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,
        ...
    },
    ...
}

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>

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。

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

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;
    }
}

我看到了John Myczek的解决方案，以及它关于ComboBox和PasswordBox兼容性的评论，所以我改进了John Myczek的解决方案，它是:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
using System.Windows.Documents;

/// <summary>
/// Class that provides the Watermark attached property
/// </summary>
public static class WatermarkService
{
    /// <summary>
    /// Watermark Attached Dependency Property
    /// </summary>
    public static readonly DependencyProperty WatermarkProperty = DependencyProperty.RegisterAttached(
       "Watermark",
       typeof(object),
       typeof(WatermarkService),
       new FrameworkPropertyMetadata((object)null, new PropertyChangedCallback(OnWatermarkChanged)));

    #region Private Fields

    /// <summary>
    /// Dictionary of ItemsControls
    /// </summary>
    private static readonly Dictionary<object, ItemsControl> itemsControls = new Dictionary<object, ItemsControl>();

    #endregion

    /// <summary>
    /// Gets the Watermark property.  This dependency property indicates the watermark for the control.
    /// </summary>
    /// <param name="d"><see cref="DependencyObject"/> to get the property from</param>
    /// <returns>The value of the Watermark property</returns>
    public static object GetWatermark(DependencyObject d)
    {
        return (object)d.GetValue(WatermarkProperty);
    }

    /// <summary>
    /// Sets the Watermark property.  This dependency property indicates the watermark for the control.
    /// </summary>
    /// <param name="d"><see cref="DependencyObject"/> to set the property on</param>
    /// <param name="value">value of the property</param>
    public static void SetWatermark(DependencyObject d, object value)
    {
        d.SetValue(WatermarkProperty, value);
    }

    /// <summary>
    /// Handles changes to the Watermark property.
    /// </summary>
    /// <param name="d"><see cref="DependencyObject"/> that fired the event</param>
    /// <param name="e">A <see cref="DependencyPropertyChangedEventArgs"/> that contains the event data.</param>
    private static void OnWatermarkChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        Control control = (Control)d;
        control.Loaded += Control_Loaded;

        if (d is TextBox || d is PasswordBox)
        {
            control.GotKeyboardFocus += Control_GotKeyboardFocus;
            control.LostKeyboardFocus += Control_Loaded;
        }
        else if (d is ComboBox)
        {
            control.GotKeyboardFocus += Control_GotKeyboardFocus;
            control.LostKeyboardFocus += Control_Loaded;
            (d as ComboBox).SelectionChanged += new SelectionChangedEventHandler(SelectionChanged);
        }
        else if (d is ItemsControl)
        {
            ItemsControl i = (ItemsControl)d;

            // for Items property  
            i.ItemContainerGenerator.ItemsChanged += ItemsChanged;
            itemsControls.Add(i.ItemContainerGenerator, i);

            // for ItemsSource property  
            DependencyPropertyDescriptor prop = DependencyPropertyDescriptor.FromProperty(ItemsControl.ItemsSourceProperty, i.GetType());
            prop.AddValueChanged(i, ItemsSourceChanged);
        }
    }

    /// <summary>
    /// Event handler for the selection changed event
    /// </summary>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">A <see cref="ItemsChangedEventArgs"/> that contains the event data.</param>
    private static void SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        Control control = (Control)sender;
        if (ShouldShowWatermark(control))
        {
            ShowWatermark(control);
        }
        else
        {
            RemoveWatermark(control);
        }
    }

    #region Event Handlers

    /// <summary>
    /// Handle the GotFocus event on the control
    /// </summary>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">A <see cref="RoutedEventArgs"/> that contains the event data.</param>
    private static void Control_GotKeyboardFocus(object sender, RoutedEventArgs e)
    {
        Control c = (Control)sender;
        if (ShouldShowWatermark(c))
        {
            RemoveWatermark(c);
        }
    }

    /// <summary>
    /// Handle the Loaded and LostFocus event on the control
    /// </summary>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">A <see cref="RoutedEventArgs"/> that contains the event data.</param>
    private static void Control_Loaded(object sender, RoutedEventArgs e)
    {
        Control control = (Control)sender;
        if (ShouldShowWatermark(control))
        {
            ShowWatermark(control);
        }
    }

    /// <summary>
    /// Event handler for the items source changed event
    /// </summary>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">A <see cref="EventArgs"/> that contains the event data.</param>
    private static void ItemsSourceChanged(object sender, EventArgs e)
    {
        ItemsControl c = (ItemsControl)sender;
        if (c.ItemsSource != null)
        {
            if (ShouldShowWatermark(c))
            {
                ShowWatermark(c);
            }
            else
            {
                RemoveWatermark(c);
            }
        }
        else
        {
            ShowWatermark(c);
        }
    }

    /// <summary>
    /// Event handler for the items changed event
    /// </summary>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">A <see cref="ItemsChangedEventArgs"/> that contains the event data.</param>
    private static void ItemsChanged(object sender, ItemsChangedEventArgs e)
    {
        ItemsControl control;
        if (itemsControls.TryGetValue(sender, out control))
        {
            if (ShouldShowWatermark(control))
            {
                ShowWatermark(control);
            }
            else
            {
                RemoveWatermark(control);
            }
        }
    }

    #endregion

    #region Helper Methods

    /// <summary>
    /// Remove the watermark from the specified element
    /// </summary>
    /// <param name="control">Element to remove the watermark from</param>
    private static void RemoveWatermark(UIElement control)
    {
        AdornerLayer layer = AdornerLayer.GetAdornerLayer(control);

        // layer could be null if control is no longer in the visual tree
        if (layer != null)
        {
            Adorner[] adorners = layer.GetAdorners(control);
            if (adorners == null)
            {
                return;
            }

            foreach (Adorner adorner in adorners)
            {
                if (adorner is WatermarkAdorner)
                {
                    adorner.Visibility = Visibility.Hidden;
                    layer.Remove(adorner);
                }
            }
        }
    }

    /// <summary>
    /// Show the watermark on the specified control
    /// </summary>
    /// <param name="control">Control to show the watermark on</param>
    private static void ShowWatermark(Control control)
    {
        AdornerLayer layer = AdornerLayer.GetAdornerLayer(control);

        // layer could be null if control is no longer in the visual tree
        if (layer != null)
        {
            layer.Add(new WatermarkAdorner(control, GetWatermark(control)));
        }
    }

    /// <summary>
    /// Indicates whether or not the watermark should be shown on the specified control
    /// </summary>
    /// <param name="c"><see cref="Control"/> to test</param>
    /// <returns>true if the watermark should be shown; false otherwise</returns>
    private static bool ShouldShowWatermark(Control c)
    {
        if (c is ComboBox)
        {
            return (c as ComboBox).SelectedItem == null;
            //return (c as ComboBox).Text == string.Empty;
        }
        else if (c is TextBoxBase)
        {
            return (c as TextBox).Text == string.Empty;
        }
        else if (c is PasswordBox)
        {
            return (c as PasswordBox).Password == string.Empty;
        }
        else if (c is ItemsControl)
        {
            return (c as ItemsControl).Items.Count == 0;
        }
        else
        {
            return false;
        }
    }

    #endregion
}

现在，一个组合框也可以编辑，密码框也可以添加水印。 别忘了用上面JoanComasFdz的评论来解决保证金问题。

当然，这都是约翰·麦切克的功劳。