Setting the scrollbar position of a ListBox

Question

Can I programatically set the position of a WPF ListBox's scrollbar? By default, I want it to go in the center.

Answer 1

To move the vertical scroll bar in a ListBox do the following:

1. Name your list box (x:Name="myListBox")
2. Add Loaded event for the Window (Loaded="Window_Loaded")
3. Implement Loaded event using method: ScrollToVerticalOffset

Here is a working sample:

XAML:

<Window x:Class="ListBoxScrollPosition.Views.MainView"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  Loaded="Window_Loaded"
  Title="Main Window" Height="100" Width="200">
  <DockPanel>
    <Grid>
      <ListBox x:Name="myListBox">
        <ListBoxItem>Zamboni</ListBoxItem>
        <ListBoxItem>Zamboni</ListBoxItem>
        <ListBoxItem>Zamboni</ListBoxItem>
        <ListBoxItem>Zamboni</ListBoxItem>
        <ListBoxItem>Zamboni</ListBoxItem>
        <ListBoxItem>Zamboni</ListBoxItem>
        <ListBoxItem>Zamboni</ListBoxItem>
        <ListBoxItem>Zamboni</ListBoxItem>
        <ListBoxItem>Zamboni</ListBoxItem>
        <ListBoxItem>Zamboni</ListBoxItem>
        <ListBoxItem>Zamboni</ListBoxItem>
        <ListBoxItem>Zamboni</ListBoxItem>
      </ListBox>
    </Grid>
  </DockPanel>
</Window>

C#

private void Window_Loaded(object sender, RoutedEventArgs e)
{
  // Get the border of the listview (first child of a listview)
  Decorator border = VisualTreeHelper.GetChild(myListBox, 0) as Decorator;
  if (border != null)
  {
    // Get scrollviewer
    ScrollViewer scrollViewer = border.Child as ScrollViewer;
    if (scrollViewer != null)
    {
      // center the Scroll Viewer...
      double center = scrollViewer.ScrollableHeight / 2.0;
      scrollViewer.ScrollToVerticalOffset(center);
    }
  }
}

Answer 2

Dim cnt as Integer = myListBox.Items.Count
Dim midPoint as Integer = cnt\2
myListBox.ScrollIntoView(myListBox.Items(midPoint))

or

myListBox.SelectedIndex = midPoint

It depends on if you want the middle item just shown, or selected.

Answer 3

I've just changed a bit code of Zamboni and added position calculation.

var border = VisualTreeHelper.GetChild(list, 0) as Decorator;
if (border == null) return;
var scrollViewer = border.Child as ScrollViewer;
if (scrollViewer == null) return;
scrollViewer.ScrollToVerticalOffset((scrollViewer.ScrollableHeight/list.Items.Count)*
                                    (list.Items.IndexOf(list.SelectedItem) + 1));

Answer 4

I don't think ListBoxes have that, but ListViews have the EnsureVisible method that moves the scrollbar to the place needed in order to make sure an item is shown.