<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。

该技术使用Background属性来显示/隐藏占位符文本框。 占位符显示事件时，文本框有焦点

工作原理:

当为空时，文本框背景设置为透明以显示占位符文本。 当不空背景设置为白色，以掩盖占位符文本。

这里有一个基本的例子。出于我自己的目的，我把它变成了一个UserControl。

<Grid>
    <Grid.Resources>
        <ux:NotEmptyConverter x:Key="NotEmptyConverter" />

        <Style TargetType="{x:Type Control}" x:Key="DefaultStyle">
            <Setter Property="FontSize" Value="20" />
            <Setter Property="Margin" Value="10"/>
            <Setter Property="VerticalAlignment" Value="Center"></Setter>
            <Setter Property="VerticalContentAlignment" Value="Center"></Setter>
        </Style>

        <Style TargetType="{x:Type TextBox}" BasedOn="{StaticResource DefaultStyle}"></Style>

    </Grid.Resources>

    <Grid.RowDefinitions>
        <RowDefinition Height="Auto"/>
    </Grid.RowDefinitions>
    <TextBox Grid.Row="0" Text="Placeholder Text Is Here" Foreground="DarkGray" />
    <TextBox Grid.Row="0" Name="TextBoxEdit" 
            Text="{Binding Path=FirstName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" >
        <TextBox.Style>
            <Style TargetType="{x:Type TextBox}" BasedOn="{StaticResource DefaultStyle}">
                <Style.Triggers>
                    <DataTrigger Binding="{Binding Path=FirstName.Length, FallbackValue=0, TargetNullValue=0}" Value="0">
                        <Setter Property="Background" Value="Transparent"/>
                    </DataTrigger>
                    <DataTrigger Binding="{Binding Path=FirstName, FallbackValue=0, TargetNullValue=0, Converter={StaticResource NotEmptyConverter}}" Value="false">
                        <Setter Property="Background" Value="White"/>
                    </DataTrigger>
                </Style.Triggers>
            </Style>
        </TextBox.Style>
    </TextBox>
</Grid>

下面是ValueConverter，用于检测DataTrigger中的非空字符串。

public class NotEmptyConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        var s = value as string;
        return string.IsNullOrEmpty(s);
    }
    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return null;
    }
}

CodeProject上有一篇关于如何在“3行XAML”中做到这一点的文章。

<Grid Background="{StaticResource brushWatermarkBackground}">
  <TextBlock Margin="5,2" Text="Type something..."
             Foreground="{StaticResource brushForeground}"
             Visibility="{Binding ElementName=txtUserEntry, Path=Text.IsEmpty,
                          Converter={StaticResource BooleanToVisibilityConverter}}" />
  <TextBox Name="txtUserEntry" Background="Transparent"
           BorderBrush="{StaticResource brushBorder}" />
</Grid>

好吧，它可能不是3行XAML格式的，但它非常简单。

需要注意的一点是:文本上的IsEmpty属性不是string的属性，而是ICollectionView的属性，并且可以显式地设置为Path=Text.(componentModel:ICollectionView.IsEmpty) (with xmlns:componentModel="clr-namespace:System.ComponentModel;assembly=WindowsBase")。这里有详细的解释。

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>

设置文本框的占位符文本在一个柔和的颜色…

public MainWindow ( )
{
    InitializeComponent ( );
    txtInput.Text = "Type something here...";
    txtInput.Foreground = Brushes.DimGray;
}

当文本框获得焦点时，清除它并更改文本颜色

private void txtInput_GotFocus ( object sender, EventArgs e )
{
    MessageBox.Show ( "got focus" );
    txtInput.Text = "";
    txtInput.Foreground = Brushes.Red;
}

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。