How to find a control in a TabItem - wpf

Question

I want to find a control within a TabItem. Using FindName it is not possible. I found a way traversing the visual tree but that looks rather cumbersome. Why does FindName not do the job?
XAM:
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<Button
Content="Click"
Click="Button_Click"
/>
<TabControl Grid.Row="1" x:Name="tabControl">
<TabControl.ContentTemplate>
<DataTemplate>
<StackPanel>
<TextBox
x:Name="textBox"
Text="{Binding DataContext.Message, ElementName=mainWindow}"
/>
</StackPanel>
</DataTemplate>
</TabControl.ContentTemplate>
<TabItem Header="One" />
<TabItem Header="Two" />
<TabItem Header="Three" />
</TabControl>
</Grid>
Code behind:
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
}
private void Button_Click(object sender, RoutedEventArgs e)
{
TabControl tabControl = FindName("tabControl") as TabControl;
TextBox textBox = FindName("textBox") as TextBox;
TabItem tabItem = tabControl.SelectedItem as TabItem;
textBox = tabItem.FindName("textBox") as TextBox;
}
}
"textBox" is null regardless whether searched from top or from selected tabItem.

Answer 1

The TextBox is not a visual child of the TabItem. If you look at the visual tree, you'll see that the contents of the currently selected tab is hosted in a ContentPresenter that is part of the TabControl's ControlTemplate.
This should work though:
private void Button_Click(object sender, RoutedEventArgs e)
{
ContentPresenter cp = tabControl.Template.FindName("PART_SelectedContentHost", tabControl) as ContentPresenter;
TextBox textBox = cp.ContentTemplate.FindName("textBox", cp) as TextBox;
}