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。

@Veton -我真的很喜欢你的解决方案的简单性，但我的声誉还没有高到足以撞到你。

@Tim Murphy -“双向绑定需要Path或XPath”的错误很容易修复…更新的代码，包括一些其他的小调整(仅WPF测试):

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Media;

public class TextBoxWatermarked : TextBox
{
  public string Watermark
  {
    get { return (string)GetValue(WaterMarkProperty); }
    set { SetValue(WaterMarkProperty, value); }
  }
  public static readonly DependencyProperty WaterMarkProperty =
      DependencyProperty.Register("Watermark", typeof(string), typeof(TextBoxWatermarked), new PropertyMetadata(new PropertyChangedCallback(OnWatermarkChanged)));

  private bool _isWatermarked = false;
  private Binding _textBinding = null;

  public TextBoxWatermarked()
  {
    Loaded += (s, ea) => ShowWatermark();
  }

  protected override void OnGotFocus(RoutedEventArgs e)
  {
    base.OnGotFocus(e);
    HideWatermark();
  }

  protected override void OnLostFocus(RoutedEventArgs e)
  {
    base.OnLostFocus(e);
    ShowWatermark();
  }

  private static void OnWatermarkChanged(DependencyObject sender, DependencyPropertyChangedEventArgs ea)
  {
    var tbw = sender as TextBoxWatermarked;
    if (tbw == null || !tbw.IsLoaded) return; //needed to check IsLoaded so that we didn't dive into the ShowWatermark() routine before initial Bindings had been made
    tbw.ShowWatermark();
  }

  private void ShowWatermark()
  {
    if (String.IsNullOrEmpty(Text) && !String.IsNullOrEmpty(Watermark))
    {
      _isWatermarked = true;

      //save the existing binding so it can be restored
      _textBinding = BindingOperations.GetBinding(this, TextProperty);

      //blank out the existing binding so we can throw in our Watermark
      BindingOperations.ClearBinding(this, TextProperty);

      //set the signature watermark gray
      Foreground = new SolidColorBrush(Colors.Gray);

      //display our watermark text
      Text = Watermark;
    }
  }

  private void HideWatermark()
  {
    if (_isWatermarked)
    {
      _isWatermarked = false;
      ClearValue(ForegroundProperty);
      Text = "";
      if (_textBinding != null) SetBinding(TextProperty, _textBinding);
    }
  }

}

这是一个演示如何在WPF中创建水印文本框的示例:

<Window x:Class="WaterMarkTextBoxDemo.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:WaterMarkTextBoxDemo"
    Height="200" Width="400">

    <Window.Resources>

        <SolidColorBrush x:Key="brushWatermarkBackground" Color="White" />
        <SolidColorBrush x:Key="brushWatermarkForeground" Color="LightSteelBlue" />
        <SolidColorBrush x:Key="brushWatermarkBorder" Color="Indigo" />

        <BooleanToVisibilityConverter x:Key="BooleanToVisibilityConverter" />
        <local:TextInputToVisibilityConverter x:Key="TextInputToVisibilityConverter" />

        <Style x:Key="EntryFieldStyle" TargetType="Grid" >
            <Setter Property="HorizontalAlignment" Value="Stretch" />
            <Setter Property="VerticalAlignment" Value="Center" />
            <Setter Property="Margin" Value="20,0" />
        </Style>

    </Window.Resources>


    <Grid Background="LightBlue">

        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition />
            <RowDefinition />
        </Grid.RowDefinitions>

        <Grid Grid.Row="0" Background="{StaticResource brushWatermarkBackground}" Style="{StaticResource EntryFieldStyle}" >
            <TextBlock Margin="5,2" Text="This prompt dissappears as you type..." Foreground="{StaticResource brushWatermarkForeground}"
                       Visibility="{Binding ElementName=txtUserEntry, Path=Text.IsEmpty, Converter={StaticResource BooleanToVisibilityConverter}}" />
            <TextBox Name="txtUserEntry" Background="Transparent" BorderBrush="{StaticResource brushWatermarkBorder}" />
        </Grid>

        <Grid Grid.Row="1" Background="{StaticResource brushWatermarkBackground}" Style="{StaticResource EntryFieldStyle}" >
            <TextBlock Margin="5,2" Text="This dissappears as the control gets focus..." Foreground="{StaticResource brushWatermarkForeground}" >
                <TextBlock.Visibility>
                    <MultiBinding Converter="{StaticResource TextInputToVisibilityConverter}">
                        <Binding ElementName="txtUserEntry2" Path="Text.IsEmpty" />
                        <Binding ElementName="txtUserEntry2" Path="IsFocused" />
                    </MultiBinding>
                </TextBlock.Visibility>
            </TextBlock>
            <TextBox Name="txtUserEntry2" Background="Transparent" BorderBrush="{StaticResource brushWatermarkBorder}" />
        </Grid>

    </Grid>

</Window>

TextInputToVisibilityConverter定义为:

using System;
using System.Windows.Data;
using System.Windows;

namespace WaterMarkTextBoxDemo
{
    public class TextInputToVisibilityConverter : IMultiValueConverter
    {
        public object Convert( object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture )
        {
            // Always test MultiValueConverter inputs for non-null
            // (to avoid crash bugs for views in the designer)
            if (values[0] is bool && values[1] is bool)
            {
                bool hasText = !(bool)values[0];
                bool hasFocus = (bool)values[1];

                if (hasFocus || hasText)
                    return Visibility.Collapsed;
            }

            return Visibility.Visible;
        }


        public object[] ConvertBack( object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture )
        {
            throw new NotImplementedException();
        }
    }
}

注意:这不是我的代码。我是在这里找到的，但我认为这是最好的方法。

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);
    }
}

如果水印的可见性不是取决于控件的焦点状态，而是取决于用户是否输入了任何文本，您可以将John Myczek的答案(从OnWatermarkChanged向下)更新为

static void OnWatermarkChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) {
    var textbox = (TextBox)d;
    textbox.Loaded += UpdateWatermark;
    textbox.TextChanged += UpdateWatermark;
}

static void UpdateWatermark(object sender, RoutedEventArgs e) {
    var textbox = (TextBox)sender;
    var layer = AdornerLayer.GetAdornerLayer(textbox);
    if (layer != null) {
        if (textbox.Text == string.Empty) {
            layer.Add(new WatermarkAdorner(textbox, GetWatermark(textbox)));
        } else {
            var adorners = layer.GetAdorners(textbox);
            if (adorners == null) {
                return;
            }

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

如果文本框在显示表单或绑定到Text属性时自动获得焦点，这就更有意义了。

同样，如果你的水印总是一个字符串，你需要水印的风格来匹配文本框的风格，那么在Adorner做:

contentPresenter = new ContentPresenter {
    Content = new TextBlock {
        Text = (string)watermark,
        Foreground = Control.Foreground,
        Background = Control.Background,
        FontFamily = Control.FontFamily,
        FontSize = Control.FontSize,
        ...
    },
    ...
}

<TextBox x:Name="OrderTxt" HorizontalAlignment="Left" VerticalAlignment="Top" VerticalContentAlignment="Center" Margin="10,10,0,0" Width="188" Height="32"/>

<Label IsHitTestVisible="False" Content="Order number" DataContext="{Binding ElementName=OrderTxt}" Foreground="DarkGray">
    <Label.Style>
        <Style TargetType="{x:Type Label}">
            <Setter Property="Visibility" Value="Collapsed"/>
            <Setter Property="Width" Value="{Binding Width}"/>
            <Setter Property="Height" Value="{Binding Height}"/>
            <Setter Property="Margin" Value="{Binding Margin}"/>
            <Setter Property="VerticalAlignment" Value="{Binding VerticalAlignment}"/>
            <Setter Property="HorizontalAlignment" Value="{Binding HorizontalAlignment}"/>
            <Setter Property="VerticalContentAlignment" Value="{Binding VerticalContentAlignment}"/>
            <Style.Triggers>
                <DataTrigger Binding="{Binding Text}" Value="">
                    <Setter Property="Visibility" Value="Visible"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </Label.Style>
</Label>