DataTemplate.LoadContent Method将resource中的datatemplate转换为UIElement，可以用于对象添加

https://msdn.microsoft.com/zh-cn/library/windows/apps/windows.ui.xaml.datatemplate.loadcontent.aspx

<StackPanel  Name="rootStackPanel">

  <StackPanel.Resources>
    <DataTemplate x:Key="oddNumberTemplate">
      <Grid>
        <Rectangle Stroke="Purple" StrokeThickness="4" />
        <TextBlock HorizontalAlignment="Center" 
                   VerticalAlignment="Center" 
                   FontSize="24" Foreground="Blue" 
                   FontWeight="Bold"/>
      </Grid>
    </DataTemplate>


    <DataTemplate x:Key="evenNumberTemplate">
      <Grid>
        <Ellipse Stroke="Green" StrokeThickness="4"/>
        <TextBlock HorizontalAlignment="Center" 
                   VerticalAlignment="Center" 
                   FontSize="24" Foreground="Red" 
                   FontWeight="Bold"  />
      </Grid>
    </DataTemplate>
  </StackPanel.Resources>

  <Border Name="selectedItemDisplay"
          Width="50" Height="50"/>

  <ListBox Name="numberList" SelectionChanged="ListBox_SelectionChanged">
    <ListBoxItem Content="1"/>
    <ListBoxItem Content="2"/>
    <ListBoxItem Content="3"/>
    <ListBoxItem Content="4"/>
    <ListBoxItem Content="5"/>
    <ListBoxItem Content="6"/>
    <ListBoxItem Content="7"/>
    <ListBoxItem Content="8"/>
    <ListBoxItem Content="9"/>
    <ListBoxItem Content="10"/>
  </ListBox>

</StackPanel>

private void ListBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    ListBoxItem lbi = ((sender as ListBox).SelectedItem as ListBoxItem);
    SelectDataTemplate(lbi.Content);
}

private void SelectDataTemplate(object value)
{
    string numberStr = value as string;

    if (numberStr != null)
    {
        int num;

        try
        {
            num = Convert.ToInt32(numberStr);
        }
        catch
        {
            return;
        }

        DataTemplate template;

        // Select one of the DataTemplate objects, based on the 
        // value of the selected item in the ComboBox.
        if (num % 2 != 0)
        {
            template = rootStackPanel.Resources["oddNumberTemplate"] as DataTemplate;
        }
        else
        {
            template = rootStackPanel.Resources["evenNumberTemplate"] as DataTemplate;
        }

        selectedItemDisplay.Child = template.LoadContent() as UIElement;
        TextBlock tb = FindVisualChild<TextBlock>(selectedItemDisplay);
        tb.Text = numberStr;
    }
}

private childItem FindVisualChild<childItem>(DependencyObject obj)
    where childItem : DependencyObject
{
    for (int i = 0; i < VisualTreeHelper.GetChildrenCount(obj); i++)
    {
        DependencyObject child = VisualTreeHelper.GetChild(obj, i);

        if (child != null && child is childItem)
        {
            return (childItem)child;
        }
        else
        {
            childItem childOfChild = FindVisualChild<childItem>(child);
            if (childOfChild != null)
                return childOfChild;
        }
    }
    return null;
}