Why is there error?
I want screen out selected item's content(In this example .Text)

XAML:

```
<ListBox Name="Lbox" SelectionChanged="Lbox_SelectionChanged"
         SelectionMode="Extended">
    <TextBlock>AAA</TextBlock>
    <TextBlock>BBB</TextBlock>
    <TextBlock>CLabel</TextBlock>
</ListBox>
<Button Click="Button_Click">Click</Button>

```

Code:

```
private void Button_Click(object sender, RoutedEventArgs e)
{
    StringBuilder str = new StringBuilder();
    foreach (ListBoxItem item in Lbox.Items)
    {
        if (item.IsSelected)
        {
            TextBlock t = item as TextBlock;  // Error, Can not cast. But why?
            str.Append(t.Text + " ");
        }
    }
    MessageBox.Show(str.ToString());
}

```