<TextBox    Grid.Column="0" Name="TextBox_SearchBar" AcceptsReturn="False" AcceptsTab="False" TextWrapping="Wrap" KeyUp="TextBox_SearchBar_KeyUp" />
<TextBox    Grid.Column="0" Name="TextBox_Watermark" Text="Search Test Sets" Foreground="Gray" GotFocus="TextBox_Watermark_GotFocus" />

private void TextBox_Watermark_GotFocus( object sender, RoutedEventArgs e ) {
    TextBox_Watermark.Visibility = Visibility.Hidden;
    TextBox_SearchBar.Focus();
}

这将创建两个文本框，一个用于接收水印的字段，另一个用于水印。接收水印的字段可以是ComboBox或其他控件(只要您获得正确的对齐并相应地获得焦点)。

水印必须是一个文本框或其他控制，可以获得焦点(TextBlock不能获得焦点，因为我学会了艰难的方式)。当水印文本框得到焦点，它隐藏自己，并给予访问其他文本框。

如果你想在搜索栏文本框失去焦点后重新显示水印，只需添加一个LostFocus事件处理程序到TextBox_SearchBar检查输入并切换TextBox_Watermark可见性为隐藏或可见，相应地。

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。

下面是我的方法是伟大的MVVM，在那里我也检查文本框是否有焦点，你也可以使用一个常规的触发器只是文本值以及重点是，当值改变时，我只是改变背景图像:

                    <TextBox.Style>
                        <Style TargetType="TextBox">

                            <Style.Triggers>
                                <MultiTrigger>
                                    <MultiTrigger.Conditions>
                                        <Condition Property="IsFocused" Value="True"/>
                                        <Condition Property="Text" Value=""/>
                                    </MultiTrigger.Conditions>
                                    <MultiTrigger.Setters>
                                        <Setter Property="Background">
                                            <Setter.Value>
                                                <ImageBrush ImageSource="/Images/Scan.PNG" Stretch="Uniform" AlignmentX="Left"/>
                                            </Setter.Value>
                                        </Setter>
                                    </MultiTrigger.Setters>
                                </MultiTrigger>

                            </Style.Triggers>
                        </Style>
                    </TextBox.Style>
                </TextBox>

MahApps。Metro for WPF有一个内置的水印控制，如果你不想自己滚动。使用起来相当简单。

 <AdornerDecorator>
            <TextBox Name="txtSomeText"
                     Width="200"
                     HorizontalAlignment="Right">
                <Controls:TextBoxHelper.Watermark>I'm a watermark!</Controls:TextBoxHelper.Watermark>
            </TextBox>
        </AdornerDecorator>

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,
        ...
    },
    ...
}