<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;
      }
    }
  }