我看到了John Myczek的解决方案，以及它关于ComboBox和PasswordBox兼容性的评论，所以我改进了John Myczek的解决方案，它是:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
using System.Windows.Documents;

/// <summary>
/// Class that provides the Watermark attached property
/// </summary>
public static class WatermarkService
{
    /// <summary>
    /// Watermark Attached Dependency Property
    /// </summary>
    public static readonly DependencyProperty WatermarkProperty = DependencyProperty.RegisterAttached(
       "Watermark",
       typeof(object),
       typeof(WatermarkService),
       new FrameworkPropertyMetadata((object)null, new PropertyChangedCallback(OnWatermarkChanged)));

    #region Private Fields

    /// <summary>
    /// Dictionary of ItemsControls
    /// </summary>
    private static readonly Dictionary<object, ItemsControl> itemsControls = new Dictionary<object, ItemsControl>();

    #endregion

    /// <summary>
    /// Gets the Watermark property.  This dependency property indicates the watermark for the control.
    /// </summary>
    /// <param name="d"><see cref="DependencyObject"/> to get the property from</param>
    /// <returns>The value of the Watermark property</returns>
    public static object GetWatermark(DependencyObject d)
    {
        return (object)d.GetValue(WatermarkProperty);
    }

    /// <summary>
    /// Sets the Watermark property.  This dependency property indicates the watermark for the control.
    /// </summary>
    /// <param name="d"><see cref="DependencyObject"/> to set the property on</param>
    /// <param name="value">value of the property</param>
    public static void SetWatermark(DependencyObject d, object value)
    {
        d.SetValue(WatermarkProperty, value);
    }

    /// <summary>
    /// Handles changes to the Watermark property.
    /// </summary>
    /// <param name="d"><see cref="DependencyObject"/> that fired the event</param>
    /// <param name="e">A <see cref="DependencyPropertyChangedEventArgs"/> that contains the event data.</param>
    private static void OnWatermarkChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        Control control = (Control)d;
        control.Loaded += Control_Loaded;

        if (d is TextBox || d is PasswordBox)
        {
            control.GotKeyboardFocus += Control_GotKeyboardFocus;
            control.LostKeyboardFocus += Control_Loaded;
        }
        else if (d is ComboBox)
        {
            control.GotKeyboardFocus += Control_GotKeyboardFocus;
            control.LostKeyboardFocus += Control_Loaded;
            (d as ComboBox).SelectionChanged += new SelectionChangedEventHandler(SelectionChanged);
        }
        else if (d is ItemsControl)
        {
            ItemsControl i = (ItemsControl)d;

            // for Items property  
            i.ItemContainerGenerator.ItemsChanged += ItemsChanged;
            itemsControls.Add(i.ItemContainerGenerator, i);

            // for ItemsSource property  
            DependencyPropertyDescriptor prop = DependencyPropertyDescriptor.FromProperty(ItemsControl.ItemsSourceProperty, i.GetType());
            prop.AddValueChanged(i, ItemsSourceChanged);
        }
    }

    /// <summary>
    /// Event handler for the selection changed event
    /// </summary>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">A <see cref="ItemsChangedEventArgs"/> that contains the event data.</param>
    private static void SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        Control control = (Control)sender;
        if (ShouldShowWatermark(control))
        {
            ShowWatermark(control);
        }
        else
        {
            RemoveWatermark(control);
        }
    }

    #region Event Handlers

    /// <summary>
    /// Handle the GotFocus event on the control
    /// </summary>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">A <see cref="RoutedEventArgs"/> that contains the event data.</param>
    private static void Control_GotKeyboardFocus(object sender, RoutedEventArgs e)
    {
        Control c = (Control)sender;
        if (ShouldShowWatermark(c))
        {
            RemoveWatermark(c);
        }
    }

    /// <summary>
    /// Handle the Loaded and LostFocus event on the control
    /// </summary>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">A <see cref="RoutedEventArgs"/> that contains the event data.</param>
    private static void Control_Loaded(object sender, RoutedEventArgs e)
    {
        Control control = (Control)sender;
        if (ShouldShowWatermark(control))
        {
            ShowWatermark(control);
        }
    }

    /// <summary>
    /// Event handler for the items source changed event
    /// </summary>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">A <see cref="EventArgs"/> that contains the event data.</param>
    private static void ItemsSourceChanged(object sender, EventArgs e)
    {
        ItemsControl c = (ItemsControl)sender;
        if (c.ItemsSource != null)
        {
            if (ShouldShowWatermark(c))
            {
                ShowWatermark(c);
            }
            else
            {
                RemoveWatermark(c);
            }
        }
        else
        {
            ShowWatermark(c);
        }
    }

    /// <summary>
    /// Event handler for the items changed event
    /// </summary>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">A <see cref="ItemsChangedEventArgs"/> that contains the event data.</param>
    private static void ItemsChanged(object sender, ItemsChangedEventArgs e)
    {
        ItemsControl control;
        if (itemsControls.TryGetValue(sender, out control))
        {
            if (ShouldShowWatermark(control))
            {
                ShowWatermark(control);
            }
            else
            {
                RemoveWatermark(control);
            }
        }
    }

    #endregion

    #region Helper Methods

    /// <summary>
    /// Remove the watermark from the specified element
    /// </summary>
    /// <param name="control">Element to remove the watermark from</param>
    private static void RemoveWatermark(UIElement control)
    {
        AdornerLayer layer = AdornerLayer.GetAdornerLayer(control);

        // layer could be null if control is no longer in the visual tree
        if (layer != null)
        {
            Adorner[] adorners = layer.GetAdorners(control);
            if (adorners == null)
            {
                return;
            }

            foreach (Adorner adorner in adorners)
            {
                if (adorner is WatermarkAdorner)
                {
                    adorner.Visibility = Visibility.Hidden;
                    layer.Remove(adorner);
                }
            }
        }
    }

    /// <summary>
    /// Show the watermark on the specified control
    /// </summary>
    /// <param name="control">Control to show the watermark on</param>
    private static void ShowWatermark(Control control)
    {
        AdornerLayer layer = AdornerLayer.GetAdornerLayer(control);

        // layer could be null if control is no longer in the visual tree
        if (layer != null)
        {
            layer.Add(new WatermarkAdorner(control, GetWatermark(control)));
        }
    }

    /// <summary>
    /// Indicates whether or not the watermark should be shown on the specified control
    /// </summary>
    /// <param name="c"><see cref="Control"/> to test</param>
    /// <returns>true if the watermark should be shown; false otherwise</returns>
    private static bool ShouldShowWatermark(Control c)
    {
        if (c is ComboBox)
        {
            return (c as ComboBox).SelectedItem == null;
            //return (c as ComboBox).Text == string.Empty;
        }
        else if (c is TextBoxBase)
        {
            return (c as TextBox).Text == string.Empty;
        }
        else if (c is PasswordBox)
        {
            return (c as PasswordBox).Password == string.Empty;
        }
        else if (c is ItemsControl)
        {
            return (c as ItemsControl).Items.Count == 0;
        }
        else
        {
            return false;
        }
    }

    #endregion
}

现在，一个组合框也可以编辑，密码框也可以添加水印。 别忘了用上面JoanComasFdz的评论来解决保证金问题。

当然，这都是约翰·麦切克的功劳。

只使用XAML，没有扩展，没有转换器:

<Grid>
    <TextBox  Width="250"  VerticalAlignment="Center" HorizontalAlignment="Left" x:Name="SearchTermTextBox" Margin="5"/>
    <TextBlock IsHitTestVisible="False" Text="Enter Search Term Here" VerticalAlignment="Center" HorizontalAlignment="Left" Margin="10,0,0,0" Foreground="DarkGray">
        <TextBlock.Style>
            <Style TargetType="{x:Type TextBlock}">
                <Setter Property="Visibility" Value="Collapsed"/>
                <Style.Triggers>
                    <DataTrigger Binding="{Binding Text, ElementName=SearchTermTextBox}" Value="">
                        <Setter Property="Visibility" Value="Visible"/>
                    </DataTrigger>
                </Style.Triggers>
            </Style>
        </TextBlock.Style>
    </TextBlock>
</Grid>

<Window.Resources>

    <Style x:Key="TextBoxUserStyle" BasedOn="{x:Null}" TargetType="{x:Type TextBox}">
      <Setter Property="Foreground" Value="Black"/>
      <Setter Property="HorizontalAlignment" Value="Center"/>
      <Setter Property="VerticalContentAlignment" Value="Center"/>
      <Setter Property="Width" Value="225"/>
      <Setter Property="Height" Value="25"/>
      <Setter Property="FontSize" Value="12"/>
      <Setter Property="Padding" Value="1"/>
      <Setter Property="Margin" Value="5"/>
      <Setter Property="AllowDrop" Value="true"/>
      <Setter Property="FocusVisualStyle" Value="{x:Null}"/>
      <Setter Property="Template">
        <Setter.Value>
          <ControlTemplate TargetType="{x:Type TextBox}">
            <Border x:Name="OuterBorder" BorderBrush="#5AFFFFFF" BorderThickness="1,1,1,1" CornerRadius="4,4,4,4">
              <Border x:Name="InnerBorder" Background="#FFFFFFFF" BorderBrush="#33000000" BorderThickness="1,1,1,1" CornerRadius="3,3,3,3">
                <ScrollViewer SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" x:Name="PART_ContentHost"/>
              </Border>
            </Border>
          </ControlTemplate>
        </Setter.Value>
      </Setter>
    </Style>

    <Style x:Key="PasswordBoxVistaStyle" BasedOn="{x:Null}" TargetType="{x:Type PasswordBox}">
      <Setter Property="Foreground" Value="Black"/>
      <Setter Property="HorizontalAlignment" Value="Center"/>
      <Setter Property="VerticalContentAlignment" Value="Center"/>
      <Setter Property="Width" Value="225"/>
      <Setter Property="Height" Value="25"/>
      <Setter Property="FontSize" Value="12"/>
      <Setter Property="Padding" Value="1"/>
      <Setter Property="Margin" Value="5"/>
      <Setter Property="AllowDrop" Value="true"/>
      <Setter Property="FocusVisualStyle" Value="{x:Null}"/>
      <Setter Property="Template">
        <Setter.Value>
          <ControlTemplate TargetType="{x:Type PasswordBox}">
            <Border x:Name="OuterBorder" BorderBrush="#5AFFFFFF" BorderThickness="1,1,1,1" CornerRadius="4,4,4,4">
              <Border x:Name="InnerBorder" Background="#FFFFFFFF" BorderBrush="#33000000" BorderThickness="1,1,1,1" CornerRadius="3,3,3,3">
                <Grid>
                  <Label x:Name="lblPwd" Content="Password" FontSize="11" VerticalAlignment="Center" Margin="2,0,0,0" FontFamily="Verdana" Foreground="#828385" Padding="0"/>
                  <ScrollViewer SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" x:Name="PART_ContentHost"/>
                </Grid>
              </Border>
            </Border>
            <ControlTemplate.Triggers>
              <Trigger Property="IsFocused" Value="True">
                <Setter Property="Visibility" TargetName="lblPwd" Value="Hidden"/>
              </Trigger>
            </ControlTemplate.Triggers>
          </ControlTemplate>
        </Setter.Value>
      </Setter>
    </Style>
  </Window.Resources>


        <PasswordBox Style="{StaticResource PasswordBoxVistaStyle}" Margin="169,143,22,0" Name="txtPassword" FontSize="14" TabIndex="2" Height="31" VerticalAlignment="Top" />

这可以帮助检查您的代码。当应用到密码框时，它将显示密码，当用户输入时，它将消失。

下面是XAML中另一个简单的解决方案:

XAML:

       <TextBox>
            <TextBox.Resources>
                <Style TargetType="TextBox">
                    <Style.Triggers>
                        <Trigger Property="IsFocused" Value="True">
                            <!--text color-->
                            <Setter Property="Foreground" Value="Black"/>
                            <Setter Property="Text" Value=""/>
                        </Trigger>
                        <Trigger Property="IsFocused" Value="False">
                            <!--placeholder color-->
                            <Setter Property="Foreground" Value="Gray"/>
                            <!--placeholder here-->
                            <Setter Property="Text" Value="Placeholder"/>
                        </Trigger>
                    </Style.Triggers>
                </Style>
            </TextBox.Resources>
        </TextBox>

还有，看看这个答案。你可以用VisualBrush和Style中的一些触发器更容易地完成这一点:

 <TextBox>
    <TextBox.Style>
        <Style TargetType="TextBox" xmlns:sys="clr-namespace:System;assembly=mscorlib">
            <Style.Resources>
                <VisualBrush x:Key="CueBannerBrush" AlignmentX="Left" AlignmentY="Center" Stretch="None">
                    <VisualBrush.Visual>
                        <Label Content="Search" Foreground="LightGray" />
                    </VisualBrush.Visual>
                </VisualBrush>
            </Style.Resources>
            <Style.Triggers>
                <Trigger Property="Text" Value="{x:Static sys:String.Empty}">
                    <Setter Property="Background" Value="{StaticResource CueBannerBrush}" />
                </Trigger>
                <Trigger Property="Text" Value="{x:Null}">
                    <Setter Property="Background" Value="{StaticResource CueBannerBrush}" />
                </Trigger>
                <Trigger Property="IsKeyboardFocused" Value="True">
                    <Setter Property="Background" Value="White" />
                </Trigger>
            </Style.Triggers>
        </Style>
    </TextBox.Style>
</TextBox>

为了增加此样式的可重用性，您还可以创建一组附加属性来控制实际的提示横幅文本、颜色、方向等。

当使用@john-myczek的代码绑定文本框时，我遇到了一些困难。由于TextBox在更新时不会引发焦点事件，水印将在新文本下面保持可见。为了解决这个问题，我简单地添加了另一个事件处理程序:

if (d is ComboBox || d is TextBox)
{
    control.GotKeyboardFocus += Control_GotKeyboardFocus;
    control.LostKeyboardFocus += Control_Loaded;

    if (d is TextBox)
        (d as TextBox).TextChanged += Control_TextChanged;
}


private static void Control_TextChanged(object sender, RoutedEventArgs e)
{
    var tb = (TextBox)sender;
    if (ShouldShowWatermark(tb))
    {
        ShowWatermark(tb);
    }
    else
    {
        RemoveWatermark(tb);
    }
}