WPF TextBox和PasswordBox添加水印

<TextBox Name="txtBoxName" Width="120" Height="23">      <TextBox.Resources>        <VisualBrush x:Key="HelpBrush" TileMode="None" Opacity="0.3" Stretch="None" AlignmentX="Left">          <VisualBrush.Visual>            <TextBlock FontStyle="Italic" Text="水印效果"/>          </VisualBrush.Visual>        </VisualBrush>      </TextBox.Resources>      <TextBox.Style>        <Style TargetType="TextBox">          <Setter Property="Height" Value="23"/>          <Setter Property="HorizontalAlignment" Value="Left"/>          <Setter Property="VerticalAlignment" Value="Top"/>          <Style.Triggers>            <Trigger Property="Text" Value="{x:Null}">              <Setter Property="Background" Value="{StaticResource HelpBrush}"/>            </Trigger>            <Trigger Property="Text" Value="">              <Setter Property="Background" Value="{StaticResource HelpBrush}"/>            </Trigger>          </Style.Triggers>        </Style>      </TextBox.Style>    </TextBox>

public class PasswordBoxMonitor : DependencyObject  {    public static bool GetIsMonitoring(DependencyObject obj)    {      return (bool)obj.GetValue(IsMonitoringProperty);    }    public static void SetIsMonitoring(DependencyObject obj, bool value)    {      obj.SetValue(IsMonitoringProperty, value);    }    public static readonly DependencyProperty IsMonitoringProperty =      DependencyProperty.RegisterAttached("IsMonitoring", typeof(bool), typeof(PasswordBoxMonitor), new UIPropertyMetadata(false, OnIsMonitoringChanged));    public static int GetPasswordLength(DependencyObject obj)    {      return (int)obj.GetValue(PasswordLengthProperty);    }    public static void SetPasswordLength(DependencyObject obj, int value)    {      obj.SetValue(PasswordLengthProperty, value);    }    public static readonly DependencyProperty PasswordLengthProperty =      DependencyProperty.RegisterAttached("PasswordLength", typeof(int), typeof(PasswordBoxMonitor), new UIPropertyMetadata(0));    private static void OnIsMonitoringChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)    {      var pb = d as PasswordBox;      if (pb == null)      {        return;      }      if ((bool)e.NewValue)      {        pb.PasswordChanged += PasswordChanged;      }      else      {        pb.PasswordChanged -= PasswordChanged;      }    }    static void PasswordChanged(object sender, RoutedEventArgs e)    {      var pb = sender as PasswordBox;      if (pb == null)      {        return;      }      SetPasswordLength(pb, pb.Password.Length);    }  }

<PasswordBox Name="pb" Width="120" VerticalAlignment="Bottom" Height="35">      <PasswordBox.Style>        <Style TargetType="PasswordBox">          <Setter Property="Height" Value="23"/>          <Setter Property="HorizontalAlignment" Value="Left"/>          <Setter Property="VerticalAlignment" Value="Top"/>          <Setter Property="local:PasswordBoxMonitor.IsMonitoring" Value="True"/>          <Setter Property="Template">            <Setter.Value>              <ControlTemplate TargetType="{x:Type PasswordBox}">                <Border Name="Bd" Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" SnapsToDevicePixels="True">                  <Grid>                    <ScrollViewer x:Name="PART_ContentHost" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>                    <StackPanel Orientation="Horizontal" Visibility="Collapsed" Name="myStackPanel">                      <TextBlock HorizontalAlignment="Left" VerticalAlignment="Center" Foreground="LightGray" Text="水印效果"/>                    </StackPanel>                  </Grid>                </Border>                <ControlTemplate.Triggers>                  <Trigger Property="IsEnabled" Value="false">                    <Setter Property="Visibility" TargetName="myStackPanel" Value="Collapsed"/>                  </Trigger>                  <Trigger Property="local:PasswordBoxMonitor.PasswordLength" Value="0">                    <Setter Property="Visibility" TargetName="myStackPanel" Value="Visible"/>                  </Trigger>                </ControlTemplate.Triggers>              </ControlTemplate>            </Setter.Value>          </Setter>        </Style>      </PasswordBox.Style>    </PasswordBox>

<TextBox x:Class="OracleCodeGenerator.watermarkTextBox"       xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"       xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"       xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"        xmlns:local="clr-namespace:OracleCodeGenerator"       mc:Ignorable="d"        d:DesignHeight="300" d:DesignWidth="300" Name="tb">  <TextBox.Resources>    <VisualBrush x:Key="HelpBrush" TileMode="None" Opacity="0.3" Stretch="None" AlignmentX="Left">      <VisualBrush.Visual>        <TextBlock Text="{Binding TbText,ElementName=tb}" FontStyle="Italic"/>      </VisualBrush.Visual>    </VisualBrush>  </TextBox.Resources>  <TextBox.Style>    <Style TargetType="TextBox">      <Setter Property="Height" Value="23"/>      <Setter Property="HorizontalAlignment" Value="Left"/>      <Setter Property="VerticalAlignment" Value="Top"/>      <Style.Triggers>        <Trigger Property="Text" Value="{x:Null}">          <Setter Property="Background" Value="{StaticResource HelpBrush}"/>        </Trigger>        <Trigger Property="Text" Value="">          <Setter Property="Background" Value="{StaticResource HelpBrush}"/>        </Trigger>      </Style.Triggers>    </Style>  </TextBox.Style></TextBox>

public partial class watermarkTextBox : TextBox  {    public watermarkTextBox()    {      InitializeComponent();    }    private string tbText;    public string TbText    {      get      {        return tbText;      }      set      {        tbText = value;      }    }  }