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>

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);
    }
}

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>

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;
    }
}

这里有一个最简单的解决方案:

            <Grid>
                <Label Content="Placeholder text" VerticalAlignment="Center" Margin="10">
                    <Label.Style>
                        <Style TargetType="Label">
                            <Setter Property="Foreground" Value="Transparent"/>
                            <Style.Triggers>
                                <DataTrigger Binding="{Binding Expression}" Value="">
                                    <Setter Property="Foreground" Value="Gray"/>
                                </DataTrigger>
                            </Style.Triggers>
                        </Style>
                    </Label.Style>
                </Label>
                <TextBox HorizontalAlignment="Stretch" Margin="5" Background="Transparent"
                 Text="{Binding Expression, UpdateSourceTrigger=PropertyChanged}" VerticalAlignment="Center" Padding="5">
                </TextBox>
        </Grid>

这是一个透明背景覆盖标签的文本框。标签的灰色文本通过数据触发器变成透明的，只要绑定的文本不是空字符串就会触发该触发器。

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;
}