WPF binding textbox to item of a string collection

Question

I have a StringCollection in my settings and want to bind 1 of the items to a label.

This is how it works.

xmlns:p="clr-namespace:MyNamespace.Properties" 

<Label Content="{Binding Path=MyStringCollection.[2], Source={x:Static p:Settings.Default}}" />

But I want to bind the index to another value and thought that this should work. But it doesnt.

<Label Content="{Binding Path=MyStringCollection.[{Binding SelectedIndex Source={x:Static p:Settings.Default}}], Source={x:Static p:Settings.Default}}" />

Can someone help me?

Answer 1

With stock WPF you'll need to use an IMultiValueConverter:

public class IndexedValueConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType,
        object parameter, CultureInfo culture)
    {
        if (values.Length < 2) return null;
        var index = Convert.ToInt32(values[1], culture);
        var array = values[0] as Array;
        if (array != null) return array.GetValue(index);
        var list = values[0] as IList;
        if (list != null) return list[index];
        var enumerable = values[0] as IEnumerable;
        if (enumerable != null)
        {
            int ii = 0;
            foreach (var item in enumerable)
            {
                if (ii++ == index) return item;
            }
        }

        return null;
    }

// ... Implement ConvertBack as desired

Then in your XAML:

<Label>
    <Label.Resources>
        <local:IndexedValueConverter x:Key="Indexer" />
    </Label.Resources>
    <Label.Content>
        <MultiBinding Converter="{StaticResource Indexer}">
            <Binding Path="MyStringCollection"
                     Source="{x:Static p:Settings.Default}" />
            <Binding Path="SelectedIndex"
                     Source="{x:Static p:Settings.Default}" />
        </MultiBinding>
    </Label.Content>
</Label>