    <TextBlock Text="Length of Fence"/>
    <TextBox Name="Metric" Width="50" Text="{Binding Path=LengthFence, Mode=TwoWay}"/>
    <TextBlock Text="Meters"/>
    <TextBox Text="{Binding ElementName=Metric, Path=Text, Converter={StaticResource MetersToInches}, StringFormat=N8}"/>
    <TextBlock Text="Inches"/>
</WrapPanel>

and the code-behind for the IValueConvertor (in MainWindow.xaml) is

    public class MetersToInches : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (value.ToString() == "")
                return 0.0;
            try
            {
                double meters = System.Convert.ToDouble(value);
                var result = meters * 39.3701;
                return result;
            }
            catch
            {
                // Catch errors when users type invalid expressions.
                return 0.0;
            }

        }

        public object ConvertBack(object value, Type targettype, object parameter, CultureInfo culture)
        {
            if (value.ToString() == "")
                return 0.0;
            try
            {
                double inches = System.Convert.ToDouble(value);
                var result = inches * 0.0254;
                return result;
            }
            catch
            {
                // Catch errors when users type invalid expressions.
                return 0.0;
            }
        }

    }

This is what this XAML looks like:

Now I have made a re-usable UserControl with three dependency properties Label for label string, Value for binding a property inside the ViewModel, and Units - a string property to show the input units.

<UserControl ...
             x:Name="parent">
    <StackPanel DataContext="{Binding ElementName=parent}">
        <TextBlock Text="{Binding Path=Label}"/>
        <TextBox Text="{Binding Path=Value}"/>
        <TextBlock Text="{Binding Path=Units}"/>
    </StackPanel>

However, this re-usable control can only tackle the first TextBox of the input. I do not know how to bind the IValueConvertor in the second TextBox. I need to do this because I want to bind other converters such as meters to feet, kg to pound, etc.

I have read that ConvertorParameter cannot be bound because it is not a dependency property and I am not sure if I can use multi-binding, mostly because I do not know how to use it properly Binding ConverterParameter.

I would be very grateful if you could show me how to do this or direct me to the appropriate link on StackOverflow or elsewhere that solves this problem. Or if there is a better way of doing this.

Many many thanks in advance.

Answer 1

First, don't bind the TextBoxes to each other (as in your original code at the begining of the question), instead, bind each TextBox to the same backing property, which, in your UserControl, is Value.

As for how to implement multiple bindings, you probably don't need a MultiBinding.

We have to pick a "standard" unit of measure to begin with- this will be the unit that will be actually stored in the property and in any database or file. I'll assume this standard unit will be meters (m). An IValueConverter can be used to convert between meters and some other unit of distance and back, using the ConverterParameter to specify which other unit to convert to/from.

Here's a good example to get you started.

public enum DistanceUnit { Meter, Foot, Inch, }

public class DistanceUnitConverter : IValueConverter
{
    private static Dictionary<DistanceUnit, double> conversions = new Dictionary<DistanceUnit, double>
    {
        { DistanceUnit.Meter, 1 },
        { DistanceUnit.Foot, 3.28084 },
        { DistanceUnit.Inch, 39.37008 }
    };

    //Converts a meter into another unit
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return conversions[(DistanceUnit)parameter] * (double)value;
    }

    //Converts some unit into a meter 
    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value == null) { return 0; }

        double v;

        var s = value as string;

        if (s == null)
        {
            v = (double)value;
        }
        else
        {
            if (s == string.Empty) { return 0; }
            v = double.Parse(s);
        }

        if (v == 0) { return 0; }

        return v / conversions[((DistanceUnit)parameter)];
    }
}

The above has a few problems. I never check if parameter really is a DistanceUnit before using it, for example. But it works.

Here's an example of how I used it:

<StackPanel>
    <StackPanel.Resources>
        <local:DistanceUnitConverter x:Key="DistCon"/>
    </StackPanel.Resources>

    <StackPanel Orientation="Horizontal">
        <TextBox Text="{Binding Distance, Converter={StaticResource DistCon}, ConverterParameter={x:Static local:DistanceUnit.Meter}}" MinWidth="20"/>
        <TextBlock>m</TextBlock>
    </StackPanel>

    <StackPanel Orientation="Horizontal">
        <TextBox Text="{Binding Distance, Converter={StaticResource DistCon}, ConverterParameter={x:Static local:DistanceUnit.Foot}}" MinWidth="20"/>
        <TextBlock>ft</TextBlock>
    </StackPanel>
</StackPanel>

The DistanceUnit enum and the internal conversions dictionary can be expanded with more units of measure. Alternatively, you can use a 3rd party library that already has all these included, like UnitsNet.