WPF 打印不显示的元素

<Window x:Class="_097打印不显示的元素.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="184" Width="269">
    <Grid Margin="5">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"></RowDefinition>
            <RowDefinition Height="Auto"></RowDefinition>
        </Grid.RowDefinitions>
        <StackPanel Orientation="Horizontal">
            <Label>Scale Percentage:</Label>
            <TextBox MinWidth="50" Name="txtScale">500</TextBox>
        </StackPanel>
        <Button Grid.Row="1" Name="btnPrint" Click="btnPrint_Click_1">Print</Button>
    </Grid>

</Window>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;


namespace _097打印不显示的元素
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }


        private void btnPrint_Click_1(object sender, RoutedEventArgs e)
        {
            PrintDialog dialog = new PrintDialog();
            if (dialog.ShowDialog() == true)
            {
                Run run = new Run("This is a test of printing function");
                TextBlock visual = new TextBlock();
                visual.Inlines.Add(run);
                visual.Margin = new Thickness(15);
                visual.TextWrapping = TextWrapping.Wrap;
                double zoom;
                if (double.TryParse(txtScale.Text, out zoom))
                {
                    visual.LayoutTransform = new ScaleTransform(zoom / 100, zoom / 100);
                    Size pageSize = new Size(dialog.PrintableAreaWidth, dialog.PrintableAreaHeight);
                    visual.Measure(pageSize);
                    visual.Arrange(new Rect(0, 0, pageSize.Width, pageSize.Height));
                    dialog.PrintVisual(visual, "A Scaled Drawing");
                }
                else
                {
                    MessageBox.Show("Invalid Scale Value");
                }
            }
        }
    }
}