如何在ListBox中获得垂直滚动条？

<ListBox x:Name="MyListBox" />

public MainWindow()
{
    InitializeComponent();

    for (int i = 1; i <= 50; i++)
    {
        MyListBox.Items.Add("Item " + i);
    }
}

<ListBox x:Name="MyListBox" ScrollViewer.VerticalScrollBarVisibility="Visible" />

<ListBox x:Name="MyListBox">
    <ListBox.Resources>
       <Style TargetType="{x:Type ScrollBar}">
           <Setter Property="Background" Value="LightGray" />
           <Setter Property="Width" Value="10" />
        </Style>
       <Style TargetType="{x:Type Thumb}">
           <Setter Property="Background" Value="Gray" />
        </Style>
       <Style TargetType="{x:Type RepeatButton}">
           <Setter Property="Background" Value="Transparent" />
        </Style>
    </ListBox.Resources>
</ListBox>