Binding in WPF to element of array specified by property

Question

Say I\'ve got some TextBlocks on my UI, something like so:

Answer 1

Another alternative is to use MultiBinding with a converter:

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApplication1"
        Title="MainWindow" Height="350" Width="525">
    <StackPanel Orientation="Vertical">
        <StackPanel.Resources>
            <local:FoodIndexConverter x:Key="foodIndexConverter" />
        </StackPanel.Resources>
        <TextBlock Text="{Binding DessertIndex}" />
        <TextBlock Text="{Binding Food[2]}" />
        <TextBlock>
                <TextBlock.Text>
                    <MultiBinding Converter="{StaticResource foodIndexConverter}">
                        <Binding Path="DessertIndex" />
                        <Binding Path="Food"/>
                    </MultiBinding>
                </TextBlock.Text>
        </TextBlock>
    </StackPanel>
</Window>

Then in the code-behind, the converter is defined something like this:

namespace WpfApplication1
{
    public class FoodIndexConverter : IMultiValueConverter
    {
        public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            if (values == null || values.Length != 2)
                return null;

            int? idx = values[0] as int?;
            object[] food = values[1] as object[];

            if (!idx.HasValue || food == null)
                return null;

            return food[idx.Value];
        }

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

Answer 2

Just To add on the great answer by Colin Thomsen.

You could also use C# dynamic keyword to make this solution work with pretty much every container type. Or even bind to multidimensional containers "{Binding Food[{Binding DessertIndex1}][{Binding DessertIndex2}]}"

public class ContainerDoubleAccessConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        try
        {
            dynamic idx1 = values[0];
            dynamic idx2 = values[1];
            dynamic container = values[2];

            return container[idx1][idx2];
        }
        catch (System.Exception err)
        {
            DebugTrace.Trace("bad conversion " + err.Message);
        }
        return null;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
    {
        return null;
    }
}

Answer 3

if you are going to the trouble of having a DesertIndex property on your DataContext, why not a property that dereferences the Food array with DesertIndex:

public object SelectedFood
{
    get { return Food[DessertIndex]; }
}    

public int DessertIndex
{
    get { return 2; }
}

public object[] Food
{
    get
    {
        return new object[]{"liver", "spam", "cake", "garlic" };
    }
}

then you can bind directly to that:

<TextBlock Text="{Binding SelectedFood}" />

This is essentially the "MVVM" approach: make the datacontext object have properties that are just right for binding to.