VB.NET WPF Listbox scroll by UP/DOWN button

I'm not familiar with VB, so I have a solution in C#.
Shouldn't be hard to translate in VB.

In the example some items are added to the listbox.
When the ListBox is Loaded, one listboxItem is measured.
The height of that listboxItem is then used to scroll up and down.

A better solution is to add a loaded event to one of the listboxItems and take the actualHeight of the Item.
If your Items are not the same height, measure every ListBoxItem and put those values in a list. Scroll according to the offsets in that list.

It's also possible to measure the position of the listboxItem relative to the ListBox.
Point relativePoint = ListBoxItem.TransformToAncestor(ListBoxTest) .Transform(new Point(0, 0));

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="auto"></ColumnDefinition>
            <ColumnDefinition></ColumnDefinition>
        </Grid.ColumnDefinitions>
        <StackPanel Orientation="Vertical">
            <Button MinWidth="100" Padding="5" Margin="5" Name="ButtonUp" Click="ButtonUp_Click">Up</Button>
            <Button MinWidth="100" Padding="5" Margin="5,0,5,5" Name="ButtonDown" Click="ButtonDown_Click">Down</Button>
        </StackPanel>
        <ScrollViewer Name="ScrollViewerTest" Grid.Column="1" VerticalScrollBarVisibility="Hidden" HorizontalScrollBarVisibility="Hidden">
            <ListBox Name="ListBoxTest" ScrollViewer.HorizontalScrollBarVisibility="Disabled" ScrollViewer.VerticalScrollBarVisibility="Disabled" Loaded="ListBoxTest_Loaded"/>
        </ScrollViewer>
    </Grid>
</Window>

public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            PopulateListBox();
        }

        private void PopulateListBox()
        {
            ListBoxTest.Items.Clear();
            for (int i=0;i<100;i++)
                ListBoxTest.Items.Add(new ListBoxItem() { Content = "Item " + i.ToString() });
        }

        double ItemOffset=0;
        private void ButtonUp_Click(object sender, RoutedEventArgs e)
        {
            ScrollViewerTest.ScrollToVerticalOffset(ScrollViewerTest.VerticalOffset - ItemOffset);
        }
        
        private void ButtonDown_Click(object sender, RoutedEventArgs e)
        {
            ScrollViewerTest.ScrollToVerticalOffset(ScrollViewerTest.VerticalOffset + ItemOffset);
        }

        private void ListBoxTest_Loaded(object sender, RoutedEventArgs e)
        {
            if (ListBoxTest.Items.Count == 0) return;

            if (ItemOffset == 0)
            {
                ((ListBoxItem)ListBoxTest.Items[0]).Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
                ItemOffset = ((ListBoxItem)ListBoxTest.Items[0]).DesiredSize.Height;
            }
        }
    }