Defining a Property in a IValueConverter class

Question

I need to define a DependencyProperty in a converter class because I need this data to make the conversion and this data is in another object, not the one I'm binding to.

My converter class is the following:

public class LEGOMaterialConverter   : DependencyObject, IValueConverter
{
    public DependencyProperty MaterialsListProperty = DependencyProperty.Register("MaterialsList", typeof(Dictionary<int, LEGOMaterial>), typeof(LEGOMaterialConverter));

    public Dictionary<int, LEGOMaterial> MaterialsList
    {
        get
        {
            return (Dictionary<int, LEGOMaterial>)GetValue(MaterialsListProperty);
        }
        set
        {
            SetValue(MaterialsListProperty, value);
        }
    }

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        LEGOMaterial material = null;

        MaterialsList.TryGetValue((int)value, out material);

        return material;
    }

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

Then I'm instanciating it on the Window.REsources area:

<Window.Resources>
    <local:LEGOMaterialConverter x:Key="MaterialsConverter" MaterialsList="{Binding Path=Materials}" />
</Window.Resources>

I'm getting the following error:

   'MaterialsList' property was already registered by 'LEGOMaterialConverter'.

Does anyone have a clue on this error?

Answer 1

Try doing it like this (just an example):

public class ValueConverterWithProperties : MarkupExtension, IValueConverter
{
    public int TrueValue { get; set; }

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        return this;
    }

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if ((int) value == TrueValue)
        {
            return true;
        }
        return false;
    }

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

notice I derive from markup extension to allow me to use it like this:

<Window x:Class="Converter.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:converter="clr-namespace:Converter"
    Title="MainWindow" Height="350" Width="525">
<Grid>
    <CheckBox IsChecked="{Binding item, Converter={converter:ValueConverterWithProperties TrueValue=5}}"></CheckBox>
    <CheckBox IsChecked="{Binding item2, Converter={converter:ValueConverterWithProperties TrueValue=10}}"></CheckBox>
</Grid>

Answer 2

Btw, this error is caused by your dependency property in the converter not being static (and after having created an instance of this converter somewhere before).

EDIT

So the problem is with this line:

public DependencyProperty MaterialsListProperty = DependencyProperty.Register("MaterialsList", typeof(Dictionary<int, LEGOMaterial>), typeof(LEGOMaterialConverter));

Here the Dependency Property MaterialsListProperty is being registered with every instantiation of an object of this type (i.e. LEGOMaterialConverter).

However Dependency Properties should be defined static, like so:

public static readonly DependencyProperty MaterialsListProperty = DependencyProperty.Register("MaterialsList", typeof(Dictionary<int, LEGOMaterial>), typeof(LEGOMaterialConverter));

A static variable is initialized (and the Dependency Property is registered) only once for all future instances of this type and that is what we need here. Failing to declare a Dependency Property as static results in the error from above.