How to display the value from the database with the desired id in the TextBlock when clicking on a tree element?

Question

WPF

How to make that when selecting a tree element in a TextBlock, the name of the corresponding test topic is displayed

public partial class PageTest : Page
    {
        private Theme _currentTheme = new Theme();
        public PageTest()
        {
            InitializeComponent();
            DataContext = _currentTheme;

        }

        private void Test1_Expanded(object sender, RoutedEventArgs e)
        {
            if (_currentTheme.id_theme == 1)
            {
            
            }
        }

<TextBlock  Margin="20, 0,10,10" Name="TxbQuestion" Height="auto" Text="{Binding themeName}"></TextBlock>
<TreeViewItem Header=" Тест 1 " Name="Test1" Expanded="Test1_Expanded" >