How to use a property of the view model in a CompositeCollection?

Question

When I have a comboBox in my view and want a empty item to be able to unselect the option, I use this code in my view:

<ComboBox.Resources>
    <CollectionViewSource x:Key="comboBoxSource" Source="{Binding ElementName=ucPrincipal, Path=DataContext.MyProperty}" />
</ComboBox.Resources>
<ComboBox.ItemsSource>
    <CompositeCollection>
        <entities:MyType ID="-1"/>
        <CollectionContainer Collection="{Binding Source={StaticResource comboBoxSource}}" />
    </CompositeCollection>
</ComboBox.ItemsSource>

In this case, is the view which sets the ID to -1 to indicate that is special item. But I don't like so much this solution because the view model depends that the view sets it correctly.

So I am thinking to have this property in my view model:

public readonly MyType MyNullItem = new MyType();

But I don't know how to use it in my composite collection in the view instead of:

<entities:MyType ID="-1"/>

Is it possible?

Thanks.

Answer 1

You need some kind of binding converter which would combine one list and one object into CompositeCollection. Some time ago I implemented similar converter with only difference that it converts multiple collections into one:

/// <summary>
/// Combines multiple collections into one CompositeCollection. This can be useful when binding to multiple item sources is needed.
/// </summary>
internal class MultiItemSourcesConverter : IMultiValueConverter {
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture) {
        var result = new CompositeCollection();

        foreach (var collection in values.OfType<IEnumerable<dynamic>>()) {
            result.Add(new CollectionContainer { Collection = collection });
        }
        return result;
    }

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

And usage of such converter looks like this in XAML:

<ComboBox.ItemsSource>
    <MultiBinding Converter="{StaticResource MultiItemSourcesConverter}">
        <Binding Path="FirstCollection" />
        <Binding Path="SecondCollection" />
    </MultiBinding>
</ComboBox.ItemsSource>