준비

<Window><!-- Windowの細かいのは省略 -->
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="50"/>
        </Grid.RowDefinitions>

        <ListBox Grid.Row="0" Name="ExampleList" Margin="10" SelectionMode="Single" ScrollViewer.VerticalScrollBarVisibility="Auto">
            <!-- SelectionMode="Single" ：リストから一つしか選択できない -->
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel Orientation="Horizontal">
                        <TextBlock Text="{Binding Id, StringFormat=IDは{0}　：}"/>
                        <TextBlock Text="{Binding Name, StringFormat= Nameは{0}　：}"/>
                        <TextBlock Text="{Binding Age, StringFormat= Ageは{0}}"/>
                    </StackPanel>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
        <Button Grid.Row="1" Name="Btn" Content="ボタン" Margin="100,10,100,10" Click="Btn_Click"/>
    </Grid>
</Window>

public partial class MainWindow : Window
{
    List<ExampleClass> list = new List<ExampleClass>();
    public MainWindow()
    {
        InitializeComponent();

        list.Add(new ExampleClass() { Id = 0, Name = "aaa", Age = 10 });
        list.Add(new ExampleClass() { Id = 1, Name = "bbb", Age = 20 });
        list.Add(new ExampleClass() { Id = 2, Name = "ccc", Age = 30 });
        list.Add(new ExampleClass() { Id = 3, Name = "ddd", Age = 40 });
        list.Add(new ExampleClass() { Id = 4, Name = "eee", Age = 50 });
        ExampleList.ItemsSource = list;
    }

    private void Btn_Click(object sender, RoutedEventArgs e)
    {

    }
}
class ExampleClass
{
    public int Id { get; set; }
    public string Name { get; set; }
    public byte Age { get; set; }
}

SelectedItem 꺼내기

    private void Btn_Click(object sender, RoutedEventArgs e)
    {
        // 選択項目が0 => メソッドを出る
        if (ExampleList.SelectedItems.Count == 0)
            return;

        // ExampleClassとして取り出す
        ExampleClass selitem = ExampleList.SelectedItem as ExampleClass;

        // 中身の確認
        Console.WriteLine("選択中の項目 Id:{0} Name:{1} Age{2}", selitem.Id, selitem.Name, selitem.Age);
    }

選択中の項目 Id:0 Name:aaa Age10