Head First C# - Kapitel 1 Beispiel

Question

I`m Tring von Head First C# lernen, aber ich erhalte diesen Fehler im ersten Kapitel:

Weitere Informationen: angegebene Sicht ist bereits ein Kind einer anderen Sicht oder die Wurzel eines CompositionTarget.

Ich bin mir ziemlich sicher, dass der Code genau wie in dem Buch ist, aber ich bin mit VS2015 und das Beispiel ist FLR VS2013:

Hier ist mein Code:

<Window x:Class="Save_the_humans.MainWindow" 
     xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
     xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
     xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
     xmlns:local="clr-namespace:Save_the_humans" 
     mc:Ignorable="d" 
     Title="Save the Humans" Height="700" Width="1000"> 
    <Window.Resources> 
     <ControlTemplate x:Key="EnemyTemplate" TargetType="{x:Type ContentControl}"> 
      <Grid> 
       <Ellipse Fill="#FFD61001" Height="100" Stroke="Black" Width="100"/> 
      </Grid> 
     </ControlTemplate> 
    </Window.Resources> 
    <Grid Background="Black"> 
     <Grid.ColumnDefinitions> 
      <ColumnDefinition Width="140"/> 
      <ColumnDefinition/> 
      <ColumnDefinition Width="160"/> 
     </Grid.ColumnDefinitions> 
     <Grid.RowDefinitions> 
      <RowDefinition/> 
      <RowDefinition Height="150"/> 
     </Grid.RowDefinitions> 
     <Button x:Name="startButton" Content="Start!" HorizontalAlignment="Center" 
       Grid.Row="1" VerticalAlignment="Center" Click="startButton_Click"/> 
     <ProgressBar x:Name="progressBar" Grid.Column="1" Grid.Row="1" Height="20"/> 
     <StackPanel Grid.Column="2" Orientation="Vertical" Grid.Row="1" HorizontalAlignment="Center" VerticalAlignment="Center"> 
      <TextBox x:Name="AvoidThese" 
       TextWrapping="Wrap" Text="Avoid These" FontSize="18" VerticalAlignment="Center" HorizontalAlignment="Center" Foreground="White" Background="Black"/> 
      <ContentControl Content="ContentControl" HorizontalAlignment="Center" VerticalAlignment="Center" Template="{DynamicResource EnemyTemplate}"/> 
     </StackPanel> 
     <Canvas x:Name="playArea" Grid.ColumnSpan="3"> 
      <Canvas.Background> 
       <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0"> 
        <GradientStop Color="#FF0F259A" Offset="0.096"/> 
        <GradientStop Color="#FF25C714" Offset="1"/> 
       </LinearGradientBrush> 
      </Canvas.Background> 
      <StackPanel x:Name="Human" Orientation="Vertical"> 
       <Ellipse Fill="White" Width="10" Height="10" Stroke="Black"/> 
       <Rectangle Fill="White" Height="25" Width="10" Stroke="Black"/> 
      </StackPanel> 
      <TextBlock x:Name="gameOverText" Canvas.Left="210" TextWrapping="Wrap" Text="Game Over" Canvas.Top="195" Height="120" Width="535" FontFamily="Arial" FontSize="100" FontStyle="Italic" FontWeight="Bold"/> 
      <Rectangle x:Name="target" Height="50" Canvas.Left="505" Stroke="Black" Canvas.Top="85" Width="50" RenderTransformOrigin="0.5,0.5"> 
       <Rectangle.RenderTransform> 
        <TransformGroup> 
         <ScaleTransform/> 
         <SkewTransform/> 
         <RotateTransform Angle="45"/> 
         <TranslateTransform/> 
        </TransformGroup> 
       </Rectangle.RenderTransform> 
       <Rectangle.Fill> 
        <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0"> 
         <GradientStop Color="#FFAE0E64" Offset="0.245"/> 
         <GradientStop Color="#FF170202" Offset="1"/> 
        </LinearGradientBrush> 
       </Rectangle.Fill> 
      </Rectangle> 
     </Canvas> 
    </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; 
using System.Windows.Media.Animation; 
using System.Windows.Threading; 
namespace Save_the_humans 
{ 
    /// <summary> 
    /// Interaction logic for MainWindow.xaml 
    /// </summary> 
    public partial class MainWindow : Window 
    { 
     Random random = new Random(); 
     DispatcherTimer enemyTimer = new DispatcherTimer(); 
     DispatcherTimer targetTimer = new DispatcherTimer(); 
     bool humanCaptured = false; 
     public MainWindow() 
     { 
      InitializeComponent(); 

      enemyTimer.Tick += enemyTimer_Tick; 
      enemyTimer.Interval = TimeSpan.FromSeconds(2); 

      targetTimer.Tick += TargetTimer_Tick; 
      targetTimer.Interval = TimeSpan.FromSeconds(.1); 

     } 

     private void TargetTimer_Tick(object sender, EventArgs e) 
     { 
      progressBar.Value += 1; 
      if (progressBar.Value >= progressBar.Maximum) 
       EndTheGame(); 

     } 

     private void EndTheGame() 
     { 
      if(!playArea.Children.Contains(gameOverText)) 
      { 
       enemyTimer.Stop(); 
       targetTimer.Stop(); 
       humanCaptured = false; 
       startButton.Visibility = Visibility.Visible; 
       playArea.Children.Add(gameOverText); 
      } 
     } 

     private void enemyTimer_Tick(object sender, EventArgs e) 
     { 
      AddEnemy(); 
     } 

     private void startButton_Click(object sender, RoutedEventArgs e) 
     { 
      StartGame(); 
     } 

     private void StartGame() 
     { 
      Human.IsHitTestVisible = true; 
      humanCaptured = false; 
      progressBar.Value = 0; 
      startButton.Visibility = Visibility.Collapsed; 
      playArea.Children.Add(target); 
      playArea.Children.Add(Human); 
      enemyTimer.Start(); 
      targetTimer.Start(); 
     } 

     private void AddEnemy() 
     { 
      ContentControl enemy = new ContentControl(); 
      enemy.Template = Resources["EnemyTemplate"] as ControlTemplate; 
      AnimateEnemy(enemy, 0, playArea.ActualWidth - 100, "(Canvas.Left)"); 
      AnimateEnemy(enemy, random.Next((int)playArea.ActualHeight - 100), 
         random.Next((int)playArea.ActualHeight - 100), "(Canvas.Top)"); 
      playArea.Children.Add(enemy); 

     } 

     private void AnimateEnemy(ContentControl enemy, double from , double to , string propertyToAnimate) 
     { 
      Storyboard storyBoard = new Storyboard() { AutoReverse = true, RepeatBehavior = RepeatBehavior.Forever }; 
      DoubleAnimation animantion = new DoubleAnimation() 
      { 
       From = from, 
       To = to, 
       Duration = new Duration(TimeSpan.FromSeconds(random.Next(4, 6))), 
      }; 
      Storyboard.SetTarget(animantion, enemy); 
      Storyboard.SetTargetProperty(animantion, new PropertyPath(propertyToAnimate)); 
      storyBoard.Children.Add(animantion); 
      storyBoard.Begin(); 
     } 
    } 
} 

Answer 1

Wenn ich diese zwei Zeilen kommentieren, erhalte ich keinen Fehler

 playArea.Children.Add(target); 
     playArea.Children.Add(Human); 

irgendwie diese zwei bereits in der Leinwand von XAML sind .. oder ich weiß nicht,