Databinding selectedindex + 1 - wpf

Question

I would like to bind a textbox to show the text of the item below the item that is selected.
Example:
Item 1 - Text = row numer one
Item 2 - Text = row number two
Item 3 - Text = row number three
I select item 2
output
Textbox 1 - Text = row number two (this is easily set up binding to selecteditem)
Textbox 2 - Text = row number three
I was thinking of a custom xpath of selectedindex + 1 but this doesn't seem to work

Answer 1

first attempt failed -- see below
You need to implement an IValueConverter and set it to the Converter attribute of the binding.
Create a class that inherits from IValueConverter, and in the Convert method, you'll cast the value parameter to ListBox (because you'll be binding the TextBox to the ListBox itself and letting the converter turn that into something meaningful).
Then get a reference to the ListBox's SelectedIndex property.
You want to return listBox.Items[selectedIndex + 1] from the method.
You can leave the ConvertBack method unimplemented.
You'll also have to handle the case where the last item in the ListBox is selected, because index + 1 will be out of bounds. Maybe you want to return the first item; maybe you want to return null or string.Empty.
update: custom ListBox
As requested, here is a sample that uses a custom ListBox with an additional [Dependency] property called "ItemAfterSelected."
First, the code for the derived control:
using System.Windows;
using System.Windows.Controls;
namespace WpfApplication1
{
public class PlusOneListBox : ListBox
{
public PlusOneListBox()
{
SelectionMode = SelectionMode.Single;
}
public object ItemAfterSelected
{
get { return GetValue(ItemAfterSelectedProperty); }
set { SetValue(ItemAfterSelectedProperty, value); }
}
public static readonly DependencyProperty ItemAfterSelectedProperty = DependencyProperty.Register(
"ItemAfterSelected", typeof (object), typeof (PlusOneListBox));
protected override void OnSelectionChanged(SelectionChangedEventArgs e)
{
var newly_selected = e.AddedItems;
if (newly_selected == null) ItemAfterSelected = null;
else
{
var last_index = Items.Count - 1;
var index = Items.IndexOf(newly_selected[0]);
ItemAfterSelected = index < last_index
? Items[index + 1]
: null;
}
base.OnSelectionChanged(e);
}
}
}
Here is a sample window that shows how to use and bind to the control (you can drop this in to an app and run it to see it in action).
<Window x:Class="WpfApplication1.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:custom="clr-namespace:WpfApplication1" xmlns:System="clr-namespace:System;assembly=mscorlib" Padding="24">
<StackPanel>
<custom:PlusOneListBox x:Name="custom_listbox">
<custom:PlusOneListBox.Items>
<System:String>one</System:String>
<System:String>two</System:String>
<System:String>three</System:String>
<System:String>four</System:String>
<System:String>five</System:String>
<System:String>six</System:String>
</custom:PlusOneListBox.Items>
</custom:PlusOneListBox>
<StackPanel Orientation="Horizontal" Margin="8">
<TextBlock Text="Selected: " />
<TextBlock Text="{Binding SelectedItem, ElementName=custom_listbox}" />
</StackPanel>
<StackPanel Orientation="Horizontal" Margin="8">
<TextBlock Text="Next: " />
<TextBlock Text="{Binding ItemAfterSelected, ElementName=custom_listbox}" />
</StackPanel>
</StackPanel>
</Window>