我不敢相信居然没有人发布Xceed的WPF扩展工具包——WatermarkTextBox。它工作得很好，并且是开源的，如果你想定制的话。

编辑:虽然仍然是开源的，但这个库现在只对非商业用途免费，请参阅价格和许可证。

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。

看看另一个简单的解决方案:

我专注于“得到专注”和“失去专注”事件。

XAML:

<Grid>
<TextBlock x:Name="DosyaİhtivaEdenDizinYansıması" Text="Hedef Dizin Belirtin" VerticalAlignment="Center" HorizontalAlignment="Center" TextAlignment="Center" Foreground="White" Background="Transparent" Width="500" MinWidth="300" Margin="10,0,0,0" Opacity="0.7"/>
<TextBox x:Name="DosyaİhtivaEdenDizin" CaretBrush="White" Foreground="White" Background="Transparent" VerticalContentAlignment="Center" HorizontalContentAlignment="Center" VerticalAlignment="Center" HorizontalAlignment="Center" MinHeight="40" BorderThickness="1" BorderBrush="White" Width="500" MinWidth="300" Margin="10,0,0,0" GotFocus="DosyaİhtivaEdenDizin_GotFocus" LostFocus="DosyaİhtivaEdenDizin_LostFocus"/>
</Grid>

C#:

    #region DosyaİhtivaEdenDizin
    private void DosyaİhtivaEdenDizin_GotFocus(object sender, RoutedEventArgs e)
    {
        if (DosyaİhtivaEdenDizin.Text.Length == 0)
        {
            DosyaİhtivaEdenDizinYansıması.Text = "";
        }
    }

    private void DosyaİhtivaEdenDizin_LostFocus(object sender, RoutedEventArgs e)
    {
        if (DosyaİhtivaEdenDizin.Text.Length == 0)
        {
            DosyaİhtivaEdenDizinYansıması.Text = "Hedef Dizin Belirtin";
        }
    }
    #endregion

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。

您可以为输入的文本保留一个单独的值，并且可以在“GotFocus”和“LostFocus”事件中设置文本框的“text”字段。当您获得焦点时，如果没有值，则需要清除文本框。当你失去焦点时，你会想要设置从文本框中获取“文本”值，然后将文本框的“文本”值重置为占位符(如果它是空的)。

private String username = "";

private void usernameTextBox_GotFocus(object sender, RoutedEventArgs e) {
  if (String.IsNullOrEmpty(username)) {
    usernameTextBox.Text = "";
  }
}

private void usernameTextBox_LostFocus(object sender, RoutedEventArgs e) {
  username = usernameTextBox.Text;
  if (String.IsNullOrEmpty(usernameTextBox.Text)) {
    usernameTextBox.Text = "Username";
  }
}

然后，您只需确保文本框的“Text”值初始化为占位符文本。

<TextBox x:Name="usernameTextBox" Text="Username" GotFocus="usernameTextBox_GotFocus" LostFocus="usernameTextBox_LostFocus" />

您可以进一步将其提取到扩展“TextBox”类的类中，然后在整个项目中重用它。

namespace UI {
  public class PlaceholderTextBox : TextBox {
    public String Value { get; set; }
    public String PlaceholderText { get; set; }
    public Brush PlaceholderBrush { get; set; }
    private Brush ValuedBrush { get; set; }

    public PlaceholderTextBox() : base() {}

    protected override void OnInitialized(EventArgs e) {
      base.OnInitialized(e);

      ValuedBrush = this.Foreground;

      if (String.IsNullOrEmpty(this.Text)) {
        this.Text = PlaceholderText;
        this.Foreground = PlaceholderBrush;
      }
    }

    protected override void OnGotFocus(System.Windows.RoutedEventArgs e) {
      this.Foreground = ValuedBrush;
      if (String.IsNullOrEmpty(Value)) {
        this.Text = "";
      }

      base.OnGotFocus(e);
    }

    protected override void OnLostFocus(System.Windows.RoutedEventArgs e) {
      Value = this.Text;
      if (String.IsNullOrEmpty(this.Text)) {
        this.Text = PlaceholderText;
        this.Foreground = PlaceholderBrush;
      }

      base.OnLostFocus(e);
    }
  }
}

然后这个可以直接添加到xaml中。

<Window x:Class="UI.LoginWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:m="clr-namespace:UI"
        Initialized="Window_Initialized">
    <Grid>
        <m:PlaceholderTextBox x:Name="usernameTextBox" PlaceholderText="Username" PlaceholderBrush="Gray" />
    </Grid>
</Window>

<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。