Binding in WPF to element of array specified by property

Question

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

<StackPanel Orientation="Vertical">
    <TextBlock Text="{Binding DessertIndex}" />
    <TextBlock Text="{Binding Food[2]}" />
    <TextBlock Text="{Binding Food[{Binding DessertIndex}]}" />
</StackPanel>

and in my code behind I've got something like this:

public partial class MainWindow : Window
{
    public int DessertIndex
    {
        get { return 2; }
    }

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

    public MainWindow()
    {
        InitializeComponent();
        DataContext = this;
    }
}

The first two TextBlocks display fine for me, displaying 2 and 'cake' respectively. The third one doesn't accomplish what I'd like, namely use the DessertIndex property to index into that array and also display 'cake'. I did a little searching here on SO for a similar question but didn't find one. Ultimately, I don't want to specify values like 2 in my .xaml file and would like to rely upon a property instead for indexing into that array. Is this possible? If so, what am I doing wrong here?

EDIT:

So what I more closely have is a situation where the data is a List of these object[] and I'm using the above StackPanel as part of a DataTemplate for a ListBox. So the idea, as Mark Heath suggests below, of using a property that dereferences the array doesn't seem to work as I'd want. Ideas?

The "dereferencing" should work. List or not.

Boppity Bop
– Boppity Bop

2024-03-04 04:56:32 +00:00
Commented Mar 4, 2024 at 4:56 — Boppity Bop
– Boppity Bop, Commented Mar 4, 2024 at 4:56

Henry Malinowski · Accepted Answer · 2021-12-16 08:35:57Z

32

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();
        }
    }
}

edited Dec 16, 2021 at 8:35

Henry Malinowski

1111 silver badge6 bronze badges

answered Nov 4, 2010 at 20:21

Colin Thomsen

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2 Comments

itsmatt Over a year ago

Thanks Colin - this works very well for my test app and suspect it'll work just as well in the real thing. It's been a good day - I learned something really cool from you. :)

Anthony Wieser Over a year ago

You may be tempted to try something different using a ConverterParameter to avoid the multibinding. Unfortunately, that will lead nowhere, as the ConverterParameter cannot use a binding as it's not a DependencyProperty, and you must use MultiBinding

noonand · Accepted Answer · 2024-09-25 14:31:27Z

17

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

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.

edited Sep 25, 2024 at 14:31

noonand

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answered Nov 4, 2010 at 19:56

Mark Heath

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2 Comments

itsmatt Over a year ago

Mark, thanks for your response. See my edit to the original question for more detail. Essentially I'm dealing with a ListBox and a List of these object[]'s so I don't know how to make the dereferencing as you suggest work in this context. The data isn't mine and I've got to deal with it as it is. Ideas?

Aminos Over a year ago

I like this solution, it's simple. When setting DessertIndex, add a call to NotifyOfPropertyChange("SelectedFood") so elements that are binded to SelectedFood can be updated

Elad Maimoni · Accepted Answer · 2020-04-20 13:26:42Z

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;
    }
}

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