嗨，我把这个任务变成了一个行为。你只需要在文本框中添加这样的东西

<i:Interaction.Behaviors>
         <Behaviors:TextBoxWatermarkBehavior Label="Test Watermark" LabelStyle="{StaticResource StyleWatermarkLabel}"/>
</i:Interaction.Behaviors>

你可以在这里找到我的博客文章

这个库有水印。

Nuget包

示例用法:

<TextBox adorners:Watermark.Text="Write something here" 
         adorners:Watermark.TextStyle="{StaticResource AdornerTextStyle}"
         adorners:Watermark.VisibleWhen="EmptyAndNotKeyboardFocused"/>

我已经创建了一个简单的代码实现，它适用于WPF和Silverlight:

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

public class TextBoxWatermarked : TextBox
{
    #region [ Dependency Properties ]

    public static DependencyProperty WatermarkProperty = DependencyProperty.Register
    (
         "Watermark",
         typeof(string),
         typeof(TextBoxWatermarked),
         new PropertyMetadata(new PropertyChangedCallback(OnWatermarkChanged))
    );
    
    #endregion
    
    
    #region [ Fields ]
    
    private bool _isWatermarked;
    private Binding _textBinding;

    #endregion


    #region [ Properties ]

    protected new Brush Foreground
    {
        get { return base.Foreground; }
        set { base.Foreground = value; }
    }

    public string Watermark
    {
        get { return (string)GetValue(WatermarkProperty); }
        set { SetValue(WatermarkProperty, value); }
    }

    #endregion


    #region [ .ctor ]
    
    public TextBoxWatermarked()
    {
        Loaded += (s, ea) => ShowWatermark();
    }
    
    #endregion


    #region [ Event Handlers ]

    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) return;
        tbw.ShowWatermark();
    }

    #endregion


    #region [ Methods ]

    private void ShowWatermark()
    {
        if (string.IsNullOrEmpty(base.Text))
        {
            _isWatermarked = true;
            base.Foreground = new SolidColorBrush(Colors.Gray);
            var bindingExpression = GetBindingExpression(TextProperty);
            _textBinding = bindingExpression == null ? null : bindingExpression.ParentBinding;
            if (bindingExpression != null)
                bindingExpression.UpdateSource();
            SetBinding(TextProperty, new Binding());
            base.Text = Watermark;
        }
    }

    private void HideWatermark()
    {
        if (_isWatermarked)
        {
            _isWatermarked = false;
            ClearValue(ForegroundProperty);
            base.Text = "";
            SetBinding(TextProperty, _textBinding ?? new Binding());
        }
    }

    #endregion
}

用法:

<TextBoxWatermarked Watermark="Some text" />

<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>

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>

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;
    }
}