为什么我无法在DataTemplate中找到元素？

Question

我的XAML设计中有一个DataTemplate。

sixth_nnd

我这样称呼它：

<DataTemplate x:Key="dt1">
    <StackPanel Orientation="Vertical" Height="388">
        <RelativePanel>
            <TextBlock x:Name="titleText" Text="{Binding Path=titleText}" Foreground="White" FontSize="40" TextLineBounds="TrimToCapHeight" Margin="0,6,0,0" />
        </RelativePanel>
    </StackPanel>
</DataTemplate>

然后，我使用下面的代码尝试在<ContentControl x:Name="r3" ContentTemplate="{StaticResource dt1}"/> 内找到titleText。

DataTemplate

但是我失败了，日志在下面：

public object FindChildInDT<T>(T inputElement, string childName)
{
    DataTemplate dt = (inputElement as ContentControl).ContentTemplate;
    DependencyObject dio = dt.LoadContent();
    foreach (var tb1 in FindVisualChildren<ContentControl>(dio))
    {
        if (tb1.Name == childName)
        {
            return tb1;
        }
    }
    return null;
}

private static IEnumerable<T> FindVisualChildren<T>(DependencyObject depObj) where T : DependencyObject
{
    if (depObj != null)
    {
        for (int i = 0; i < VisualTreeHelper.GetChildrenCount(depObj); i++)
        {
            DependencyObject child = VisualTreeHelper.GetChild(depObj, i);
            if (child != null && child is T)
            {
                yield return (T)child;
            }

            foreach (T childOfChild in FindVisualChildren<T>(child))
            {
                yield return childOfChild;
            }
        }
    }
}

private void Grid_Loaded(object sender, RoutedEventArgs e)
{
    TextBlock tts = FindChildInDT(r3, "titleText") as TextBlock;
    if (tts == null)
        Debug.WriteLine("tts is null");
    else
        Debug.WriteLine("tts is not null");
}