我决定通过一个行为来解决这个问题。它使用Hint属性定义要显示的文本(如果您愿意，也可以是一个对象)，并使用Value属性计算提示是否应该可见。

行为声明如下:

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

    public class HintBehavior : Behavior<ContentControl>
    {
        public static readonly DependencyProperty HintProperty = DependencyProperty
            .Register("Hint", typeof (string), typeof (HintBehavior)
            //, new FrameworkPropertyMetadata(null, OnHintChanged)
            );

        public string Hint
        {
            get { return (string) GetValue(HintProperty); }
            set { SetValue(HintProperty, value); }
        }

        public static readonly DependencyProperty ValueProperty = DependencyProperty
            .Register("Value", typeof (object), typeof (HintBehavior)
                , new FrameworkPropertyMetadata(null, OnValueChanged));

        private static void OnValueChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var visible = e.NewValue == null;
            d.SetValue(VisibilityProperty, visible ? Visibility.Visible : Visibility.Collapsed);
        }

        public object Value
        {
            get { return GetValue(ValueProperty); }
            set { SetValue(ValueProperty, value); }
        }

        public static readonly DependencyProperty VisibilityProperty = DependencyProperty
            .Register("Visibility", typeof (Visibility), typeof (HintBehavior)
                , new FrameworkPropertyMetadata(Visibility.Visible
                    //, new PropertyChangedCallback(OnVisibilityChanged)
                    ));

        public Visibility Visibility
        {
            get { return (Visibility) GetValue(VisibilityProperty); }
            set { SetValue(VisibilityProperty, value); }
        }

        public static readonly DependencyProperty ForegroundProperty = DependencyProperty
            .Register("Foreground", typeof (Brush), typeof (HintBehavior)
                , new FrameworkPropertyMetadata(new SolidColorBrush(Colors.DarkGray)
                    //, new PropertyChangedCallback(OnForegroundChanged)
                    ));

        public Brush Foreground
        {
            get { return (Brush) GetValue(ForegroundProperty); }
            set { SetValue(ForegroundProperty, value); }
        }

        public static readonly DependencyProperty MarginProperty = DependencyProperty
            .Register("Margin", typeof (Thickness), typeof (HintBehavior)
                , new FrameworkPropertyMetadata(new Thickness(4, 5, 0, 0)
                    //, new PropertyChangedCallback(OnMarginChanged)
                    ));

        public Thickness Margin
        {
            get { return (Thickness) GetValue(MarginProperty); }
            set { SetValue(MarginProperty, value); }
        }


        private static ResourceDictionary _hintBehaviorResources;

        public static ResourceDictionary HintBehaviorResources
        {
            get
            {
                if (_hintBehaviorResources == null)
                {
                    var res = new ResourceDictionary
                    {
                        Source = new Uri("/Mayflower.Client.Core;component/Behaviors/HintBehaviorResources.xaml",
                            UriKind.RelativeOrAbsolute)
                    };
                    _hintBehaviorResources = res;
                }
                return _hintBehaviorResources;
            }
        }


        protected override void OnAttached()
        {
            base.OnAttached();
            var t = (ControlTemplate) HintBehaviorResources["HintBehaviorWrapper"];
            AssociatedObject.Template = t;
            AssociatedObject.Loaded += OnLoaded;
        }

        private void OnLoaded(object sender, RoutedEventArgs e)
        {
            AssociatedObject.Loaded -= OnLoaded;
            var label = (Label) AssociatedObject.Template.FindName("PART_HintLabel", AssociatedObject);
            label.DataContext = this;
            //label.Content = "Hello...";
            label.SetBinding(UIElement.VisibilityProperty, new Binding("Visibility") {Source = this, Mode = BindingMode.OneWay});
            label.SetBinding(ContentControl.ContentProperty, new Binding("Hint") {Source = this, Mode = BindingMode.OneWay});
            label.SetBinding(Control.ForegroundProperty, new Binding("Foreground") {Source = this, Mode = BindingMode.OneWay});
            label.SetBinding(FrameworkElement.MarginProperty, new Binding("Margin") {Source = this, Mode = BindingMode.OneWay});
        }
    }

它用自己的模板包装目标，并向其添加一个标签:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <ControlTemplate x:Key="HintBehaviorWrapper" TargetType="{x:Type ContentControl}">
        <Grid>
            <ContentPresenter Content="{TemplateBinding Content}" />
            <Label x:Name="PART_HintLabel" IsHitTestVisible="False" Padding="0" />
        </Grid>
    </ControlTemplate>
</ResourceDictionary>

要使用它，只需将其添加为一个行为并绑定你的值(在我的情况下，我将它添加在一个ControlTemplate中，因此绑定):

<ContentControl>
    <i:Interaction.Behaviors>
        <behaviors:HintBehavior Value="{Binding Property, RelativeSource={RelativeSource TemplatedParent}}"
                                                        Hint="{Binding Hint, RelativeSource={RelativeSource TemplatedParent}}" />
    </i:Interaction.Behaviors>
    <TextBox ... />
</ContentControl>

如果这是一个干净的解决方案，我希望得到反馈。它不需要静态字典，因此没有内存泄漏。

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;
    }
}

我的解决办法很简单。

在我的登录窗口。xaml是这样的。

 <DockPanel HorizontalAlignment="Center" VerticalAlignment="Center" Height="80" Width="300" LastChildFill="True">
        <Button Margin="5,0,0,0" Click="login_Click" DockPanel.Dock="Right"  VerticalAlignment="Center" ToolTip="Login to system">
            Login
        </Button>
        <StackPanel>
            <TextBox x:Name="userNameWatermarked" Height="25" Foreground="Gray" Text="UserName" GotFocus="userNameWatermarked_GotFocus"></TextBox>
            <TextBox x:Name="userName" Height="25"  TextChanged="loginElement_TextChanged" Visibility="Collapsed" LostFocus="userName_LostFocus" ></TextBox>
            <TextBox x:Name="passwordWatermarked" Height="25" Foreground="Gray" Text="Password"  Margin="0,5,0,5" GotFocus="passwordWatermarked_GotFocus"></TextBox>
            <PasswordBox x:Name="password" Height="25" PasswordChanged="password_PasswordChanged" KeyUp="password_KeyUp" LostFocus="password_LostFocus" Margin="0,5,0,5" Visibility="Collapsed"></PasswordBox>
            <TextBlock x:Name="loginError" Visibility="Hidden" Foreground="Red" FontSize="12"></TextBlock>
        </StackPanel>
    </DockPanel>

代码是这样的。

private void userNameWatermarked_GotFocus(object sender, RoutedEventArgs e)
    {
        userNameWatermarked.Visibility = System.Windows.Visibility.Collapsed;
        userName.Visibility = System.Windows.Visibility.Visible;
        userName.Focus();
    }

    private void userName_LostFocus(object sender, RoutedEventArgs e)
    {
        if (string.IsNullOrEmpty(this.userName.Text))
        {
            userName.Visibility = System.Windows.Visibility.Collapsed;
            userNameWatermarked.Visibility = System.Windows.Visibility.Visible;
        }
    }

    private void passwordWatermarked_GotFocus(object sender, RoutedEventArgs e)
    {
        passwordWatermarked.Visibility = System.Windows.Visibility.Collapsed;
        password.Visibility = System.Windows.Visibility.Visible;
        password.Focus();
    }

    private void password_LostFocus(object sender, RoutedEventArgs e)
    {
        if (string.IsNullOrEmpty(this.password.Password))
        {
            password.Visibility = System.Windows.Visibility.Collapsed;
            passwordWatermarked.Visibility = System.Windows.Visibility.Visible;
        }
    }

仅仅决定隐藏或显示水印文本框就足够了。虽然不漂亮，但工作得很好。

<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>

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